1 ===================== 1 ============================
2 LINUX KERNEL MEMORY B 2 LINUX KERNEL MEMORY BARRIERS
3 ===================== 3 ============================
4 4
5 By: David Howells <dhowells@redhat.com> 5 By: David Howells <dhowells@redhat.com>
6 Paul E. McKenney <paulmck@linux.ibm.com> 6 Paul E. McKenney <paulmck@linux.ibm.com>
7 Will Deacon <will.deacon@arm.com> 7 Will Deacon <will.deacon@arm.com>
8 Peter Zijlstra <peterz@infradead.org> 8 Peter Zijlstra <peterz@infradead.org>
9 9
10 ========== 10 ==========
11 DISCLAIMER 11 DISCLAIMER
12 ========== 12 ==========
13 13
14 This document is not a specification; it is in 14 This document is not a specification; it is intentionally (for the sake of
15 brevity) and unintentionally (due to being hum 15 brevity) and unintentionally (due to being human) incomplete. This document is
16 meant as a guide to using the various memory b 16 meant as a guide to using the various memory barriers provided by Linux, but
17 in case of any doubt (and there are many) plea 17 in case of any doubt (and there are many) please ask. Some doubts may be
18 resolved by referring to the formal memory con 18 resolved by referring to the formal memory consistency model and related
19 documentation at tools/memory-model/. Neverth 19 documentation at tools/memory-model/. Nevertheless, even this memory
20 model should be viewed as the collective opini 20 model should be viewed as the collective opinion of its maintainers rather
21 than as an infallible oracle. 21 than as an infallible oracle.
22 22
23 To repeat, this document is not a specificatio 23 To repeat, this document is not a specification of what Linux expects from
24 hardware. 24 hardware.
25 25
26 The purpose of this document is twofold: 26 The purpose of this document is twofold:
27 27
28 (1) to specify the minimum functionality that 28 (1) to specify the minimum functionality that one can rely on for any
29 particular barrier, and 29 particular barrier, and
30 30
31 (2) to provide a guide as to how to use the b 31 (2) to provide a guide as to how to use the barriers that are available.
32 32
33 Note that an architecture can provide more tha 33 Note that an architecture can provide more than the minimum requirement
34 for any particular barrier, but if the archite 34 for any particular barrier, but if the architecture provides less than
35 that, that architecture is incorrect. 35 that, that architecture is incorrect.
36 36
37 Note also that it is possible that a barrier m 37 Note also that it is possible that a barrier may be a no-op for an
38 architecture because the way that arch works r 38 architecture because the way that arch works renders an explicit barrier
39 unnecessary in that case. 39 unnecessary in that case.
40 40
41 41
42 ======== 42 ========
43 CONTENTS 43 CONTENTS
44 ======== 44 ========
45 45
46 (*) Abstract memory access model. 46 (*) Abstract memory access model.
47 47
48 - Device operations. 48 - Device operations.
49 - Guarantees. 49 - Guarantees.
50 50
51 (*) What are memory barriers? 51 (*) What are memory barriers?
52 52
53 - Varieties of memory barrier. 53 - Varieties of memory barrier.
54 - What may not be assumed about memory ba 54 - What may not be assumed about memory barriers?
55 - Address-dependency barriers (historical 55 - Address-dependency barriers (historical).
56 - Control dependencies. 56 - Control dependencies.
57 - SMP barrier pairing. 57 - SMP barrier pairing.
58 - Examples of memory barrier sequences. 58 - Examples of memory barrier sequences.
59 - Read memory barriers vs load speculatio 59 - Read memory barriers vs load speculation.
60 - Multicopy atomicity. 60 - Multicopy atomicity.
61 61
62 (*) Explicit kernel barriers. 62 (*) Explicit kernel barriers.
63 63
64 - Compiler barrier. 64 - Compiler barrier.
65 - CPU memory barriers. 65 - CPU memory barriers.
66 66
67 (*) Implicit kernel memory barriers. 67 (*) Implicit kernel memory barriers.
68 68
69 - Lock acquisition functions. 69 - Lock acquisition functions.
70 - Interrupt disabling functions. 70 - Interrupt disabling functions.
71 - Sleep and wake-up functions. 71 - Sleep and wake-up functions.
72 - Miscellaneous functions. 72 - Miscellaneous functions.
73 73
74 (*) Inter-CPU acquiring barrier effects. 74 (*) Inter-CPU acquiring barrier effects.
75 75
76 - Acquires vs memory accesses. 76 - Acquires vs memory accesses.
77 77
78 (*) Where are memory barriers needed? 78 (*) Where are memory barriers needed?
79 79
80 - Interprocessor interaction. 80 - Interprocessor interaction.
81 - Atomic operations. 81 - Atomic operations.
82 - Accessing devices. 82 - Accessing devices.
83 - Interrupts. 83 - Interrupts.
84 84
85 (*) Kernel I/O barrier effects. 85 (*) Kernel I/O barrier effects.
86 86
87 (*) Assumed minimum execution ordering model. 87 (*) Assumed minimum execution ordering model.
88 88
89 (*) The effects of the cpu cache. 89 (*) The effects of the cpu cache.
90 90
91 - Cache coherency. 91 - Cache coherency.
92 - Cache coherency vs DMA. 92 - Cache coherency vs DMA.
93 - Cache coherency vs MMIO. 93 - Cache coherency vs MMIO.
94 94
95 (*) The things CPUs get up to. 95 (*) The things CPUs get up to.
96 96
97 - And then there's the Alpha. 97 - And then there's the Alpha.
98 - Virtual Machine Guests. 98 - Virtual Machine Guests.
99 99
100 (*) Example uses. 100 (*) Example uses.
101 101
102 - Circular buffers. 102 - Circular buffers.
103 103
104 (*) References. 104 (*) References.
105 105
106 106
107 ============================ 107 ============================
108 ABSTRACT MEMORY ACCESS MODEL 108 ABSTRACT MEMORY ACCESS MODEL
109 ============================ 109 ============================
110 110
111 Consider the following abstract model of the s 111 Consider the following abstract model of the system:
112 112
113 : : 113 : :
114 : : 114 : :
115 : : 115 : :
116 +-------+ : +--------+ : 116 +-------+ : +--------+ : +-------+
117 | | : | | : 117 | | : | | : | |
118 | | : | | : 118 | | : | | : | |
119 | CPU 1 |<----->| Memory |<--- 119 | CPU 1 |<----->| Memory |<----->| CPU 2 |
120 | | : | | : 120 | | : | | : | |
121 | | : | | : 121 | | : | | : | |
122 +-------+ : +--------+ : 122 +-------+ : +--------+ : +-------+
123 ^ : ^ : 123 ^ : ^ : ^
124 | : | : 124 | : | : |
125 | : | : 125 | : | : |
126 | : v : 126 | : v : |
127 | : +--------+ : 127 | : +--------+ : |
128 | : | | : 128 | : | | : |
129 | : | | : 129 | : | | : |
130 +---------->| Device |<--- 130 +---------->| Device |<----------+
131 : | | : 131 : | | :
132 : | | : 132 : | | :
133 : +--------+ : 133 : +--------+ :
134 : : 134 : :
135 135
136 Each CPU executes a program that generates mem 136 Each CPU executes a program that generates memory access operations. In the
137 abstract CPU, memory operation ordering is ver 137 abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
138 perform the memory operations in any order it 138 perform the memory operations in any order it likes, provided program causality
139 appears to be maintained. Similarly, the comp 139 appears to be maintained. Similarly, the compiler may also arrange the
140 instructions it emits in any order it likes, p 140 instructions it emits in any order it likes, provided it doesn't affect the
141 apparent operation of the program. 141 apparent operation of the program.
142 142
143 So in the above diagram, the effects of the me 143 So in the above diagram, the effects of the memory operations performed by a
144 CPU are perceived by the rest of the system as 144 CPU are perceived by the rest of the system as the operations cross the
145 interface between the CPU and rest of the syst 145 interface between the CPU and rest of the system (the dotted lines).
146 146
147 147
148 For example, consider the following sequence o 148 For example, consider the following sequence of events:
149 149
150 CPU 1 CPU 2 150 CPU 1 CPU 2
151 =============== =============== 151 =============== ===============
152 { A == 1; B == 2 } 152 { A == 1; B == 2 }
153 A = 3; x = B; 153 A = 3; x = B;
154 B = 4; y = A; 154 B = 4; y = A;
155 155
156 The set of accesses as seen by the memory syst 156 The set of accesses as seen by the memory system in the middle can be arranged
157 in 24 different combinations: 157 in 24 different combinations:
158 158
159 STORE A=3, STORE B=4, y=LOAD 159 STORE A=3, STORE B=4, y=LOAD A->3, x=LOAD B->4
160 STORE A=3, STORE B=4, x=LOAD 160 STORE A=3, STORE B=4, x=LOAD B->4, y=LOAD A->3
161 STORE A=3, y=LOAD A->3, STORE 161 STORE A=3, y=LOAD A->3, STORE B=4, x=LOAD B->4
162 STORE A=3, y=LOAD A->3, x=LOAD 162 STORE A=3, y=LOAD A->3, x=LOAD B->2, STORE B=4
163 STORE A=3, x=LOAD B->2, STORE 163 STORE A=3, x=LOAD B->2, STORE B=4, y=LOAD A->3
164 STORE A=3, x=LOAD B->2, y=LOAD 164 STORE A=3, x=LOAD B->2, y=LOAD A->3, STORE B=4
165 STORE B=4, STORE A=3, y=LOAD 165 STORE B=4, STORE A=3, y=LOAD A->3, x=LOAD B->4
166 STORE B=4, ... 166 STORE B=4, ...
167 ... 167 ...
168 168
169 and can thus result in four different combinat 169 and can thus result in four different combinations of values:
170 170
171 x == 2, y == 1 171 x == 2, y == 1
172 x == 2, y == 3 172 x == 2, y == 3
173 x == 4, y == 1 173 x == 4, y == 1
174 x == 4, y == 3 174 x == 4, y == 3
175 175
176 176
177 Furthermore, the stores committed by a CPU to 177 Furthermore, the stores committed by a CPU to the memory system may not be
178 perceived by the loads made by another CPU in 178 perceived by the loads made by another CPU in the same order as the stores were
179 committed. 179 committed.
180 180
181 181
182 As a further example, consider this sequence o 182 As a further example, consider this sequence of events:
183 183
184 CPU 1 CPU 2 184 CPU 1 CPU 2
185 =============== =============== 185 =============== ===============
186 { A == 1, B == 2, C == 3, P == &A, Q = 186 { A == 1, B == 2, C == 3, P == &A, Q == &C }
187 B = 4; Q = P; 187 B = 4; Q = P;
188 P = &B; D = *Q; 188 P = &B; D = *Q;
189 189
190 There is an obvious address dependency here, a 190 There is an obvious address dependency here, as the value loaded into D depends
191 on the address retrieved from P by CPU 2. At 191 on the address retrieved from P by CPU 2. At the end of the sequence, any of
192 the following results are possible: 192 the following results are possible:
193 193
194 (Q == &A) and (D == 1) 194 (Q == &A) and (D == 1)
195 (Q == &B) and (D == 2) 195 (Q == &B) and (D == 2)
196 (Q == &B) and (D == 4) 196 (Q == &B) and (D == 4)
197 197
198 Note that CPU 2 will never try and load C into 198 Note that CPU 2 will never try and load C into D because the CPU will load P
199 into Q before issuing the load of *Q. 199 into Q before issuing the load of *Q.
200 200
201 201
202 DEVICE OPERATIONS 202 DEVICE OPERATIONS
203 ----------------- 203 -----------------
204 204
205 Some devices present their control interfaces 205 Some devices present their control interfaces as collections of memory
206 locations, but the order in which the control 206 locations, but the order in which the control registers are accessed is very
207 important. For instance, imagine an ethernet 207 important. For instance, imagine an ethernet card with a set of internal
208 registers that are accessed through an address 208 registers that are accessed through an address port register (A) and a data
209 port register (D). To read internal register 209 port register (D). To read internal register 5, the following code might then
210 be used: 210 be used:
211 211
212 *A = 5; 212 *A = 5;
213 x = *D; 213 x = *D;
214 214
215 but this might show up as either of the follow 215 but this might show up as either of the following two sequences:
216 216
217 STORE *A = 5, x = LOAD *D 217 STORE *A = 5, x = LOAD *D
218 x = LOAD *D, STORE *A = 5 218 x = LOAD *D, STORE *A = 5
219 219
220 the second of which will almost certainly resu 220 the second of which will almost certainly result in a malfunction, since it set
221 the address _after_ attempting to read the reg 221 the address _after_ attempting to read the register.
222 222
223 223
224 GUARANTEES 224 GUARANTEES
225 ---------- 225 ----------
226 226
227 There are some minimal guarantees that may be 227 There are some minimal guarantees that may be expected of a CPU:
228 228
229 (*) On any given CPU, dependent memory access 229 (*) On any given CPU, dependent memory accesses will be issued in order, with
230 respect to itself. This means that for: 230 respect to itself. This means that for:
231 231
232 Q = READ_ONCE(P); D = READ_ONCE(*Q); 232 Q = READ_ONCE(P); D = READ_ONCE(*Q);
233 233
234 the CPU will issue the following memory o 234 the CPU will issue the following memory operations:
235 235
236 Q = LOAD P, D = LOAD *Q 236 Q = LOAD P, D = LOAD *Q
237 237
238 and always in that order. However, on DE 238 and always in that order. However, on DEC Alpha, READ_ONCE() also
239 emits a memory-barrier instruction, so th 239 emits a memory-barrier instruction, so that a DEC Alpha CPU will
240 instead issue the following memory operat 240 instead issue the following memory operations:
241 241
242 Q = LOAD P, MEMORY_BARRIER, D = LOAD * 242 Q = LOAD P, MEMORY_BARRIER, D = LOAD *Q, MEMORY_BARRIER
243 243
244 Whether on DEC Alpha or not, the READ_ONC 244 Whether on DEC Alpha or not, the READ_ONCE() also prevents compiler
245 mischief. 245 mischief.
246 246
247 (*) Overlapping loads and stores within a par 247 (*) Overlapping loads and stores within a particular CPU will appear to be
248 ordered within that CPU. This means that 248 ordered within that CPU. This means that for:
249 249
250 a = READ_ONCE(*X); WRITE_ONCE(*X, b); 250 a = READ_ONCE(*X); WRITE_ONCE(*X, b);
251 251
252 the CPU will only issue the following seq 252 the CPU will only issue the following sequence of memory operations:
253 253
254 a = LOAD *X, STORE *X = b 254 a = LOAD *X, STORE *X = b
255 255
256 And for: 256 And for:
257 257
258 WRITE_ONCE(*X, c); d = READ_ONCE(*X); 258 WRITE_ONCE(*X, c); d = READ_ONCE(*X);
259 259
260 the CPU will only issue: 260 the CPU will only issue:
261 261
262 STORE *X = c, d = LOAD *X 262 STORE *X = c, d = LOAD *X
263 263
264 (Loads and stores overlap if they are tar 264 (Loads and stores overlap if they are targeted at overlapping pieces of
265 memory). 265 memory).
266 266
267 And there are a number of things that _must_ o 267 And there are a number of things that _must_ or _must_not_ be assumed:
268 268
269 (*) It _must_not_ be assumed that the compile 269 (*) It _must_not_ be assumed that the compiler will do what you want
270 with memory references that are not prote 270 with memory references that are not protected by READ_ONCE() and
271 WRITE_ONCE(). Without them, the compiler 271 WRITE_ONCE(). Without them, the compiler is within its rights to
272 do all sorts of "creative" transformation 272 do all sorts of "creative" transformations, which are covered in
273 the COMPILER BARRIER section. 273 the COMPILER BARRIER section.
274 274
275 (*) It _must_not_ be assumed that independent 275 (*) It _must_not_ be assumed that independent loads and stores will be issued
276 in the order given. This means that for: 276 in the order given. This means that for:
277 277
278 X = *A; Y = *B; *D = Z; 278 X = *A; Y = *B; *D = Z;
279 279
280 we may get any of the following sequences 280 we may get any of the following sequences:
281 281
282 X = LOAD *A, Y = LOAD *B, STORE *D = 282 X = LOAD *A, Y = LOAD *B, STORE *D = Z
283 X = LOAD *A, STORE *D = Z, Y = LOAD * 283 X = LOAD *A, STORE *D = Z, Y = LOAD *B
284 Y = LOAD *B, X = LOAD *A, STORE *D = 284 Y = LOAD *B, X = LOAD *A, STORE *D = Z
285 Y = LOAD *B, STORE *D = Z, X = LOAD * 285 Y = LOAD *B, STORE *D = Z, X = LOAD *A
286 STORE *D = Z, X = LOAD *A, Y = LOAD * 286 STORE *D = Z, X = LOAD *A, Y = LOAD *B
287 STORE *D = Z, Y = LOAD *B, X = LOAD * 287 STORE *D = Z, Y = LOAD *B, X = LOAD *A
288 288
289 (*) It _must_ be assumed that overlapping mem 289 (*) It _must_ be assumed that overlapping memory accesses may be merged or
290 discarded. This means that for: 290 discarded. This means that for:
291 291
292 X = *A; Y = *(A + 4); 292 X = *A; Y = *(A + 4);
293 293
294 we may get any one of the following seque 294 we may get any one of the following sequences:
295 295
296 X = LOAD *A; Y = LOAD *(A + 4); 296 X = LOAD *A; Y = LOAD *(A + 4);
297 Y = LOAD *(A + 4); X = LOAD *A; 297 Y = LOAD *(A + 4); X = LOAD *A;
298 {X, Y} = LOAD {*A, *(A + 4) }; 298 {X, Y} = LOAD {*A, *(A + 4) };
299 299
300 And for: 300 And for:
301 301
302 *A = X; *(A + 4) = Y; 302 *A = X; *(A + 4) = Y;
303 303
304 we may get any of: 304 we may get any of:
305 305
306 STORE *A = X; STORE *(A + 4) = Y; 306 STORE *A = X; STORE *(A + 4) = Y;
307 STORE *(A + 4) = Y; STORE *A = X; 307 STORE *(A + 4) = Y; STORE *A = X;
308 STORE {*A, *(A + 4) } = {X, Y}; 308 STORE {*A, *(A + 4) } = {X, Y};
309 309
310 And there are anti-guarantees: 310 And there are anti-guarantees:
311 311
312 (*) These guarantees do not apply to bitfield 312 (*) These guarantees do not apply to bitfields, because compilers often
313 generate code to modify these using non-a 313 generate code to modify these using non-atomic read-modify-write
314 sequences. Do not attempt to use bitfiel 314 sequences. Do not attempt to use bitfields to synchronize parallel
315 algorithms. 315 algorithms.
316 316
317 (*) Even in cases where bitfields are protect 317 (*) Even in cases where bitfields are protected by locks, all fields
318 in a given bitfield must be protected by 318 in a given bitfield must be protected by one lock. If two fields
319 in a given bitfield are protected by diff 319 in a given bitfield are protected by different locks, the compiler's
320 non-atomic read-modify-write sequences ca 320 non-atomic read-modify-write sequences can cause an update to one
321 field to corrupt the value of an adjacent 321 field to corrupt the value of an adjacent field.
322 322
323 (*) These guarantees apply only to properly a 323 (*) These guarantees apply only to properly aligned and sized scalar
324 variables. "Properly sized" currently me 324 variables. "Properly sized" currently means variables that are
325 the same size as "char", "short", "int" a 325 the same size as "char", "short", "int" and "long". "Properly
326 aligned" means the natural alignment, thu 326 aligned" means the natural alignment, thus no constraints for
327 "char", two-byte alignment for "short", f 327 "char", two-byte alignment for "short", four-byte alignment for
328 "int", and either four-byte or eight-byte 328 "int", and either four-byte or eight-byte alignment for "long",
329 on 32-bit and 64-bit systems, respectivel 329 on 32-bit and 64-bit systems, respectively. Note that these
330 guarantees were introduced into the C11 s 330 guarantees were introduced into the C11 standard, so beware when
331 using older pre-C11 compilers (for exampl 331 using older pre-C11 compilers (for example, gcc 4.6). The portion
332 of the standard containing this guarantee 332 of the standard containing this guarantee is Section 3.14, which
333 defines "memory location" as follows: 333 defines "memory location" as follows:
334 334
335 memory location 335 memory location
336 either an object of scalar typ 336 either an object of scalar type, or a maximal sequence
337 of adjacent bit-fields all hav 337 of adjacent bit-fields all having nonzero width
338 338
339 NOTE 1: Two threads of executi 339 NOTE 1: Two threads of execution can update and access
340 separate memory locations with 340 separate memory locations without interfering with
341 each other. 341 each other.
342 342
343 NOTE 2: A bit-field and an adj 343 NOTE 2: A bit-field and an adjacent non-bit-field member
344 are in separate memory locatio 344 are in separate memory locations. The same applies
345 to two bit-fields, if one is d 345 to two bit-fields, if one is declared inside a nested
346 structure declaration and the 346 structure declaration and the other is not, or if the two
347 are separated by a zero-length 347 are separated by a zero-length bit-field declaration,
348 or if they are separated by a 348 or if they are separated by a non-bit-field member
349 declaration. It is not safe to 349 declaration. It is not safe to concurrently update two
350 bit-fields in the same structu 350 bit-fields in the same structure if all members declared
351 between them are also bit-fiel 351 between them are also bit-fields, no matter what the
352 sizes of those intervening bit 352 sizes of those intervening bit-fields happen to be.
353 353
354 354
355 ========================= 355 =========================
356 WHAT ARE MEMORY BARRIERS? 356 WHAT ARE MEMORY BARRIERS?
357 ========================= 357 =========================
358 358
359 As can be seen above, independent memory opera 359 As can be seen above, independent memory operations are effectively performed
360 in random order, but this can be a problem for 360 in random order, but this can be a problem for CPU-CPU interaction and for I/O.
361 What is required is some way of intervening to 361 What is required is some way of intervening to instruct the compiler and the
362 CPU to restrict the order. 362 CPU to restrict the order.
363 363
364 Memory barriers are such interventions. They 364 Memory barriers are such interventions. They impose a perceived partial
365 ordering over the memory operations on either 365 ordering over the memory operations on either side of the barrier.
366 366
367 Such enforcement is important because the CPUs 367 Such enforcement is important because the CPUs and other devices in a system
368 can use a variety of tricks to improve perform 368 can use a variety of tricks to improve performance, including reordering,
369 deferral and combination of memory operations; 369 deferral and combination of memory operations; speculative loads; speculative
370 branch prediction and various types of caching 370 branch prediction and various types of caching. Memory barriers are used to
371 override or suppress these tricks, allowing th 371 override or suppress these tricks, allowing the code to sanely control the
372 interaction of multiple CPUs and/or devices. 372 interaction of multiple CPUs and/or devices.
373 373
374 374
375 VARIETIES OF MEMORY BARRIER 375 VARIETIES OF MEMORY BARRIER
376 --------------------------- 376 ---------------------------
377 377
378 Memory barriers come in four basic varieties: 378 Memory barriers come in four basic varieties:
379 379
380 (1) Write (or store) memory barriers. 380 (1) Write (or store) memory barriers.
381 381
382 A write memory barrier gives a guarantee 382 A write memory barrier gives a guarantee that all the STORE operations
383 specified before the barrier will appear 383 specified before the barrier will appear to happen before all the STORE
384 operations specified after the barrier wi 384 operations specified after the barrier with respect to the other
385 components of the system. 385 components of the system.
386 386
387 A write barrier is a partial ordering on 387 A write barrier is a partial ordering on stores only; it is not required
388 to have any effect on loads. 388 to have any effect on loads.
389 389
390 A CPU can be viewed as committing a seque 390 A CPU can be viewed as committing a sequence of store operations to the
391 memory system as time progresses. All st 391 memory system as time progresses. All stores _before_ a write barrier
392 will occur _before_ all the stores after 392 will occur _before_ all the stores after the write barrier.
393 393
394 [!] Note that write barriers should norma 394 [!] Note that write barriers should normally be paired with read or
395 address-dependency barriers; see the "SMP 395 address-dependency barriers; see the "SMP barrier pairing" subsection.
396 396
397 397
398 (2) Address-dependency barriers (historical). 398 (2) Address-dependency barriers (historical).
399 [!] This section is marked as HISTORICAL: <<
400 smp_read_barrier_depends() macro, the sem <<
401 implicit in all marked accesses. For mor <<
402 including how compiler transformations ca <<
403 dependencies, see Documentation/RCU/rcu_d <<
404 399
405 An address-dependency barrier is a weaker 400 An address-dependency barrier is a weaker form of read barrier. In the
406 case where two loads are performed such t 401 case where two loads are performed such that the second depends on the
407 result of the first (eg: the first load r 402 result of the first (eg: the first load retrieves the address to which
408 the second load will be directed), an add 403 the second load will be directed), an address-dependency barrier would
409 be required to make sure that the target 404 be required to make sure that the target of the second load is updated
410 after the address obtained by the first l 405 after the address obtained by the first load is accessed.
411 406
412 An address-dependency barrier is a partia 407 An address-dependency barrier is a partial ordering on interdependent
413 loads only; it is not required to have an 408 loads only; it is not required to have any effect on stores, independent
414 loads or overlapping loads. 409 loads or overlapping loads.
415 410
416 As mentioned in (1), the other CPUs in th 411 As mentioned in (1), the other CPUs in the system can be viewed as
417 committing sequences of stores to the mem 412 committing sequences of stores to the memory system that the CPU being
418 considered can then perceive. An address 413 considered can then perceive. An address-dependency barrier issued by
419 the CPU under consideration guarantees th 414 the CPU under consideration guarantees that for any load preceding it,
420 if that load touches one of a sequence of 415 if that load touches one of a sequence of stores from another CPU, then
421 by the time the barrier completes, the ef 416 by the time the barrier completes, the effects of all the stores prior to
422 that touched by the load will be percepti 417 that touched by the load will be perceptible to any loads issued after
423 the address-dependency barrier. 418 the address-dependency barrier.
424 419
425 See the "Examples of memory barrier seque 420 See the "Examples of memory barrier sequences" subsection for diagrams
426 showing the ordering constraints. 421 showing the ordering constraints.
427 422
428 [!] Note that the first load really has t 423 [!] Note that the first load really has to have an _address_ dependency and
429 not a control dependency. If the address 424 not a control dependency. If the address for the second load is dependent
430 on the first load, but the dependency is 425 on the first load, but the dependency is through a conditional rather than
431 actually loading the address itself, then 426 actually loading the address itself, then it's a _control_ dependency and
432 a full read barrier or better is required 427 a full read barrier or better is required. See the "Control dependencies"
433 subsection for more information. 428 subsection for more information.
434 429
435 [!] Note that address-dependency barriers 430 [!] Note that address-dependency barriers should normally be paired with
436 write barriers; see the "SMP barrier pair 431 write barriers; see the "SMP barrier pairing" subsection.
437 432
438 [!] Kernel release v5.9 removed kernel AP 433 [!] Kernel release v5.9 removed kernel APIs for explicit address-
439 dependency barriers. Nowadays, APIs for 434 dependency barriers. Nowadays, APIs for marking loads from shared
440 variables such as READ_ONCE() and rcu_der 435 variables such as READ_ONCE() and rcu_dereference() provide implicit
441 address-dependency barriers. 436 address-dependency barriers.
442 437
443 (3) Read (or load) memory barriers. 438 (3) Read (or load) memory barriers.
444 439
445 A read barrier is an address-dependency b 440 A read barrier is an address-dependency barrier plus a guarantee that all
446 the LOAD operations specified before the 441 the LOAD operations specified before the barrier will appear to happen
447 before all the LOAD operations specified 442 before all the LOAD operations specified after the barrier with respect to
448 the other components of the system. 443 the other components of the system.
449 444
450 A read barrier is a partial ordering on l 445 A read barrier is a partial ordering on loads only; it is not required to
451 have any effect on stores. 446 have any effect on stores.
452 447
453 Read memory barriers imply address-depend 448 Read memory barriers imply address-dependency barriers, and so can
454 substitute for them. 449 substitute for them.
455 450
456 [!] Note that read barriers should normal 451 [!] Note that read barriers should normally be paired with write barriers;
457 see the "SMP barrier pairing" subsection. 452 see the "SMP barrier pairing" subsection.
458 453
459 454
460 (4) General memory barriers. 455 (4) General memory barriers.
461 456
462 A general memory barrier gives a guarante 457 A general memory barrier gives a guarantee that all the LOAD and STORE
463 operations specified before the barrier w 458 operations specified before the barrier will appear to happen before all
464 the LOAD and STORE operations specified a 459 the LOAD and STORE operations specified after the barrier with respect to
465 the other components of the system. 460 the other components of the system.
466 461
467 A general memory barrier is a partial ord 462 A general memory barrier is a partial ordering over both loads and stores.
468 463
469 General memory barriers imply both read a 464 General memory barriers imply both read and write memory barriers, and so
470 can substitute for either. 465 can substitute for either.
471 466
472 467
473 And a couple of implicit varieties: 468 And a couple of implicit varieties:
474 469
475 (5) ACQUIRE operations. 470 (5) ACQUIRE operations.
476 471
477 This acts as a one-way permeable barrier. 472 This acts as a one-way permeable barrier. It guarantees that all memory
478 operations after the ACQUIRE operation wi 473 operations after the ACQUIRE operation will appear to happen after the
479 ACQUIRE operation with respect to the oth 474 ACQUIRE operation with respect to the other components of the system.
480 ACQUIRE operations include LOCK operation 475 ACQUIRE operations include LOCK operations and both smp_load_acquire()
481 and smp_cond_load_acquire() operations. 476 and smp_cond_load_acquire() operations.
482 477
483 Memory operations that occur before an AC 478 Memory operations that occur before an ACQUIRE operation may appear to
484 happen after it completes. 479 happen after it completes.
485 480
486 An ACQUIRE operation should almost always 481 An ACQUIRE operation should almost always be paired with a RELEASE
487 operation. 482 operation.
488 483
489 484
490 (6) RELEASE operations. 485 (6) RELEASE operations.
491 486
492 This also acts as a one-way permeable bar 487 This also acts as a one-way permeable barrier. It guarantees that all
493 memory operations before the RELEASE oper 488 memory operations before the RELEASE operation will appear to happen
494 before the RELEASE operation with respect 489 before the RELEASE operation with respect to the other components of the
495 system. RELEASE operations include UNLOCK 490 system. RELEASE operations include UNLOCK operations and
496 smp_store_release() operations. 491 smp_store_release() operations.
497 492
498 Memory operations that occur after a RELE 493 Memory operations that occur after a RELEASE operation may appear to
499 happen before it completes. 494 happen before it completes.
500 495
501 The use of ACQUIRE and RELEASE operations 496 The use of ACQUIRE and RELEASE operations generally precludes the need
502 for other sorts of memory barrier. In ad 497 for other sorts of memory barrier. In addition, a RELEASE+ACQUIRE pair is
503 -not- guaranteed to act as a full memory 498 -not- guaranteed to act as a full memory barrier. However, after an
504 ACQUIRE on a given variable, all memory a 499 ACQUIRE on a given variable, all memory accesses preceding any prior
505 RELEASE on that same variable are guarant 500 RELEASE on that same variable are guaranteed to be visible. In other
506 words, within a given variable's critical 501 words, within a given variable's critical section, all accesses of all
507 previous critical sections for that varia 502 previous critical sections for that variable are guaranteed to have
508 completed. 503 completed.
509 504
510 This means that ACQUIRE acts as a minimal 505 This means that ACQUIRE acts as a minimal "acquire" operation and
511 RELEASE acts as a minimal "release" opera 506 RELEASE acts as a minimal "release" operation.
512 507
513 A subset of the atomic operations described in 508 A subset of the atomic operations described in atomic_t.txt have ACQUIRE and
514 RELEASE variants in addition to fully-ordered 509 RELEASE variants in addition to fully-ordered and relaxed (no barrier
515 semantics) definitions. For compound atomics 510 semantics) definitions. For compound atomics performing both a load and a
516 store, ACQUIRE semantics apply only to the loa 511 store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
517 only to the store portion of the operation. 512 only to the store portion of the operation.
518 513
519 Memory barriers are only required where there' 514 Memory barriers are only required where there's a possibility of interaction
520 between two CPUs or between a CPU and a device 515 between two CPUs or between a CPU and a device. If it can be guaranteed that
521 there won't be any such interaction in any par 516 there won't be any such interaction in any particular piece of code, then
522 memory barriers are unnecessary in that piece 517 memory barriers are unnecessary in that piece of code.
523 518
524 519
525 Note that these are the _minimum_ guarantees. 520 Note that these are the _minimum_ guarantees. Different architectures may give
526 more substantial guarantees, but they may _not 521 more substantial guarantees, but they may _not_ be relied upon outside of arch
527 specific code. 522 specific code.
528 523
529 524
530 WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS? 525 WHAT MAY NOT BE ASSUMED ABOUT MEMORY BARRIERS?
531 ---------------------------------------------- 526 ----------------------------------------------
532 527
533 There are certain things that the Linux kernel 528 There are certain things that the Linux kernel memory barriers do not guarantee:
534 529
535 (*) There is no guarantee that any of the mem 530 (*) There is no guarantee that any of the memory accesses specified before a
536 memory barrier will be _complete_ by the 531 memory barrier will be _complete_ by the completion of a memory barrier
537 instruction; the barrier can be considere 532 instruction; the barrier can be considered to draw a line in that CPU's
538 access queue that accesses of the appropr 533 access queue that accesses of the appropriate type may not cross.
539 534
540 (*) There is no guarantee that issuing a memo 535 (*) There is no guarantee that issuing a memory barrier on one CPU will have
541 any direct effect on another CPU or any o 536 any direct effect on another CPU or any other hardware in the system. The
542 indirect effect will be the order in whic 537 indirect effect will be the order in which the second CPU sees the effects
543 of the first CPU's accesses occur, but se 538 of the first CPU's accesses occur, but see the next point:
544 539
545 (*) There is no guarantee that a CPU will see 540 (*) There is no guarantee that a CPU will see the correct order of effects
546 from a second CPU's accesses, even _if_ t 541 from a second CPU's accesses, even _if_ the second CPU uses a memory
547 barrier, unless the first CPU _also_ uses 542 barrier, unless the first CPU _also_ uses a matching memory barrier (see
548 the subsection on "SMP Barrier Pairing"). 543 the subsection on "SMP Barrier Pairing").
549 544
550 (*) There is no guarantee that some interveni 545 (*) There is no guarantee that some intervening piece of off-the-CPU
551 hardware[*] will not reorder the memory a 546 hardware[*] will not reorder the memory accesses. CPU cache coherency
552 mechanisms should propagate the indirect 547 mechanisms should propagate the indirect effects of a memory barrier
553 between CPUs, but might not do so in orde 548 between CPUs, but might not do so in order.
554 549
555 [*] For information on bus mastering D 550 [*] For information on bus mastering DMA and coherency please read:
556 551
557 Documentation/driver-api/pci/pci.r 552 Documentation/driver-api/pci/pci.rst
558 Documentation/core-api/dma-api-how 553 Documentation/core-api/dma-api-howto.rst
559 Documentation/core-api/dma-api.rst 554 Documentation/core-api/dma-api.rst
560 555
561 556
562 ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) 557 ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
563 ---------------------------------------- 558 ----------------------------------------
564 [!] This section is marked as HISTORICAL: it c <<
565 smp_read_barrier_depends() macro, the semantic <<
566 in all marked accesses. For more up-to-date i <<
567 how compiler transformations can sometimes bre <<
568 see Documentation/RCU/rcu_dereference.rst. <<
569 559
570 As of v4.15 of the Linux kernel, an smp_mb() w 560 As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
571 DEC Alpha, which means that about the only peo 561 DEC Alpha, which means that about the only people who need to pay attention
572 to this section are those working on DEC Alpha 562 to this section are those working on DEC Alpha architecture-specific code
573 and those working on READ_ONCE() itself. For 563 and those working on READ_ONCE() itself. For those who need it, and for
574 those who are interested in the history, here 564 those who are interested in the history, here is the story of
575 address-dependency barriers. 565 address-dependency barriers.
576 566
577 [!] While address dependencies are observed in 567 [!] While address dependencies are observed in both load-to-load and
578 load-to-store relations, address-dependency ba 568 load-to-store relations, address-dependency barriers are not necessary
579 for load-to-store situations. 569 for load-to-store situations.
580 570
581 The requirement of address-dependency barriers 571 The requirement of address-dependency barriers is a little subtle, and
582 it's not always obvious that they're needed. 572 it's not always obvious that they're needed. To illustrate, consider the
583 following sequence of events: 573 following sequence of events:
584 574
585 CPU 1 CPU 2 575 CPU 1 CPU 2
586 =============== =============== 576 =============== ===============
587 { A == 1, B == 2, C == 3, P == &A, Q = 577 { A == 1, B == 2, C == 3, P == &A, Q == &C }
588 B = 4; 578 B = 4;
589 <write barrier> 579 <write barrier>
590 WRITE_ONCE(P, &B); 580 WRITE_ONCE(P, &B);
591 Q = READ_ONCE_OL 581 Q = READ_ONCE_OLD(P);
592 D = *Q; 582 D = *Q;
593 583
594 [!] READ_ONCE_OLD() corresponds to READ_ONCE() 584 [!] READ_ONCE_OLD() corresponds to READ_ONCE() of pre-4.15 kernel, which
595 doesn't imply an address-dependency barrier. 585 doesn't imply an address-dependency barrier.
596 586
597 There's a clear address dependency here, and i 587 There's a clear address dependency here, and it would seem that by the end of
598 the sequence, Q must be either &A or &B, and t 588 the sequence, Q must be either &A or &B, and that:
599 589
600 (Q == &A) implies (D == 1) 590 (Q == &A) implies (D == 1)
601 (Q == &B) implies (D == 4) 591 (Q == &B) implies (D == 4)
602 592
603 But! CPU 2's perception of P may be updated _ 593 But! CPU 2's perception of P may be updated _before_ its perception of B, thus
604 leading to the following situation: 594 leading to the following situation:
605 595
606 (Q == &B) and (D == 2) ???? 596 (Q == &B) and (D == 2) ????
607 597
608 While this may seem like a failure of coherenc 598 While this may seem like a failure of coherency or causality maintenance, it
609 isn't, and this behaviour can be observed on c 599 isn't, and this behaviour can be observed on certain real CPUs (such as the DEC
610 Alpha). 600 Alpha).
611 601
612 To deal with this, READ_ONCE() provides an imp 602 To deal with this, READ_ONCE() provides an implicit address-dependency barrier
613 since kernel release v4.15: 603 since kernel release v4.15:
614 604
615 CPU 1 CPU 2 605 CPU 1 CPU 2
616 =============== =============== 606 =============== ===============
617 { A == 1, B == 2, C == 3, P == &A, Q = 607 { A == 1, B == 2, C == 3, P == &A, Q == &C }
618 B = 4; 608 B = 4;
619 <write barrier> 609 <write barrier>
620 WRITE_ONCE(P, &B); 610 WRITE_ONCE(P, &B);
621 Q = READ_ONCE(P) 611 Q = READ_ONCE(P);
622 <implicit addres 612 <implicit address-dependency barrier>
623 D = *Q; 613 D = *Q;
624 614
625 This enforces the occurrence of one of the two 615 This enforces the occurrence of one of the two implications, and prevents the
626 third possibility from arising. 616 third possibility from arising.
627 617
628 618
629 [!] Note that this extremely counterintuitive 619 [!] Note that this extremely counterintuitive situation arises most easily on
630 machines with split caches, so that, for examp 620 machines with split caches, so that, for example, one cache bank processes
631 even-numbered cache lines and the other bank p 621 even-numbered cache lines and the other bank processes odd-numbered cache
632 lines. The pointer P might be stored in an od 622 lines. The pointer P might be stored in an odd-numbered cache line, and the
633 variable B might be stored in an even-numbered 623 variable B might be stored in an even-numbered cache line. Then, if the
634 even-numbered bank of the reading CPU's cache 624 even-numbered bank of the reading CPU's cache is extremely busy while the
635 odd-numbered bank is idle, one can see the new 625 odd-numbered bank is idle, one can see the new value of the pointer P (&B),
636 but the old value of the variable B (2). 626 but the old value of the variable B (2).
637 627
638 628
639 An address-dependency barrier is not required 629 An address-dependency barrier is not required to order dependent writes
640 because the CPUs that the Linux kernel support 630 because the CPUs that the Linux kernel supports don't do writes until they
641 are certain (1) that the write will actually h 631 are certain (1) that the write will actually happen, (2) of the location of
642 the write, and (3) of the value to be written. 632 the write, and (3) of the value to be written.
643 But please carefully read the "CONTROL DEPENDE 633 But please carefully read the "CONTROL DEPENDENCIES" section and the
644 Documentation/RCU/rcu_dereference.rst file: T 634 Documentation/RCU/rcu_dereference.rst file: The compiler can and does break
645 dependencies in a great many highly creative w 635 dependencies in a great many highly creative ways.
646 636
647 CPU 1 CPU 2 637 CPU 1 CPU 2
648 =============== =============== 638 =============== ===============
649 { A == 1, B == 2, C = 3, P == &A, Q == 639 { A == 1, B == 2, C = 3, P == &A, Q == &C }
650 B = 4; 640 B = 4;
651 <write barrier> 641 <write barrier>
652 WRITE_ONCE(P, &B); 642 WRITE_ONCE(P, &B);
653 Q = READ_ONCE_OL 643 Q = READ_ONCE_OLD(P);
654 WRITE_ONCE(*Q, 5 644 WRITE_ONCE(*Q, 5);
655 645
656 Therefore, no address-dependency barrier is re 646 Therefore, no address-dependency barrier is required to order the read into
657 Q with the store into *Q. In other words, thi 647 Q with the store into *Q. In other words, this outcome is prohibited,
658 even without an implicit address-dependency ba 648 even without an implicit address-dependency barrier of modern READ_ONCE():
659 649
660 (Q == &B) && (B == 4) 650 (Q == &B) && (B == 4)
661 651
662 Please note that this pattern should be rare. 652 Please note that this pattern should be rare. After all, the whole point
663 of dependency ordering is to -prevent- writes 653 of dependency ordering is to -prevent- writes to the data structure, along
664 with the expensive cache misses associated wit 654 with the expensive cache misses associated with those writes. This pattern
665 can be used to record rare error conditions an 655 can be used to record rare error conditions and the like, and the CPUs'
666 naturally occurring ordering prevents such rec 656 naturally occurring ordering prevents such records from being lost.
667 657
668 658
669 Note well that the ordering provided by an add 659 Note well that the ordering provided by an address dependency is local to
670 the CPU containing it. See the section on "Mu 660 the CPU containing it. See the section on "Multicopy atomicity" for
671 more information. 661 more information.
672 662
673 663
674 The address-dependency barrier is very importa 664 The address-dependency barrier is very important to the RCU system,
675 for example. See rcu_assign_pointer() and rcu 665 for example. See rcu_assign_pointer() and rcu_dereference() in
676 include/linux/rcupdate.h. This permits the cu 666 include/linux/rcupdate.h. This permits the current target of an RCU'd
677 pointer to be replaced with a new modified tar 667 pointer to be replaced with a new modified target, without the replacement
678 target appearing to be incompletely initialise 668 target appearing to be incompletely initialised.
679 669
680 See also the subsection on "Cache Coherency" f 670 See also the subsection on "Cache Coherency" for a more thorough example.
681 671
682 672
683 CONTROL DEPENDENCIES 673 CONTROL DEPENDENCIES
684 -------------------- 674 --------------------
685 675
686 Control dependencies can be a bit tricky becau 676 Control dependencies can be a bit tricky because current compilers do
687 not understand them. The purpose of this sect 677 not understand them. The purpose of this section is to help you prevent
688 the compiler's ignorance from breaking your co 678 the compiler's ignorance from breaking your code.
689 679
690 A load-load control dependency requires a full 680 A load-load control dependency requires a full read memory barrier, not
691 simply an (implicit) address-dependency barrie 681 simply an (implicit) address-dependency barrier to make it work correctly.
692 Consider the following bit of code: 682 Consider the following bit of code:
693 683
694 q = READ_ONCE(a); 684 q = READ_ONCE(a);
695 <implicit address-dependency barrier> 685 <implicit address-dependency barrier>
696 if (q) { 686 if (q) {
697 /* BUG: No address dependency! 687 /* BUG: No address dependency!!! */
698 p = READ_ONCE(b); 688 p = READ_ONCE(b);
699 } 689 }
700 690
701 This will not have the desired effect because 691 This will not have the desired effect because there is no actual address
702 dependency, but rather a control dependency th 692 dependency, but rather a control dependency that the CPU may short-circuit
703 by attempting to predict the outcome in advanc 693 by attempting to predict the outcome in advance, so that other CPUs see
704 the load from b as having happened before the 694 the load from b as having happened before the load from a. In such a case
705 what's actually required is: 695 what's actually required is:
706 696
707 q = READ_ONCE(a); 697 q = READ_ONCE(a);
708 if (q) { 698 if (q) {
709 <read barrier> 699 <read barrier>
710 p = READ_ONCE(b); 700 p = READ_ONCE(b);
711 } 701 }
712 702
713 However, stores are not speculated. This mean 703 However, stores are not speculated. This means that ordering -is- provided
714 for load-store control dependencies, as in the 704 for load-store control dependencies, as in the following example:
715 705
716 q = READ_ONCE(a); 706 q = READ_ONCE(a);
717 if (q) { 707 if (q) {
718 WRITE_ONCE(b, 1); 708 WRITE_ONCE(b, 1);
719 } 709 }
720 710
721 Control dependencies pair normally with other 711 Control dependencies pair normally with other types of barriers.
722 That said, please note that neither READ_ONCE( 712 That said, please note that neither READ_ONCE() nor WRITE_ONCE()
723 are optional! Without the READ_ONCE(), the com 713 are optional! Without the READ_ONCE(), the compiler might combine the
724 load from 'a' with other loads from 'a'. With 714 load from 'a' with other loads from 'a'. Without the WRITE_ONCE(),
725 the compiler might combine the store to 'b' wi 715 the compiler might combine the store to 'b' with other stores to 'b'.
726 Either can result in highly counterintuitive e 716 Either can result in highly counterintuitive effects on ordering.
727 717
728 Worse yet, if the compiler is able to prove (s 718 Worse yet, if the compiler is able to prove (say) that the value of
729 variable 'a' is always non-zero, it would be w 719 variable 'a' is always non-zero, it would be well within its rights
730 to optimize the original example by eliminatin 720 to optimize the original example by eliminating the "if" statement
731 as follows: 721 as follows:
732 722
733 q = a; 723 q = a;
734 b = 1; /* BUG: Compiler and CPU can b 724 b = 1; /* BUG: Compiler and CPU can both reorder!!! */
735 725
736 So don't leave out the READ_ONCE(). 726 So don't leave out the READ_ONCE().
737 727
738 It is tempting to try to enforce ordering on i 728 It is tempting to try to enforce ordering on identical stores on both
739 branches of the "if" statement as follows: 729 branches of the "if" statement as follows:
740 730
741 q = READ_ONCE(a); 731 q = READ_ONCE(a);
742 if (q) { 732 if (q) {
743 barrier(); 733 barrier();
744 WRITE_ONCE(b, 1); 734 WRITE_ONCE(b, 1);
745 do_something(); 735 do_something();
746 } else { 736 } else {
747 barrier(); 737 barrier();
748 WRITE_ONCE(b, 1); 738 WRITE_ONCE(b, 1);
749 do_something_else(); 739 do_something_else();
750 } 740 }
751 741
752 Unfortunately, current compilers will transfor 742 Unfortunately, current compilers will transform this as follows at high
753 optimization levels: 743 optimization levels:
754 744
755 q = READ_ONCE(a); 745 q = READ_ONCE(a);
756 barrier(); 746 barrier();
757 WRITE_ONCE(b, 1); /* BUG: No ordering 747 WRITE_ONCE(b, 1); /* BUG: No ordering vs. load from a!!! */
758 if (q) { 748 if (q) {
759 /* WRITE_ONCE(b, 1); -- moved 749 /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
760 do_something(); 750 do_something();
761 } else { 751 } else {
762 /* WRITE_ONCE(b, 1); -- moved 752 /* WRITE_ONCE(b, 1); -- moved up, BUG!!! */
763 do_something_else(); 753 do_something_else();
764 } 754 }
765 755
766 Now there is no conditional between the load f 756 Now there is no conditional between the load from 'a' and the store to
767 'b', which means that the CPU is within its ri 757 'b', which means that the CPU is within its rights to reorder them:
768 The conditional is absolutely required, and mu 758 The conditional is absolutely required, and must be present in the
769 assembly code even after all compiler optimiza 759 assembly code even after all compiler optimizations have been applied.
770 Therefore, if you need ordering in this exampl 760 Therefore, if you need ordering in this example, you need explicit
771 memory barriers, for example, smp_store_releas 761 memory barriers, for example, smp_store_release():
772 762
773 q = READ_ONCE(a); 763 q = READ_ONCE(a);
774 if (q) { 764 if (q) {
775 smp_store_release(&b, 1); 765 smp_store_release(&b, 1);
776 do_something(); 766 do_something();
777 } else { 767 } else {
778 smp_store_release(&b, 1); 768 smp_store_release(&b, 1);
779 do_something_else(); 769 do_something_else();
780 } 770 }
781 771
782 In contrast, without explicit memory barriers, 772 In contrast, without explicit memory barriers, two-legged-if control
783 ordering is guaranteed only when the stores di 773 ordering is guaranteed only when the stores differ, for example:
784 774
785 q = READ_ONCE(a); 775 q = READ_ONCE(a);
786 if (q) { 776 if (q) {
787 WRITE_ONCE(b, 1); 777 WRITE_ONCE(b, 1);
788 do_something(); 778 do_something();
789 } else { 779 } else {
790 WRITE_ONCE(b, 2); 780 WRITE_ONCE(b, 2);
791 do_something_else(); 781 do_something_else();
792 } 782 }
793 783
794 The initial READ_ONCE() is still required to p 784 The initial READ_ONCE() is still required to prevent the compiler from
795 proving the value of 'a'. 785 proving the value of 'a'.
796 786
797 In addition, you need to be careful what you d 787 In addition, you need to be careful what you do with the local variable 'q',
798 otherwise the compiler might be able to guess 788 otherwise the compiler might be able to guess the value and again remove
799 the needed conditional. For example: 789 the needed conditional. For example:
800 790
801 q = READ_ONCE(a); 791 q = READ_ONCE(a);
802 if (q % MAX) { 792 if (q % MAX) {
803 WRITE_ONCE(b, 1); 793 WRITE_ONCE(b, 1);
804 do_something(); 794 do_something();
805 } else { 795 } else {
806 WRITE_ONCE(b, 2); 796 WRITE_ONCE(b, 2);
807 do_something_else(); 797 do_something_else();
808 } 798 }
809 799
810 If MAX is defined to be 1, then the compiler k 800 If MAX is defined to be 1, then the compiler knows that (q % MAX) is
811 equal to zero, in which case the compiler is w 801 equal to zero, in which case the compiler is within its rights to
812 transform the above code into the following: 802 transform the above code into the following:
813 803
814 q = READ_ONCE(a); 804 q = READ_ONCE(a);
815 WRITE_ONCE(b, 2); 805 WRITE_ONCE(b, 2);
816 do_something_else(); 806 do_something_else();
817 807
818 Given this transformation, the CPU is not requ 808 Given this transformation, the CPU is not required to respect the ordering
819 between the load from variable 'a' and the sto 809 between the load from variable 'a' and the store to variable 'b'. It is
820 tempting to add a barrier(), but this does not 810 tempting to add a barrier(), but this does not help. The conditional
821 is gone, and the barrier won't bring it back. 811 is gone, and the barrier won't bring it back. Therefore, if you are
822 relying on this ordering, you should make sure 812 relying on this ordering, you should make sure that MAX is greater than
823 one, perhaps as follows: 813 one, perhaps as follows:
824 814
825 q = READ_ONCE(a); 815 q = READ_ONCE(a);
826 BUILD_BUG_ON(MAX <= 1); /* Order load 816 BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
827 if (q % MAX) { 817 if (q % MAX) {
828 WRITE_ONCE(b, 1); 818 WRITE_ONCE(b, 1);
829 do_something(); 819 do_something();
830 } else { 820 } else {
831 WRITE_ONCE(b, 2); 821 WRITE_ONCE(b, 2);
832 do_something_else(); 822 do_something_else();
833 } 823 }
834 824
835 Please note once again that the stores to 'b' 825 Please note once again that the stores to 'b' differ. If they were
836 identical, as noted earlier, the compiler coul 826 identical, as noted earlier, the compiler could pull this store outside
837 of the 'if' statement. 827 of the 'if' statement.
838 828
839 You must also be careful not to rely too much 829 You must also be careful not to rely too much on boolean short-circuit
840 evaluation. Consider this example: 830 evaluation. Consider this example:
841 831
842 q = READ_ONCE(a); 832 q = READ_ONCE(a);
843 if (q || 1 > 0) 833 if (q || 1 > 0)
844 WRITE_ONCE(b, 1); 834 WRITE_ONCE(b, 1);
845 835
846 Because the first condition cannot fault and t 836 Because the first condition cannot fault and the second condition is
847 always true, the compiler can transform this e 837 always true, the compiler can transform this example as following,
848 defeating control dependency: 838 defeating control dependency:
849 839
850 q = READ_ONCE(a); 840 q = READ_ONCE(a);
851 WRITE_ONCE(b, 1); 841 WRITE_ONCE(b, 1);
852 842
853 This example underscores the need to ensure th 843 This example underscores the need to ensure that the compiler cannot
854 out-guess your code. More generally, although 844 out-guess your code. More generally, although READ_ONCE() does force
855 the compiler to actually emit code for a given 845 the compiler to actually emit code for a given load, it does not force
856 the compiler to use the results. 846 the compiler to use the results.
857 847
858 In addition, control dependencies apply only t 848 In addition, control dependencies apply only to the then-clause and
859 else-clause of the if-statement in question. 849 else-clause of the if-statement in question. In particular, it does
860 not necessarily apply to code following the if 850 not necessarily apply to code following the if-statement:
861 851
862 q = READ_ONCE(a); 852 q = READ_ONCE(a);
863 if (q) { 853 if (q) {
864 WRITE_ONCE(b, 1); 854 WRITE_ONCE(b, 1);
865 } else { 855 } else {
866 WRITE_ONCE(b, 2); 856 WRITE_ONCE(b, 2);
867 } 857 }
868 WRITE_ONCE(c, 1); /* BUG: No ordering 858 WRITE_ONCE(c, 1); /* BUG: No ordering against the read from 'a'. */
869 859
870 It is tempting to argue that there in fact is 860 It is tempting to argue that there in fact is ordering because the
871 compiler cannot reorder volatile accesses and 861 compiler cannot reorder volatile accesses and also cannot reorder
872 the writes to 'b' with the condition. Unfortu 862 the writes to 'b' with the condition. Unfortunately for this line
873 of reasoning, the compiler might compile the t 863 of reasoning, the compiler might compile the two writes to 'b' as
874 conditional-move instructions, as in this fanc 864 conditional-move instructions, as in this fanciful pseudo-assembly
875 language: 865 language:
876 866
877 ld r1,a 867 ld r1,a
878 cmp r1,$0 868 cmp r1,$0
879 cmov,ne r4,$1 869 cmov,ne r4,$1
880 cmov,eq r4,$2 870 cmov,eq r4,$2
881 st r4,b 871 st r4,b
882 st $1,c 872 st $1,c
883 873
884 A weakly ordered CPU would have no dependency 874 A weakly ordered CPU would have no dependency of any sort between the load
885 from 'a' and the store to 'c'. The control de 875 from 'a' and the store to 'c'. The control dependencies would extend
886 only to the pair of cmov instructions and the 876 only to the pair of cmov instructions and the store depending on them.
887 In short, control dependencies apply only to t 877 In short, control dependencies apply only to the stores in the then-clause
888 and else-clause of the if-statement in questio 878 and else-clause of the if-statement in question (including functions
889 invoked by those two clauses), not to code fol 879 invoked by those two clauses), not to code following that if-statement.
890 880
891 881
892 Note well that the ordering provided by a cont 882 Note well that the ordering provided by a control dependency is local
893 to the CPU containing it. See the section on 883 to the CPU containing it. See the section on "Multicopy atomicity"
894 for more information. 884 for more information.
895 885
896 886
897 In summary: 887 In summary:
898 888
899 (*) Control dependencies can order prior loa 889 (*) Control dependencies can order prior loads against later stores.
900 However, they do -not- guarantee any oth 890 However, they do -not- guarantee any other sort of ordering:
901 Not prior loads against later loads, nor 891 Not prior loads against later loads, nor prior stores against
902 later anything. If you need these other 892 later anything. If you need these other forms of ordering,
903 use smp_rmb(), smp_wmb(), or, in the cas 893 use smp_rmb(), smp_wmb(), or, in the case of prior stores and
904 later loads, smp_mb(). 894 later loads, smp_mb().
905 895
906 (*) If both legs of the "if" statement begin 896 (*) If both legs of the "if" statement begin with identical stores to
907 the same variable, then those stores mus 897 the same variable, then those stores must be ordered, either by
908 preceding both of them with smp_mb() or 898 preceding both of them with smp_mb() or by using smp_store_release()
909 to carry out the stores. Please note th 899 to carry out the stores. Please note that it is -not- sufficient
910 to use barrier() at beginning of each le 900 to use barrier() at beginning of each leg of the "if" statement
911 because, as shown by the example above, 901 because, as shown by the example above, optimizing compilers can
912 destroy the control dependency while res 902 destroy the control dependency while respecting the letter of the
913 barrier() law. 903 barrier() law.
914 904
915 (*) Control dependencies require at least on 905 (*) Control dependencies require at least one run-time conditional
916 between the prior load and the subsequen 906 between the prior load and the subsequent store, and this
917 conditional must involve the prior load. 907 conditional must involve the prior load. If the compiler is able
918 to optimize the conditional away, it wil 908 to optimize the conditional away, it will have also optimized
919 away the ordering. Careful use of READ_ 909 away the ordering. Careful use of READ_ONCE() and WRITE_ONCE()
920 can help to preserve the needed conditio 910 can help to preserve the needed conditional.
921 911
922 (*) Control dependencies require that the co 912 (*) Control dependencies require that the compiler avoid reordering the
923 dependency into nonexistence. Careful u 913 dependency into nonexistence. Careful use of READ_ONCE() or
924 atomic{,64}_read() can help to preserve 914 atomic{,64}_read() can help to preserve your control dependency.
925 Please see the COMPILER BARRIER section 915 Please see the COMPILER BARRIER section for more information.
926 916
927 (*) Control dependencies apply only to the t 917 (*) Control dependencies apply only to the then-clause and else-clause
928 of the if-statement containing the contr 918 of the if-statement containing the control dependency, including
929 any functions that these two clauses cal 919 any functions that these two clauses call. Control dependencies
930 do -not- apply to code following the if- 920 do -not- apply to code following the if-statement containing the
931 control dependency. 921 control dependency.
932 922
933 (*) Control dependencies pair normally with 923 (*) Control dependencies pair normally with other types of barriers.
934 924
935 (*) Control dependencies do -not- provide mu 925 (*) Control dependencies do -not- provide multicopy atomicity. If you
936 need all the CPUs to see a given store a 926 need all the CPUs to see a given store at the same time, use smp_mb().
937 927
938 (*) Compilers do not understand control depe 928 (*) Compilers do not understand control dependencies. It is therefore
939 your job to ensure that they do not brea 929 your job to ensure that they do not break your code.
940 930
941 931
942 SMP BARRIER PAIRING 932 SMP BARRIER PAIRING
943 ------------------- 933 -------------------
944 934
945 When dealing with CPU-CPU interactions, certai 935 When dealing with CPU-CPU interactions, certain types of memory barrier should
946 always be paired. A lack of appropriate pairi 936 always be paired. A lack of appropriate pairing is almost certainly an error.
947 937
948 General barriers pair with each other, though 938 General barriers pair with each other, though they also pair with most
949 other types of barriers, albeit without multic 939 other types of barriers, albeit without multicopy atomicity. An acquire
950 barrier pairs with a release barrier, but both 940 barrier pairs with a release barrier, but both may also pair with other
951 barriers, including of course general barriers 941 barriers, including of course general barriers. A write barrier pairs
952 with an address-dependency barrier, a control 942 with an address-dependency barrier, a control dependency, an acquire barrier,
953 a release barrier, a read barrier, or a genera 943 a release barrier, a read barrier, or a general barrier. Similarly a
954 read barrier, control dependency, or an addres 944 read barrier, control dependency, or an address-dependency barrier pairs
955 with a write barrier, an acquire barrier, a re 945 with a write barrier, an acquire barrier, a release barrier, or a
956 general barrier: 946 general barrier:
957 947
958 CPU 1 CPU 2 948 CPU 1 CPU 2
959 =============== =============== 949 =============== ===============
960 WRITE_ONCE(a, 1); 950 WRITE_ONCE(a, 1);
961 <write barrier> 951 <write barrier>
962 WRITE_ONCE(b, 2); x = READ_ONCE(b) 952 WRITE_ONCE(b, 2); x = READ_ONCE(b);
963 <read barrier> 953 <read barrier>
964 y = READ_ONCE(a) 954 y = READ_ONCE(a);
965 955
966 Or: 956 Or:
967 957
968 CPU 1 CPU 2 958 CPU 1 CPU 2
969 =============== ================ 959 =============== ===============================
970 a = 1; 960 a = 1;
971 <write barrier> 961 <write barrier>
972 WRITE_ONCE(b, &a); x = READ_ONCE(b) 962 WRITE_ONCE(b, &a); x = READ_ONCE(b);
973 <implicit addres 963 <implicit address-dependency barrier>
974 y = *x; 964 y = *x;
975 965
976 Or even: 966 Or even:
977 967
978 CPU 1 CPU 2 968 CPU 1 CPU 2
979 =============== ================ 969 =============== ===============================
980 r1 = READ_ONCE(y); 970 r1 = READ_ONCE(y);
981 <general barrier> 971 <general barrier>
982 WRITE_ONCE(x, 1); if (r2 = READ_ON 972 WRITE_ONCE(x, 1); if (r2 = READ_ONCE(x)) {
983 <implicit con 973 <implicit control dependency>
984 WRITE_ONCE(y, 974 WRITE_ONCE(y, 1);
985 } 975 }
986 976
987 assert(r1 == 0 || r2 == 0); 977 assert(r1 == 0 || r2 == 0);
988 978
989 Basically, the read barrier always has to be t 979 Basically, the read barrier always has to be there, even though it can be of
990 the "weaker" type. 980 the "weaker" type.
991 981
992 [!] Note that the stores before the write barr 982 [!] Note that the stores before the write barrier would normally be expected to
993 match the loads after the read barrier or the 983 match the loads after the read barrier or the address-dependency barrier, and
994 vice versa: 984 vice versa:
995 985
996 CPU 1 CP 986 CPU 1 CPU 2
997 =================== == 987 =================== ===================
998 WRITE_ONCE(a, 1); }---- --->{ v 988 WRITE_ONCE(a, 1); }---- --->{ v = READ_ONCE(c);
999 WRITE_ONCE(b, 2); } \ / { w 989 WRITE_ONCE(b, 2); } \ / { w = READ_ONCE(d);
1000 <write barrier> \ < 990 <write barrier> \ <read barrier>
1001 WRITE_ONCE(c, 3); } / \ { x 991 WRITE_ONCE(c, 3); } / \ { x = READ_ONCE(a);
1002 WRITE_ONCE(d, 4); }---- --->{ y 992 WRITE_ONCE(d, 4); }---- --->{ y = READ_ONCE(b);
1003 993
1004 994
1005 EXAMPLES OF MEMORY BARRIER SEQUENCES 995 EXAMPLES OF MEMORY BARRIER SEQUENCES
1006 ------------------------------------ 996 ------------------------------------
1007 997
1008 Firstly, write barriers act as partial orderi 998 Firstly, write barriers act as partial orderings on store operations.
1009 Consider the following sequence of events: 999 Consider the following sequence of events:
1010 1000
1011 CPU 1 1001 CPU 1
1012 ======================= 1002 =======================
1013 STORE A = 1 1003 STORE A = 1
1014 STORE B = 2 1004 STORE B = 2
1015 STORE C = 3 1005 STORE C = 3
1016 <write barrier> 1006 <write barrier>
1017 STORE D = 4 1007 STORE D = 4
1018 STORE E = 5 1008 STORE E = 5
1019 1009
1020 This sequence of events is committed to the m 1010 This sequence of events is committed to the memory coherence system in an order
1021 that the rest of the system might perceive as 1011 that the rest of the system might perceive as the unordered set of { STORE A,
1022 STORE B, STORE C } all occurring before the u 1012 STORE B, STORE C } all occurring before the unordered set of { STORE D, STORE E
1023 }: 1013 }:
1024 1014
1025 +-------+ : : 1015 +-------+ : :
1026 | | +------+ 1016 | | +------+
1027 | |------>| C=3 | } /\ 1017 | |------>| C=3 | } /\
1028 | | : +------+ }----- 1018 | | : +------+ }----- \ -----> Events perceptible to
1029 | | : | A=1 | } 1019 | | : | A=1 | } \/ the rest of the system
1030 | | : +------+ } 1020 | | : +------+ }
1031 | CPU 1 | : | B=2 | } 1021 | CPU 1 | : | B=2 | }
1032 | | +------+ } 1022 | | +------+ }
1033 | | wwwwwwwwwwwwwwww } <--- 1023 | | wwwwwwwwwwwwwwww } <--- At this point the write barrier
1034 | | +------+ } 1024 | | +------+ } requires all stores prior to the
1035 | | : | E=5 | } 1025 | | : | E=5 | } barrier to be committed before
1036 | | : +------+ } 1026 | | : +------+ } further stores may take place
1037 | |------>| D=4 | } 1027 | |------>| D=4 | }
1038 | | +------+ 1028 | | +------+
1039 +-------+ : : 1029 +-------+ : :
1040 | 1030 |
1041 | Sequence in whic 1031 | Sequence in which stores are committed to the
1042 | memory system by 1032 | memory system by CPU 1
1043 V 1033 V
1044 1034
1045 1035
1046 Secondly, address-dependency barriers act as 1036 Secondly, address-dependency barriers act as partial orderings on address-
1047 dependent loads. Consider the following sequ 1037 dependent loads. Consider the following sequence of events:
1048 1038
1049 CPU 1 CPU 2 1039 CPU 1 CPU 2
1050 ======================= ============= 1040 ======================= =======================
1051 { B = 7; X = 9; Y = 8; C = &Y 1041 { B = 7; X = 9; Y = 8; C = &Y }
1052 STORE A = 1 1042 STORE A = 1
1053 STORE B = 2 1043 STORE B = 2
1054 <write barrier> 1044 <write barrier>
1055 STORE C = &B LOAD X 1045 STORE C = &B LOAD X
1056 STORE D = 4 LOAD C (gets 1046 STORE D = 4 LOAD C (gets &B)
1057 LOAD *C (read 1047 LOAD *C (reads B)
1058 1048
1059 Without intervention, CPU 2 may perceive the 1049 Without intervention, CPU 2 may perceive the events on CPU 1 in some
1060 effectively random order, despite the write b 1050 effectively random order, despite the write barrier issued by CPU 1:
1061 1051
1062 +-------+ : : 1052 +-------+ : : : :
1063 | | +------+ 1053 | | +------+ +-------+ | Sequence of update
1064 | |------>| B=2 |----- - 1054 | |------>| B=2 |----- --->| Y->8 | | of perception on
1065 | | : +------+ \ 1055 | | : +------+ \ +-------+ | CPU 2
1066 | CPU 1 | : | A=1 | \ - 1056 | CPU 1 | : | A=1 | \ --->| C->&Y | V
1067 | | +------+ | 1057 | | +------+ | +-------+
1068 | | wwwwwwwwwwwwwwww | 1058 | | wwwwwwwwwwwwwwww | : :
1069 | | +------+ | 1059 | | +------+ | : :
1070 | | : | C=&B |--- | 1060 | | : | C=&B |--- | : : +-------+
1071 | | : +------+ \ | 1061 | | : +------+ \ | +-------+ | |
1072 | |------>| D=4 | --------- 1062 | |------>| D=4 | ----------->| C->&B |------>| |
1073 | | +------+ | 1063 | | +------+ | +-------+ | |
1074 +-------+ : : | 1064 +-------+ : : | : : | |
1075 | 1065 | : : | |
1076 | 1066 | : : | CPU 2 |
1077 | 1067 | +-------+ | |
1078 Apparently incorrect ---> | 1068 Apparently incorrect ---> | | B->7 |------>| |
1079 perception of B (!) | 1069 perception of B (!) | +-------+ | |
1080 | 1070 | : : | |
1081 | 1071 | +-------+ | |
1082 The load of X holds ---> \ 1072 The load of X holds ---> \ | X->9 |------>| |
1083 up the maintenance \ 1073 up the maintenance \ +-------+ | |
1084 of coherence of B --- 1074 of coherence of B ----->| B->2 | +-------+
1085 1075 +-------+
1086 1076 : :
1087 1077
1088 1078
1089 In the above example, CPU 2 perceives that B 1079 In the above example, CPU 2 perceives that B is 7, despite the load of *C
1090 (which would be B) coming after the LOAD of C 1080 (which would be B) coming after the LOAD of C.
1091 1081
1092 If, however, an address-dependency barrier we 1082 If, however, an address-dependency barrier were to be placed between the load
1093 of C and the load of *C (ie: B) on CPU 2: 1083 of C and the load of *C (ie: B) on CPU 2:
1094 1084
1095 CPU 1 CPU 2 1085 CPU 1 CPU 2
1096 ======================= ============= 1086 ======================= =======================
1097 { B = 7; X = 9; Y = 8; C = &Y 1087 { B = 7; X = 9; Y = 8; C = &Y }
1098 STORE A = 1 1088 STORE A = 1
1099 STORE B = 2 1089 STORE B = 2
1100 <write barrier> 1090 <write barrier>
1101 STORE C = &B LOAD X 1091 STORE C = &B LOAD X
1102 STORE D = 4 LOAD C (gets 1092 STORE D = 4 LOAD C (gets &B)
1103 <address-depe 1093 <address-dependency barrier>
1104 LOAD *C (read 1094 LOAD *C (reads B)
1105 1095
1106 then the following will occur: 1096 then the following will occur:
1107 1097
1108 +-------+ : : 1098 +-------+ : : : :
1109 | | +------+ 1099 | | +------+ +-------+
1110 | |------>| B=2 |----- - 1100 | |------>| B=2 |----- --->| Y->8 |
1111 | | : +------+ \ 1101 | | : +------+ \ +-------+
1112 | CPU 1 | : | A=1 | \ - 1102 | CPU 1 | : | A=1 | \ --->| C->&Y |
1113 | | +------+ | 1103 | | +------+ | +-------+
1114 | | wwwwwwwwwwwwwwww | 1104 | | wwwwwwwwwwwwwwww | : :
1115 | | +------+ | 1105 | | +------+ | : :
1116 | | : | C=&B |--- | 1106 | | : | C=&B |--- | : : +-------+
1117 | | : +------+ \ | 1107 | | : +------+ \ | +-------+ | |
1118 | |------>| D=4 | --------- 1108 | |------>| D=4 | ----------->| C->&B |------>| |
1119 | | +------+ | 1109 | | +------+ | +-------+ | |
1120 +-------+ : : | 1110 +-------+ : : | : : | |
1121 | 1111 | : : | |
1122 | 1112 | : : | CPU 2 |
1123 | 1113 | +-------+ | |
1124 | 1114 | | X->9 |------>| |
1125 | 1115 | +-------+ | |
1126 Makes sure all effects ---> \ a 1116 Makes sure all effects ---> \ aaaaaaaaaaaaaaaaa | |
1127 prior to the store of C \ 1117 prior to the store of C \ +-------+ | |
1128 are perceptible to --- 1118 are perceptible to ----->| B->2 |------>| |
1129 subsequent loads 1119 subsequent loads +-------+ | |
1130 1120 : : +-------+
1131 1121
1132 1122
1133 And thirdly, a read barrier acts as a partial 1123 And thirdly, a read barrier acts as a partial order on loads. Consider the
1134 following sequence of events: 1124 following sequence of events:
1135 1125
1136 CPU 1 CPU 2 1126 CPU 1 CPU 2
1137 ======================= ============= 1127 ======================= =======================
1138 { A = 0, B = 9 } 1128 { A = 0, B = 9 }
1139 STORE A=1 1129 STORE A=1
1140 <write barrier> 1130 <write barrier>
1141 STORE B=2 1131 STORE B=2
1142 LOAD B 1132 LOAD B
1143 LOAD A 1133 LOAD A
1144 1134
1145 Without intervention, CPU 2 may then choose t 1135 Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
1146 some effectively random order, despite the wr 1136 some effectively random order, despite the write barrier issued by CPU 1:
1147 1137
1148 +-------+ : : 1138 +-------+ : : : :
1149 | | +------+ 1139 | | +------+ +-------+
1150 | |------>| A=1 |------ - 1140 | |------>| A=1 |------ --->| A->0 |
1151 | | +------+ \ 1141 | | +------+ \ +-------+
1152 | CPU 1 | wwwwwwwwwwwwwwww \ - 1142 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1153 | | +------+ | 1143 | | +------+ | +-------+
1154 | |------>| B=2 |--- | 1144 | |------>| B=2 |--- | : :
1155 | | +------+ \ | 1145 | | +------+ \ | : : +-------+
1156 +-------+ : : \ | 1146 +-------+ : : \ | +-------+ | |
1157 -------- 1147 ---------->| B->2 |------>| |
1158 | 1148 | +-------+ | CPU 2 |
1159 | 1149 | | A->0 |------>| |
1160 | 1150 | +-------+ | |
1161 | 1151 | : : +-------+
1162 \ 1152 \ : :
1163 \ 1153 \ +-------+
1164 -- 1154 ---->| A->1 |
1165 1155 +-------+
1166 1156 : :
1167 1157
1168 1158
1169 If, however, a read barrier were to be placed 1159 If, however, a read barrier were to be placed between the load of B and the
1170 load of A on CPU 2: 1160 load of A on CPU 2:
1171 1161
1172 CPU 1 CPU 2 1162 CPU 1 CPU 2
1173 ======================= ============= 1163 ======================= =======================
1174 { A = 0, B = 9 } 1164 { A = 0, B = 9 }
1175 STORE A=1 1165 STORE A=1
1176 <write barrier> 1166 <write barrier>
1177 STORE B=2 1167 STORE B=2
1178 LOAD B 1168 LOAD B
1179 <read barrier 1169 <read barrier>
1180 LOAD A 1170 LOAD A
1181 1171
1182 then the partial ordering imposed by CPU 1 wi 1172 then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
1183 2: 1173 2:
1184 1174
1185 +-------+ : : 1175 +-------+ : : : :
1186 | | +------+ 1176 | | +------+ +-------+
1187 | |------>| A=1 |------ - 1177 | |------>| A=1 |------ --->| A->0 |
1188 | | +------+ \ 1178 | | +------+ \ +-------+
1189 | CPU 1 | wwwwwwwwwwwwwwww \ - 1179 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1190 | | +------+ | 1180 | | +------+ | +-------+
1191 | |------>| B=2 |--- | 1181 | |------>| B=2 |--- | : :
1192 | | +------+ \ | 1182 | | +------+ \ | : : +-------+
1193 +-------+ : : \ | 1183 +-------+ : : \ | +-------+ | |
1194 -------- 1184 ---------->| B->2 |------>| |
1195 | 1185 | +-------+ | CPU 2 |
1196 | 1186 | : : | |
1197 | 1187 | : : | |
1198 At this point the read ----> \ r 1188 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1199 barrier causes all effects \ 1189 barrier causes all effects \ +-------+ | |
1200 prior to the storage of B -- 1190 prior to the storage of B ---->| A->1 |------>| |
1201 to be perceptible to CPU 2 1191 to be perceptible to CPU 2 +-------+ | |
1202 1192 : : +-------+
1203 1193
1204 1194
1205 To illustrate this more completely, consider 1195 To illustrate this more completely, consider what could happen if the code
1206 contained a load of A either side of the read 1196 contained a load of A either side of the read barrier:
1207 1197
1208 CPU 1 CPU 2 1198 CPU 1 CPU 2
1209 ======================= ============= 1199 ======================= =======================
1210 { A = 0, B = 9 } 1200 { A = 0, B = 9 }
1211 STORE A=1 1201 STORE A=1
1212 <write barrier> 1202 <write barrier>
1213 STORE B=2 1203 STORE B=2
1214 LOAD B 1204 LOAD B
1215 LOAD A [first 1205 LOAD A [first load of A]
1216 <read barrier 1206 <read barrier>
1217 LOAD A [secon 1207 LOAD A [second load of A]
1218 1208
1219 Even though the two loads of A both occur aft 1209 Even though the two loads of A both occur after the load of B, they may both
1220 come up with different values: 1210 come up with different values:
1221 1211
1222 +-------+ : : 1212 +-------+ : : : :
1223 | | +------+ 1213 | | +------+ +-------+
1224 | |------>| A=1 |------ - 1214 | |------>| A=1 |------ --->| A->0 |
1225 | | +------+ \ 1215 | | +------+ \ +-------+
1226 | CPU 1 | wwwwwwwwwwwwwwww \ - 1216 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1227 | | +------+ | 1217 | | +------+ | +-------+
1228 | |------>| B=2 |--- | 1218 | |------>| B=2 |--- | : :
1229 | | +------+ \ | 1219 | | +------+ \ | : : +-------+
1230 +-------+ : : \ | 1220 +-------+ : : \ | +-------+ | |
1231 -------- 1221 ---------->| B->2 |------>| |
1232 | 1222 | +-------+ | CPU 2 |
1233 | 1223 | : : | |
1234 | 1224 | : : | |
1235 | 1225 | +-------+ | |
1236 | 1226 | | A->0 |------>| 1st |
1237 | 1227 | +-------+ | |
1238 At this point the read ----> \ r 1228 At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
1239 barrier causes all effects \ 1229 barrier causes all effects \ +-------+ | |
1240 prior to the storage of B -- 1230 prior to the storage of B ---->| A->1 |------>| 2nd |
1241 to be perceptible to CPU 2 1231 to be perceptible to CPU 2 +-------+ | |
1242 1232 : : +-------+
1243 1233
1244 1234
1245 But it may be that the update to A from CPU 1 1235 But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
1246 before the read barrier completes anyway: 1236 before the read barrier completes anyway:
1247 1237
1248 +-------+ : : 1238 +-------+ : : : :
1249 | | +------+ 1239 | | +------+ +-------+
1250 | |------>| A=1 |------ - 1240 | |------>| A=1 |------ --->| A->0 |
1251 | | +------+ \ 1241 | | +------+ \ +-------+
1252 | CPU 1 | wwwwwwwwwwwwwwww \ - 1242 | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
1253 | | +------+ | 1243 | | +------+ | +-------+
1254 | |------>| B=2 |--- | 1244 | |------>| B=2 |--- | : :
1255 | | +------+ \ | 1245 | | +------+ \ | : : +-------+
1256 +-------+ : : \ | 1246 +-------+ : : \ | +-------+ | |
1257 -------- 1247 ---------->| B->2 |------>| |
1258 | 1248 | +-------+ | CPU 2 |
1259 | 1249 | : : | |
1260 \ 1250 \ : : | |
1261 \ 1251 \ +-------+ | |
1262 -- 1252 ---->| A->1 |------>| 1st |
1263 1253 +-------+ | |
1264 r 1254 rrrrrrrrrrrrrrrrr | |
1265 1255 +-------+ | |
1266 1256 | A->1 |------>| 2nd |
1267 1257 +-------+ | |
1268 1258 : : +-------+
1269 1259
1270 1260
1271 The guarantee is that the second load will al 1261 The guarantee is that the second load will always come up with A == 1 if the
1272 load of B came up with B == 2. No such guara 1262 load of B came up with B == 2. No such guarantee exists for the first load of
1273 A; that may come up with either A == 0 or A = 1263 A; that may come up with either A == 0 or A == 1.
1274 1264
1275 1265
1276 READ MEMORY BARRIERS VS LOAD SPECULATION 1266 READ MEMORY BARRIERS VS LOAD SPECULATION
1277 ---------------------------------------- 1267 ----------------------------------------
1278 1268
1279 Many CPUs speculate with loads: that is they 1269 Many CPUs speculate with loads: that is they see that they will need to load an
1280 item from memory, and they find a time where 1270 item from memory, and they find a time where they're not using the bus for any
1281 other loads, and so do the load in advance - 1271 other loads, and so do the load in advance - even though they haven't actually
1282 got to that point in the instruction executio 1272 got to that point in the instruction execution flow yet. This permits the
1283 actual load instruction to potentially comple 1273 actual load instruction to potentially complete immediately because the CPU
1284 already has the value to hand. 1274 already has the value to hand.
1285 1275
1286 It may turn out that the CPU didn't actually 1276 It may turn out that the CPU didn't actually need the value - perhaps because a
1287 branch circumvented the load - in which case 1277 branch circumvented the load - in which case it can discard the value or just
1288 cache it for later use. 1278 cache it for later use.
1289 1279
1290 Consider: 1280 Consider:
1291 1281
1292 CPU 1 CPU 2 1282 CPU 1 CPU 2
1293 ======================= ============= 1283 ======================= =======================
1294 LOAD B 1284 LOAD B
1295 DIVIDE 1285 DIVIDE } Divide instructions generally
1296 DIVIDE 1286 DIVIDE } take a long time to perform
1297 LOAD A 1287 LOAD A
1298 1288
1299 Which might appear as this: 1289 Which might appear as this:
1300 1290
1301 1291 : : +-------+
1302 1292 +-------+ | |
1303 - 1293 --->| B->2 |------>| |
1304 1294 +-------+ | CPU 2 |
1305 1295 : :DIVIDE | |
1306 1296 +-------+ | |
1307 The CPU being busy doing a ---> - 1297 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1308 division speculates on the 1298 division speculates on the +-------+ ~ | |
1309 LOAD of A 1299 LOAD of A : : ~ | |
1310 1300 : :DIVIDE | |
1311 1301 : : ~ | |
1312 Once the divisions are complete --> 1302 Once the divisions are complete --> : : ~-->| |
1313 the CPU can then perform the 1303 the CPU can then perform the : : | |
1314 LOAD with immediate effect 1304 LOAD with immediate effect : : +-------+
1315 1305
1316 1306
1317 Placing a read barrier or an address-dependen 1307 Placing a read barrier or an address-dependency barrier just before the second
1318 load: 1308 load:
1319 1309
1320 CPU 1 CPU 2 1310 CPU 1 CPU 2
1321 ======================= ============= 1311 ======================= =======================
1322 LOAD B 1312 LOAD B
1323 DIVIDE 1313 DIVIDE
1324 DIVIDE 1314 DIVIDE
1325 <read barrier 1315 <read barrier>
1326 LOAD A 1316 LOAD A
1327 1317
1328 will force any value speculatively obtained t 1318 will force any value speculatively obtained to be reconsidered to an extent
1329 dependent on the type of barrier used. If th 1319 dependent on the type of barrier used. If there was no change made to the
1330 speculated memory location, then the speculat 1320 speculated memory location, then the speculated value will just be used:
1331 1321
1332 1322 : : +-------+
1333 1323 +-------+ | |
1334 - 1324 --->| B->2 |------>| |
1335 1325 +-------+ | CPU 2 |
1336 1326 : :DIVIDE | |
1337 1327 +-------+ | |
1338 The CPU being busy doing a ---> - 1328 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1339 division speculates on the 1329 division speculates on the +-------+ ~ | |
1340 LOAD of A 1330 LOAD of A : : ~ | |
1341 1331 : :DIVIDE | |
1342 1332 : : ~ | |
1343 1333 : : ~ | |
1344 r 1334 rrrrrrrrrrrrrrrr~ | |
1345 1335 : : ~ | |
1346 1336 : : ~-->| |
1347 1337 : : | |
1348 1338 : : +-------+
1349 1339
1350 1340
1351 but if there was an update or an invalidation 1341 but if there was an update or an invalidation from another CPU pending, then
1352 the speculation will be cancelled and the val 1342 the speculation will be cancelled and the value reloaded:
1353 1343
1354 1344 : : +-------+
1355 1345 +-------+ | |
1356 - 1346 --->| B->2 |------>| |
1357 1347 +-------+ | CPU 2 |
1358 1348 : :DIVIDE | |
1359 1349 +-------+ | |
1360 The CPU being busy doing a ---> - 1350 The CPU being busy doing a ---> --->| A->0 |~~~~ | |
1361 division speculates on the 1351 division speculates on the +-------+ ~ | |
1362 LOAD of A 1352 LOAD of A : : ~ | |
1363 1353 : :DIVIDE | |
1364 1354 : : ~ | |
1365 1355 : : ~ | |
1366 r 1356 rrrrrrrrrrrrrrrrr | |
1367 1357 +-------+ | |
1368 The speculation is discarded ---> - 1358 The speculation is discarded ---> --->| A->1 |------>| |
1369 and an updated value is 1359 and an updated value is +-------+ | |
1370 retrieved 1360 retrieved : : +-------+
1371 1361
1372 1362
1373 MULTICOPY ATOMICITY 1363 MULTICOPY ATOMICITY
1374 -------------------- 1364 --------------------
1375 1365
1376 Multicopy atomicity is a deeply intuitive not 1366 Multicopy atomicity is a deeply intuitive notion about ordering that is
1377 not always provided by real computer systems, 1367 not always provided by real computer systems, namely that a given store
1378 becomes visible at the same time to all CPUs, 1368 becomes visible at the same time to all CPUs, or, alternatively, that all
1379 CPUs agree on the order in which all stores b 1369 CPUs agree on the order in which all stores become visible. However,
1380 support of full multicopy atomicity would rul 1370 support of full multicopy atomicity would rule out valuable hardware
1381 optimizations, so a weaker form called ``othe 1371 optimizations, so a weaker form called ``other multicopy atomicity''
1382 instead guarantees only that a given store be 1372 instead guarantees only that a given store becomes visible at the same
1383 time to all -other- CPUs. The remainder of t 1373 time to all -other- CPUs. The remainder of this document discusses this
1384 weaker form, but for brevity will call it sim 1374 weaker form, but for brevity will call it simply ``multicopy atomicity''.
1385 1375
1386 The following example demonstrates multicopy 1376 The following example demonstrates multicopy atomicity:
1387 1377
1388 CPU 1 CPU 2 1378 CPU 1 CPU 2 CPU 3
1389 ======================= ============= 1379 ======================= ======================= =======================
1390 { X = 0, Y = 0 } 1380 { X = 0, Y = 0 }
1391 STORE X=1 r1=LOAD X (re 1381 STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1)
1392 <general barr 1382 <general barrier> <read barrier>
1393 STORE Y=r1 1383 STORE Y=r1 LOAD X
1394 1384
1395 Suppose that CPU 2's load from X returns 1, w 1385 Suppose that CPU 2's load from X returns 1, which it then stores to Y,
1396 and CPU 3's load from Y returns 1. This indi 1386 and CPU 3's load from Y returns 1. This indicates that CPU 1's store
1397 to X precedes CPU 2's load from X and that CP 1387 to X precedes CPU 2's load from X and that CPU 2's store to Y precedes
1398 CPU 3's load from Y. In addition, the memory 1388 CPU 3's load from Y. In addition, the memory barriers guarantee that
1399 CPU 2 executes its load before its store, and 1389 CPU 2 executes its load before its store, and CPU 3 loads from Y before
1400 it loads from X. The question is then "Can C 1390 it loads from X. The question is then "Can CPU 3's load from X return 0?"
1401 1391
1402 Because CPU 3's load from X in some sense com 1392 Because CPU 3's load from X in some sense comes after CPU 2's load, it
1403 is natural to expect that CPU 3's load from X 1393 is natural to expect that CPU 3's load from X must therefore return 1.
1404 This expectation follows from multicopy atomi 1394 This expectation follows from multicopy atomicity: if a load executing
1405 on CPU B follows a load from the same variabl 1395 on CPU B follows a load from the same variable executing on CPU A (and
1406 CPU A did not originally store the value whic 1396 CPU A did not originally store the value which it read), then on
1407 multicopy-atomic systems, CPU B's load must r 1397 multicopy-atomic systems, CPU B's load must return either the same value
1408 that CPU A's load did or some later value. H 1398 that CPU A's load did or some later value. However, the Linux kernel
1409 does not require systems to be multicopy atom 1399 does not require systems to be multicopy atomic.
1410 1400
1411 The use of a general memory barrier in the ex 1401 The use of a general memory barrier in the example above compensates
1412 for any lack of multicopy atomicity. In the 1402 for any lack of multicopy atomicity. In the example, if CPU 2's load
1413 from X returns 1 and CPU 3's load from Y retu 1403 from X returns 1 and CPU 3's load from Y returns 1, then CPU 3's load
1414 from X must indeed also return 1. 1404 from X must indeed also return 1.
1415 1405
1416 However, dependencies, read barriers, and wri 1406 However, dependencies, read barriers, and write barriers are not always
1417 able to compensate for non-multicopy atomicit 1407 able to compensate for non-multicopy atomicity. For example, suppose
1418 that CPU 2's general barrier is removed from 1408 that CPU 2's general barrier is removed from the above example, leaving
1419 only the data dependency shown below: 1409 only the data dependency shown below:
1420 1410
1421 CPU 1 CPU 2 1411 CPU 1 CPU 2 CPU 3
1422 ======================= ============= 1412 ======================= ======================= =======================
1423 { X = 0, Y = 0 } 1413 { X = 0, Y = 0 }
1424 STORE X=1 r1=LOAD X (re 1414 STORE X=1 r1=LOAD X (reads 1) LOAD Y (reads 1)
1425 <data depende 1415 <data dependency> <read barrier>
1426 STORE Y=r1 1416 STORE Y=r1 LOAD X (reads 0)
1427 1417
1428 This substitution allows non-multicopy atomic 1418 This substitution allows non-multicopy atomicity to run rampant: in
1429 this example, it is perfectly legal for CPU 2 1419 this example, it is perfectly legal for CPU 2's load from X to return 1,
1430 CPU 3's load from Y to return 1, and its load 1420 CPU 3's load from Y to return 1, and its load from X to return 0.
1431 1421
1432 The key point is that although CPU 2's data d 1422 The key point is that although CPU 2's data dependency orders its load
1433 and store, it does not guarantee to order CPU 1423 and store, it does not guarantee to order CPU 1's store. Thus, if this
1434 example runs on a non-multicopy-atomic system 1424 example runs on a non-multicopy-atomic system where CPUs 1 and 2 share a
1435 store buffer or a level of cache, CPU 2 might 1425 store buffer or a level of cache, CPU 2 might have early access to CPU 1's
1436 writes. General barriers are therefore requi 1426 writes. General barriers are therefore required to ensure that all CPUs
1437 agree on the combined order of multiple acces 1427 agree on the combined order of multiple accesses.
1438 1428
1439 General barriers can compensate not only for 1429 General barriers can compensate not only for non-multicopy atomicity,
1440 but can also generate additional ordering tha 1430 but can also generate additional ordering that can ensure that -all-
1441 CPUs will perceive the same order of -all- op 1431 CPUs will perceive the same order of -all- operations. In contrast, a
1442 chain of release-acquire pairs do not provide 1432 chain of release-acquire pairs do not provide this additional ordering,
1443 which means that only those CPUs on the chain 1433 which means that only those CPUs on the chain are guaranteed to agree
1444 on the combined order of the accesses. For e 1434 on the combined order of the accesses. For example, switching to C code
1445 in deference to the ghost of Herman Hollerith 1435 in deference to the ghost of Herman Hollerith:
1446 1436
1447 int u, v, x, y, z; 1437 int u, v, x, y, z;
1448 1438
1449 void cpu0(void) 1439 void cpu0(void)
1450 { 1440 {
1451 r0 = smp_load_acquire(&x); 1441 r0 = smp_load_acquire(&x);
1452 WRITE_ONCE(u, 1); 1442 WRITE_ONCE(u, 1);
1453 smp_store_release(&y, 1); 1443 smp_store_release(&y, 1);
1454 } 1444 }
1455 1445
1456 void cpu1(void) 1446 void cpu1(void)
1457 { 1447 {
1458 r1 = smp_load_acquire(&y); 1448 r1 = smp_load_acquire(&y);
1459 r4 = READ_ONCE(v); 1449 r4 = READ_ONCE(v);
1460 r5 = READ_ONCE(u); 1450 r5 = READ_ONCE(u);
1461 smp_store_release(&z, 1); 1451 smp_store_release(&z, 1);
1462 } 1452 }
1463 1453
1464 void cpu2(void) 1454 void cpu2(void)
1465 { 1455 {
1466 r2 = smp_load_acquire(&z); 1456 r2 = smp_load_acquire(&z);
1467 smp_store_release(&x, 1); 1457 smp_store_release(&x, 1);
1468 } 1458 }
1469 1459
1470 void cpu3(void) 1460 void cpu3(void)
1471 { 1461 {
1472 WRITE_ONCE(v, 1); 1462 WRITE_ONCE(v, 1);
1473 smp_mb(); 1463 smp_mb();
1474 r3 = READ_ONCE(u); 1464 r3 = READ_ONCE(u);
1475 } 1465 }
1476 1466
1477 Because cpu0(), cpu1(), and cpu2() participat 1467 Because cpu0(), cpu1(), and cpu2() participate in a chain of
1478 smp_store_release()/smp_load_acquire() pairs, 1468 smp_store_release()/smp_load_acquire() pairs, the following outcome
1479 is prohibited: 1469 is prohibited:
1480 1470
1481 r0 == 1 && r1 == 1 && r2 == 1 1471 r0 == 1 && r1 == 1 && r2 == 1
1482 1472
1483 Furthermore, because of the release-acquire r 1473 Furthermore, because of the release-acquire relationship between cpu0()
1484 and cpu1(), cpu1() must see cpu0()'s writes, 1474 and cpu1(), cpu1() must see cpu0()'s writes, so that the following
1485 outcome is prohibited: 1475 outcome is prohibited:
1486 1476
1487 r1 == 1 && r5 == 0 1477 r1 == 1 && r5 == 0
1488 1478
1489 However, the ordering provided by a release-a 1479 However, the ordering provided by a release-acquire chain is local
1490 to the CPUs participating in that chain and d 1480 to the CPUs participating in that chain and does not apply to cpu3(),
1491 at least aside from stores. Therefore, the f 1481 at least aside from stores. Therefore, the following outcome is possible:
1492 1482
1493 r0 == 0 && r1 == 1 && r2 == 1 && r3 = 1483 r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
1494 1484
1495 As an aside, the following outcome is also po 1485 As an aside, the following outcome is also possible:
1496 1486
1497 r0 == 0 && r1 == 1 && r2 == 1 && r3 = 1487 r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
1498 1488
1499 Although cpu0(), cpu1(), and cpu2() will see 1489 Although cpu0(), cpu1(), and cpu2() will see their respective reads and
1500 writes in order, CPUs not involved in the rel 1490 writes in order, CPUs not involved in the release-acquire chain might
1501 well disagree on the order. This disagreemen 1491 well disagree on the order. This disagreement stems from the fact that
1502 the weak memory-barrier instructions used to 1492 the weak memory-barrier instructions used to implement smp_load_acquire()
1503 and smp_store_release() are not required to o 1493 and smp_store_release() are not required to order prior stores against
1504 subsequent loads in all cases. This means th 1494 subsequent loads in all cases. This means that cpu3() can see cpu0()'s
1505 store to u as happening -after- cpu1()'s load 1495 store to u as happening -after- cpu1()'s load from v, even though
1506 both cpu0() and cpu1() agree that these two o 1496 both cpu0() and cpu1() agree that these two operations occurred in the
1507 intended order. 1497 intended order.
1508 1498
1509 However, please keep in mind that smp_load_ac 1499 However, please keep in mind that smp_load_acquire() is not magic.
1510 In particular, it simply reads from its argum 1500 In particular, it simply reads from its argument with ordering. It does
1511 -not- ensure that any particular value will b 1501 -not- ensure that any particular value will be read. Therefore, the
1512 following outcome is possible: 1502 following outcome is possible:
1513 1503
1514 r0 == 0 && r1 == 0 && r2 == 0 && r5 = 1504 r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
1515 1505
1516 Note that this outcome can happen even on a m 1506 Note that this outcome can happen even on a mythical sequentially
1517 consistent system where nothing is ever reord 1507 consistent system where nothing is ever reordered.
1518 1508
1519 To reiterate, if your code requires full orde 1509 To reiterate, if your code requires full ordering of all operations,
1520 use general barriers throughout. 1510 use general barriers throughout.
1521 1511
1522 1512
1523 ======================== 1513 ========================
1524 EXPLICIT KERNEL BARRIERS 1514 EXPLICIT KERNEL BARRIERS
1525 ======================== 1515 ========================
1526 1516
1527 The Linux kernel has a variety of different b 1517 The Linux kernel has a variety of different barriers that act at different
1528 levels: 1518 levels:
1529 1519
1530 (*) Compiler barrier. 1520 (*) Compiler barrier.
1531 1521
1532 (*) CPU memory barriers. 1522 (*) CPU memory barriers.
1533 1523
1534 1524
1535 COMPILER BARRIER 1525 COMPILER BARRIER
1536 ---------------- 1526 ----------------
1537 1527
1538 The Linux kernel has an explicit compiler bar 1528 The Linux kernel has an explicit compiler barrier function that prevents the
1539 compiler from moving the memory accesses eith 1529 compiler from moving the memory accesses either side of it to the other side:
1540 1530
1541 barrier(); 1531 barrier();
1542 1532
1543 This is a general barrier -- there are no rea 1533 This is a general barrier -- there are no read-read or write-write
1544 variants of barrier(). However, READ_ONCE() 1534 variants of barrier(). However, READ_ONCE() and WRITE_ONCE() can be
1545 thought of as weak forms of barrier() that af 1535 thought of as weak forms of barrier() that affect only the specific
1546 accesses flagged by the READ_ONCE() or WRITE_ 1536 accesses flagged by the READ_ONCE() or WRITE_ONCE().
1547 1537
1548 The barrier() function has the following effe 1538 The barrier() function has the following effects:
1549 1539
1550 (*) Prevents the compiler from reordering ac 1540 (*) Prevents the compiler from reordering accesses following the
1551 barrier() to precede any accesses preced 1541 barrier() to precede any accesses preceding the barrier().
1552 One example use for this property is to 1542 One example use for this property is to ease communication between
1553 interrupt-handler code and the code that 1543 interrupt-handler code and the code that was interrupted.
1554 1544
1555 (*) Within a loop, forces the compiler to lo 1545 (*) Within a loop, forces the compiler to load the variables used
1556 in that loop's conditional on each pass 1546 in that loop's conditional on each pass through that loop.
1557 1547
1558 The READ_ONCE() and WRITE_ONCE() functions ca 1548 The READ_ONCE() and WRITE_ONCE() functions can prevent any number of
1559 optimizations that, while perfectly safe in s 1549 optimizations that, while perfectly safe in single-threaded code, can
1560 be fatal in concurrent code. Here are some e 1550 be fatal in concurrent code. Here are some examples of these sorts
1561 of optimizations: 1551 of optimizations:
1562 1552
1563 (*) The compiler is within its rights to reo 1553 (*) The compiler is within its rights to reorder loads and stores
1564 to the same variable, and in some cases, 1554 to the same variable, and in some cases, the CPU is within its
1565 rights to reorder loads to the same vari 1555 rights to reorder loads to the same variable. This means that
1566 the following code: 1556 the following code:
1567 1557
1568 a[0] = x; 1558 a[0] = x;
1569 a[1] = x; 1559 a[1] = x;
1570 1560
1571 Might result in an older value of x stor 1561 Might result in an older value of x stored in a[1] than in a[0].
1572 Prevent both the compiler and the CPU fr 1562 Prevent both the compiler and the CPU from doing this as follows:
1573 1563
1574 a[0] = READ_ONCE(x); 1564 a[0] = READ_ONCE(x);
1575 a[1] = READ_ONCE(x); 1565 a[1] = READ_ONCE(x);
1576 1566
1577 In short, READ_ONCE() and WRITE_ONCE() p 1567 In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
1578 accesses from multiple CPUs to a single 1568 accesses from multiple CPUs to a single variable.
1579 1569
1580 (*) The compiler is within its rights to mer 1570 (*) The compiler is within its rights to merge successive loads from
1581 the same variable. Such merging can cau 1571 the same variable. Such merging can cause the compiler to "optimize"
1582 the following code: 1572 the following code:
1583 1573
1584 while (tmp = a) 1574 while (tmp = a)
1585 do_something_with(tmp); 1575 do_something_with(tmp);
1586 1576
1587 into the following code, which, although 1577 into the following code, which, although in some sense legitimate
1588 for single-threaded code, is almost cert 1578 for single-threaded code, is almost certainly not what the developer
1589 intended: 1579 intended:
1590 1580
1591 if (tmp = a) 1581 if (tmp = a)
1592 for (;;) 1582 for (;;)
1593 do_something_with(tmp 1583 do_something_with(tmp);
1594 1584
1595 Use READ_ONCE() to prevent the compiler 1585 Use READ_ONCE() to prevent the compiler from doing this to you:
1596 1586
1597 while (tmp = READ_ONCE(a)) 1587 while (tmp = READ_ONCE(a))
1598 do_something_with(tmp); 1588 do_something_with(tmp);
1599 1589
1600 (*) The compiler is within its rights to rel 1590 (*) The compiler is within its rights to reload a variable, for example,
1601 in cases where high register pressure pr 1591 in cases where high register pressure prevents the compiler from
1602 keeping all data of interest in register 1592 keeping all data of interest in registers. The compiler might
1603 therefore optimize the variable 'tmp' ou 1593 therefore optimize the variable 'tmp' out of our previous example:
1604 1594
1605 while (tmp = a) 1595 while (tmp = a)
1606 do_something_with(tmp); 1596 do_something_with(tmp);
1607 1597
1608 This could result in the following code, 1598 This could result in the following code, which is perfectly safe in
1609 single-threaded code, but can be fatal i 1599 single-threaded code, but can be fatal in concurrent code:
1610 1600
1611 while (a) 1601 while (a)
1612 do_something_with(a); 1602 do_something_with(a);
1613 1603
1614 For example, the optimized version of th 1604 For example, the optimized version of this code could result in
1615 passing a zero to do_something_with() in 1605 passing a zero to do_something_with() in the case where the variable
1616 a was modified by some other CPU between 1606 a was modified by some other CPU between the "while" statement and
1617 the call to do_something_with(). 1607 the call to do_something_with().
1618 1608
1619 Again, use READ_ONCE() to prevent the co 1609 Again, use READ_ONCE() to prevent the compiler from doing this:
1620 1610
1621 while (tmp = READ_ONCE(a)) 1611 while (tmp = READ_ONCE(a))
1622 do_something_with(tmp); 1612 do_something_with(tmp);
1623 1613
1624 Note that if the compiler runs short of 1614 Note that if the compiler runs short of registers, it might save
1625 tmp onto the stack. The overhead of thi 1615 tmp onto the stack. The overhead of this saving and later restoring
1626 is why compilers reload variables. Doin 1616 is why compilers reload variables. Doing so is perfectly safe for
1627 single-threaded code, so you need to tel 1617 single-threaded code, so you need to tell the compiler about cases
1628 where it is not safe. 1618 where it is not safe.
1629 1619
1630 (*) The compiler is within its rights to omi 1620 (*) The compiler is within its rights to omit a load entirely if it knows
1631 what the value will be. For example, if 1621 what the value will be. For example, if the compiler can prove that
1632 the value of variable 'a' is always zero 1622 the value of variable 'a' is always zero, it can optimize this code:
1633 1623
1634 while (tmp = a) 1624 while (tmp = a)
1635 do_something_with(tmp); 1625 do_something_with(tmp);
1636 1626
1637 Into this: 1627 Into this:
1638 1628
1639 do { } while (0); 1629 do { } while (0);
1640 1630
1641 This transformation is a win for single- 1631 This transformation is a win for single-threaded code because it
1642 gets rid of a load and a branch. The pr 1632 gets rid of a load and a branch. The problem is that the compiler
1643 will carry out its proof assuming that t 1633 will carry out its proof assuming that the current CPU is the only
1644 one updating variable 'a'. If variable 1634 one updating variable 'a'. If variable 'a' is shared, then the
1645 compiler's proof will be erroneous. Use 1635 compiler's proof will be erroneous. Use READ_ONCE() to tell the
1646 compiler that it doesn't know as much as 1636 compiler that it doesn't know as much as it thinks it does:
1647 1637
1648 while (tmp = READ_ONCE(a)) 1638 while (tmp = READ_ONCE(a))
1649 do_something_with(tmp); 1639 do_something_with(tmp);
1650 1640
1651 But please note that the compiler is als 1641 But please note that the compiler is also closely watching what you
1652 do with the value after the READ_ONCE(). 1642 do with the value after the READ_ONCE(). For example, suppose you
1653 do the following and MAX is a preprocess 1643 do the following and MAX is a preprocessor macro with the value 1:
1654 1644
1655 while ((tmp = READ_ONCE(a)) % MAX) 1645 while ((tmp = READ_ONCE(a)) % MAX)
1656 do_something_with(tmp); 1646 do_something_with(tmp);
1657 1647
1658 Then the compiler knows that the result 1648 Then the compiler knows that the result of the "%" operator applied
1659 to MAX will always be zero, again allowi 1649 to MAX will always be zero, again allowing the compiler to optimize
1660 the code into near-nonexistence. (It wi 1650 the code into near-nonexistence. (It will still load from the
1661 variable 'a'.) 1651 variable 'a'.)
1662 1652
1663 (*) Similarly, the compiler is within its ri 1653 (*) Similarly, the compiler is within its rights to omit a store entirely
1664 if it knows that the variable already ha 1654 if it knows that the variable already has the value being stored.
1665 Again, the compiler assumes that the cur 1655 Again, the compiler assumes that the current CPU is the only one
1666 storing into the variable, which can cau 1656 storing into the variable, which can cause the compiler to do the
1667 wrong thing for shared variables. For e 1657 wrong thing for shared variables. For example, suppose you have
1668 the following: 1658 the following:
1669 1659
1670 a = 0; 1660 a = 0;
1671 ... Code that does not store to varia 1661 ... Code that does not store to variable a ...
1672 a = 0; 1662 a = 0;
1673 1663
1674 The compiler sees that the value of vari 1664 The compiler sees that the value of variable 'a' is already zero, so
1675 it might well omit the second store. Th 1665 it might well omit the second store. This would come as a fatal
1676 surprise if some other CPU might have st 1666 surprise if some other CPU might have stored to variable 'a' in the
1677 meantime. 1667 meantime.
1678 1668
1679 Use WRITE_ONCE() to prevent the compiler 1669 Use WRITE_ONCE() to prevent the compiler from making this sort of
1680 wrong guess: 1670 wrong guess:
1681 1671
1682 WRITE_ONCE(a, 0); 1672 WRITE_ONCE(a, 0);
1683 ... Code that does not store to varia 1673 ... Code that does not store to variable a ...
1684 WRITE_ONCE(a, 0); 1674 WRITE_ONCE(a, 0);
1685 1675
1686 (*) The compiler is within its rights to reo 1676 (*) The compiler is within its rights to reorder memory accesses unless
1687 you tell it not to. For example, consid 1677 you tell it not to. For example, consider the following interaction
1688 between process-level code and an interr 1678 between process-level code and an interrupt handler:
1689 1679
1690 void process_level(void) 1680 void process_level(void)
1691 { 1681 {
1692 msg = get_message(); 1682 msg = get_message();
1693 flag = true; 1683 flag = true;
1694 } 1684 }
1695 1685
1696 void interrupt_handler(void) 1686 void interrupt_handler(void)
1697 { 1687 {
1698 if (flag) 1688 if (flag)
1699 process_message(msg); 1689 process_message(msg);
1700 } 1690 }
1701 1691
1702 There is nothing to prevent the compiler 1692 There is nothing to prevent the compiler from transforming
1703 process_level() to the following, in fac 1693 process_level() to the following, in fact, this might well be a
1704 win for single-threaded code: 1694 win for single-threaded code:
1705 1695
1706 void process_level(void) 1696 void process_level(void)
1707 { 1697 {
1708 flag = true; 1698 flag = true;
1709 msg = get_message(); 1699 msg = get_message();
1710 } 1700 }
1711 1701
1712 If the interrupt occurs between these tw 1702 If the interrupt occurs between these two statement, then
1713 interrupt_handler() might be passed a ga 1703 interrupt_handler() might be passed a garbled msg. Use WRITE_ONCE()
1714 to prevent this as follows: 1704 to prevent this as follows:
1715 1705
1716 void process_level(void) 1706 void process_level(void)
1717 { 1707 {
1718 WRITE_ONCE(msg, get_message() 1708 WRITE_ONCE(msg, get_message());
1719 WRITE_ONCE(flag, true); 1709 WRITE_ONCE(flag, true);
1720 } 1710 }
1721 1711
1722 void interrupt_handler(void) 1712 void interrupt_handler(void)
1723 { 1713 {
1724 if (READ_ONCE(flag)) 1714 if (READ_ONCE(flag))
1725 process_message(READ_ 1715 process_message(READ_ONCE(msg));
1726 } 1716 }
1727 1717
1728 Note that the READ_ONCE() and WRITE_ONCE 1718 Note that the READ_ONCE() and WRITE_ONCE() wrappers in
1729 interrupt_handler() are needed if this i 1719 interrupt_handler() are needed if this interrupt handler can itself
1730 be interrupted by something that also ac 1720 be interrupted by something that also accesses 'flag' and 'msg',
1731 for example, a nested interrupt or an NM 1721 for example, a nested interrupt or an NMI. Otherwise, READ_ONCE()
1732 and WRITE_ONCE() are not needed in inter 1722 and WRITE_ONCE() are not needed in interrupt_handler() other than
1733 for documentation purposes. (Note also 1723 for documentation purposes. (Note also that nested interrupts
1734 do not typically occur in modern Linux k 1724 do not typically occur in modern Linux kernels, in fact, if an
1735 interrupt handler returns with interrupt 1725 interrupt handler returns with interrupts enabled, you will get a
1736 WARN_ONCE() splat.) 1726 WARN_ONCE() splat.)
1737 1727
1738 You should assume that the compiler can 1728 You should assume that the compiler can move READ_ONCE() and
1739 WRITE_ONCE() past code not containing RE 1729 WRITE_ONCE() past code not containing READ_ONCE(), WRITE_ONCE(),
1740 barrier(), or similar primitives. 1730 barrier(), or similar primitives.
1741 1731
1742 This effect could also be achieved using 1732 This effect could also be achieved using barrier(), but READ_ONCE()
1743 and WRITE_ONCE() are more selective: Wi 1733 and WRITE_ONCE() are more selective: With READ_ONCE() and
1744 WRITE_ONCE(), the compiler need only for 1734 WRITE_ONCE(), the compiler need only forget the contents of the
1745 indicated memory locations, while with b 1735 indicated memory locations, while with barrier() the compiler must
1746 discard the value of all memory location 1736 discard the value of all memory locations that it has currently
1747 cached in any machine registers. Of cou 1737 cached in any machine registers. Of course, the compiler must also
1748 respect the order in which the READ_ONCE 1738 respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
1749 though the CPU of course need not do so. 1739 though the CPU of course need not do so.
1750 1740
1751 (*) The compiler is within its rights to inv 1741 (*) The compiler is within its rights to invent stores to a variable,
1752 as in the following example: 1742 as in the following example:
1753 1743
1754 if (a) 1744 if (a)
1755 b = a; 1745 b = a;
1756 else 1746 else
1757 b = 42; 1747 b = 42;
1758 1748
1759 The compiler might save a branch by opti 1749 The compiler might save a branch by optimizing this as follows:
1760 1750
1761 b = 42; 1751 b = 42;
1762 if (a) 1752 if (a)
1763 b = a; 1753 b = a;
1764 1754
1765 In single-threaded code, this is not onl 1755 In single-threaded code, this is not only safe, but also saves
1766 a branch. Unfortunately, in concurrent 1756 a branch. Unfortunately, in concurrent code, this optimization
1767 could cause some other CPU to see a spur 1757 could cause some other CPU to see a spurious value of 42 -- even
1768 if variable 'a' was never zero -- when l 1758 if variable 'a' was never zero -- when loading variable 'b'.
1769 Use WRITE_ONCE() to prevent this as foll 1759 Use WRITE_ONCE() to prevent this as follows:
1770 1760
1771 if (a) 1761 if (a)
1772 WRITE_ONCE(b, a); 1762 WRITE_ONCE(b, a);
1773 else 1763 else
1774 WRITE_ONCE(b, 42); 1764 WRITE_ONCE(b, 42);
1775 1765
1776 The compiler can also invent loads. The 1766 The compiler can also invent loads. These are usually less
1777 damaging, but they can result in cache-l 1767 damaging, but they can result in cache-line bouncing and thus in
1778 poor performance and scalability. Use R 1768 poor performance and scalability. Use READ_ONCE() to prevent
1779 invented loads. 1769 invented loads.
1780 1770
1781 (*) For aligned memory locations whose size 1771 (*) For aligned memory locations whose size allows them to be accessed
1782 with a single memory-reference instructi 1772 with a single memory-reference instruction, prevents "load tearing"
1783 and "store tearing," in which a single l 1773 and "store tearing," in which a single large access is replaced by
1784 multiple smaller accesses. For example, 1774 multiple smaller accesses. For example, given an architecture having
1785 16-bit store instructions with 7-bit imm 1775 16-bit store instructions with 7-bit immediate fields, the compiler
1786 might be tempted to use two 16-bit store 1776 might be tempted to use two 16-bit store-immediate instructions to
1787 implement the following 32-bit store: 1777 implement the following 32-bit store:
1788 1778
1789 p = 0x00010002; 1779 p = 0x00010002;
1790 1780
1791 Please note that GCC really does use thi 1781 Please note that GCC really does use this sort of optimization,
1792 which is not surprising given that it wo 1782 which is not surprising given that it would likely take more
1793 than two instructions to build the const 1783 than two instructions to build the constant and then store it.
1794 This optimization can therefore be a win 1784 This optimization can therefore be a win in single-threaded code.
1795 In fact, a recent bug (since fixed) caus 1785 In fact, a recent bug (since fixed) caused GCC to incorrectly use
1796 this optimization in a volatile store. 1786 this optimization in a volatile store. In the absence of such bugs,
1797 use of WRITE_ONCE() prevents store teari 1787 use of WRITE_ONCE() prevents store tearing in the following example:
1798 1788
1799 WRITE_ONCE(p, 0x00010002); 1789 WRITE_ONCE(p, 0x00010002);
1800 1790
1801 Use of packed structures can also result 1791 Use of packed structures can also result in load and store tearing,
1802 as in this example: 1792 as in this example:
1803 1793
1804 struct __attribute__((__packed__)) fo 1794 struct __attribute__((__packed__)) foo {
1805 short a; 1795 short a;
1806 int b; 1796 int b;
1807 short c; 1797 short c;
1808 }; 1798 };
1809 struct foo foo1, foo2; 1799 struct foo foo1, foo2;
1810 ... 1800 ...
1811 1801
1812 foo2.a = foo1.a; 1802 foo2.a = foo1.a;
1813 foo2.b = foo1.b; 1803 foo2.b = foo1.b;
1814 foo2.c = foo1.c; 1804 foo2.c = foo1.c;
1815 1805
1816 Because there are no READ_ONCE() or WRIT 1806 Because there are no READ_ONCE() or WRITE_ONCE() wrappers and no
1817 volatile markings, the compiler would be 1807 volatile markings, the compiler would be well within its rights to
1818 implement these three assignment stateme 1808 implement these three assignment statements as a pair of 32-bit
1819 loads followed by a pair of 32-bit store 1809 loads followed by a pair of 32-bit stores. This would result in
1820 load tearing on 'foo1.b' and store teari 1810 load tearing on 'foo1.b' and store tearing on 'foo2.b'. READ_ONCE()
1821 and WRITE_ONCE() again prevent tearing i 1811 and WRITE_ONCE() again prevent tearing in this example:
1822 1812
1823 foo2.a = foo1.a; 1813 foo2.a = foo1.a;
1824 WRITE_ONCE(foo2.b, READ_ONCE(foo1.b)) 1814 WRITE_ONCE(foo2.b, READ_ONCE(foo1.b));
1825 foo2.c = foo1.c; 1815 foo2.c = foo1.c;
1826 1816
1827 All that aside, it is never necessary to use 1817 All that aside, it is never necessary to use READ_ONCE() and
1828 WRITE_ONCE() on a variable that has been mark 1818 WRITE_ONCE() on a variable that has been marked volatile. For example,
1829 because 'jiffies' is marked volatile, it is n 1819 because 'jiffies' is marked volatile, it is never necessary to
1830 say READ_ONCE(jiffies). The reason for this 1820 say READ_ONCE(jiffies). The reason for this is that READ_ONCE() and
1831 WRITE_ONCE() are implemented as volatile cast 1821 WRITE_ONCE() are implemented as volatile casts, which has no effect when
1832 its argument is already marked volatile. 1822 its argument is already marked volatile.
1833 1823
1834 Please note that these compiler barriers have 1824 Please note that these compiler barriers have no direct effect on the CPU,
1835 which may then reorder things however it wish 1825 which may then reorder things however it wishes.
1836 1826
1837 1827
1838 CPU MEMORY BARRIERS 1828 CPU MEMORY BARRIERS
1839 ------------------- 1829 -------------------
1840 1830
1841 The Linux kernel has seven basic CPU memory b 1831 The Linux kernel has seven basic CPU memory barriers:
1842 1832
1843 TYPE MANDATORY 1833 TYPE MANDATORY SMP CONDITIONAL
1844 ======================= ============= 1834 ======================= =============== ===============
1845 GENERAL mb() 1835 GENERAL mb() smp_mb()
1846 WRITE wmb() 1836 WRITE wmb() smp_wmb()
1847 READ rmb() 1837 READ rmb() smp_rmb()
1848 ADDRESS DEPENDENCY 1838 ADDRESS DEPENDENCY READ_ONCE()
1849 1839
1850 1840
1851 All memory barriers except the address-depend 1841 All memory barriers except the address-dependency barriers imply a compiler
1852 barrier. Address dependencies do not impose 1842 barrier. Address dependencies do not impose any additional compiler ordering.
1853 1843
1854 Aside: In the case of address dependencies, t 1844 Aside: In the case of address dependencies, the compiler would be expected
1855 to issue the loads in the correct order (eg. 1845 to issue the loads in the correct order (eg. `a[b]` would have to load
1856 the value of b before loading a[b]), however 1846 the value of b before loading a[b]), however there is no guarantee in
1857 the C specification that the compiler may not 1847 the C specification that the compiler may not speculate the value of b
1858 (eg. is equal to 1) and load a[b] before b (e 1848 (eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1)
1859 tmp = a[b]; ). There is also the problem of 1849 tmp = a[b]; ). There is also the problem of a compiler reloading b after
1860 having loaded a[b], thus having a newer copy 1850 having loaded a[b], thus having a newer copy of b than a[b]. A consensus
1861 has not yet been reached about these problems 1851 has not yet been reached about these problems, however the READ_ONCE()
1862 macro is a good place to start looking. 1852 macro is a good place to start looking.
1863 1853
1864 SMP memory barriers are reduced to compiler b 1854 SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
1865 systems because it is assumed that a CPU will 1855 systems because it is assumed that a CPU will appear to be self-consistent,
1866 and will order overlapping accesses correctly 1856 and will order overlapping accesses correctly with respect to itself.
1867 However, see the subsection on "Virtual Machi 1857 However, see the subsection on "Virtual Machine Guests" below.
1868 1858
1869 [!] Note that SMP memory barriers _must_ be u 1859 [!] Note that SMP memory barriers _must_ be used to control the ordering of
1870 references to shared memory on SMP systems, t 1860 references to shared memory on SMP systems, though the use of locking instead
1871 is sufficient. 1861 is sufficient.
1872 1862
1873 Mandatory barriers should not be used to cont 1863 Mandatory barriers should not be used to control SMP effects, since mandatory
1874 barriers impose unnecessary overhead on both 1864 barriers impose unnecessary overhead on both SMP and UP systems. They may,
1875 however, be used to control MMIO effects on a 1865 however, be used to control MMIO effects on accesses through relaxed memory I/O
1876 windows. These barriers are required even on 1866 windows. These barriers are required even on non-SMP systems as they affect
1877 the order in which memory operations appear t 1867 the order in which memory operations appear to a device by prohibiting both the
1878 compiler and the CPU from reordering them. 1868 compiler and the CPU from reordering them.
1879 1869
1880 1870
1881 There are some more advanced barrier function 1871 There are some more advanced barrier functions:
1882 1872
1883 (*) smp_store_mb(var, value) 1873 (*) smp_store_mb(var, value)
1884 1874
1885 This assigns the value to the variable a 1875 This assigns the value to the variable and then inserts a full memory
1886 barrier after it. It isn't guaranteed t 1876 barrier after it. It isn't guaranteed to insert anything more than a
1887 compiler barrier in a UP compilation. 1877 compiler barrier in a UP compilation.
1888 1878
1889 1879
1890 (*) smp_mb__before_atomic(); 1880 (*) smp_mb__before_atomic();
1891 (*) smp_mb__after_atomic(); 1881 (*) smp_mb__after_atomic();
1892 1882
1893 These are for use with atomic RMW functi 1883 These are for use with atomic RMW functions that do not imply memory
1894 barriers, but where the code needs a mem 1884 barriers, but where the code needs a memory barrier. Examples for atomic
1895 RMW functions that do not imply a memory 1885 RMW functions that do not imply a memory barrier are e.g. add,
1896 subtract, (failed) conditional operation 1886 subtract, (failed) conditional operations, _relaxed functions,
1897 but not atomic_read or atomic_set. A com 1887 but not atomic_read or atomic_set. A common example where a memory
1898 barrier may be required is when atomic o 1888 barrier may be required is when atomic ops are used for reference
1899 counting. 1889 counting.
1900 1890
1901 These are also used for atomic RMW bitop 1891 These are also used for atomic RMW bitop functions that do not imply a
1902 memory barrier (such as set_bit and clea 1892 memory barrier (such as set_bit and clear_bit).
1903 1893
1904 As an example, consider a piece of code 1894 As an example, consider a piece of code that marks an object as being dead
1905 and then decrements the object's referen 1895 and then decrements the object's reference count:
1906 1896
1907 obj->dead = 1; 1897 obj->dead = 1;
1908 smp_mb__before_atomic(); 1898 smp_mb__before_atomic();
1909 atomic_dec(&obj->ref_count); 1899 atomic_dec(&obj->ref_count);
1910 1900
1911 This makes sure that the death mark on t 1901 This makes sure that the death mark on the object is perceived to be set
1912 *before* the reference counter is decrem 1902 *before* the reference counter is decremented.
1913 1903
1914 See Documentation/atomic_{t,bitops}.txt 1904 See Documentation/atomic_{t,bitops}.txt for more information.
1915 1905
1916 1906
1917 (*) dma_wmb(); 1907 (*) dma_wmb();
1918 (*) dma_rmb(); 1908 (*) dma_rmb();
1919 (*) dma_mb(); 1909 (*) dma_mb();
1920 1910
1921 These are for use with consistent memory 1911 These are for use with consistent memory to guarantee the ordering
1922 of writes or reads of shared memory acce 1912 of writes or reads of shared memory accessible to both the CPU and a
1923 DMA capable device. See Documentation/co !! 1913 DMA capable device.
1924 information about consistent memory. <<
1925 1914
1926 For example, consider a device driver th 1915 For example, consider a device driver that shares memory with a device
1927 and uses a descriptor status value to in 1916 and uses a descriptor status value to indicate if the descriptor belongs
1928 to the device or the CPU, and a doorbell 1917 to the device or the CPU, and a doorbell to notify it when new
1929 descriptors are available: 1918 descriptors are available:
1930 1919
1931 if (desc->status != DEVICE_OWN) { 1920 if (desc->status != DEVICE_OWN) {
1932 /* do not read data until we 1921 /* do not read data until we own descriptor */
1933 dma_rmb(); 1922 dma_rmb();
1934 1923
1935 /* read/modify data */ 1924 /* read/modify data */
1936 read_data = desc->data; 1925 read_data = desc->data;
1937 desc->data = write_data; 1926 desc->data = write_data;
1938 1927
1939 /* flush modifications before 1928 /* flush modifications before status update */
1940 dma_wmb(); 1929 dma_wmb();
1941 1930
1942 /* assign ownership */ 1931 /* assign ownership */
1943 desc->status = DEVICE_OWN; 1932 desc->status = DEVICE_OWN;
1944 1933
1945 /* Make descriptor status vis !! 1934 /* notify device of new descriptors */
1946 * notify device of new descr <<
1947 */ <<
1948 writel(DESC_NOTIFY, doorbell) 1935 writel(DESC_NOTIFY, doorbell);
1949 } 1936 }
1950 1937
1951 The dma_rmb() allows us to guarantee tha !! 1938 The dma_rmb() allows us guarantee the device has released ownership
1952 before we read the data from the descrip 1939 before we read the data from the descriptor, and the dma_wmb() allows
1953 us to guarantee the data is written to t 1940 us to guarantee the data is written to the descriptor before the device
1954 can see it now has ownership. The dma_m 1941 can see it now has ownership. The dma_mb() implies both a dma_rmb() and
1955 a dma_wmb(). !! 1942 a dma_wmb(). Note that, when using writel(), a prior wmb() is not needed
1956 !! 1943 to guarantee that the cache coherent memory writes have completed before
1957 Note that the dma_*() barriers do not pr !! 1944 writing to the MMIO region. The cheaper writel_relaxed() does not provide
1958 accesses to MMIO regions. See the later !! 1945 this guarantee and must not be used here.
1959 subsection for more information about I/ !! 1946
>> 1947 See the subsection "Kernel I/O barrier effects" for more information on
>> 1948 relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for
>> 1949 more information on consistent memory.
1960 1950
1961 (*) pmem_wmb(); 1951 (*) pmem_wmb();
1962 1952
1963 This is for use with persistent memory t 1953 This is for use with persistent memory to ensure that stores for which
1964 modifications are written to persistent 1954 modifications are written to persistent storage reached a platform
1965 durability domain. 1955 durability domain.
1966 1956
1967 For example, after a non-temporal write 1957 For example, after a non-temporal write to pmem region, we use pmem_wmb()
1968 to ensure that stores have reached a pla 1958 to ensure that stores have reached a platform durability domain. This ensures
1969 that stores have updated persistent stor 1959 that stores have updated persistent storage before any data access or
1970 data transfer caused by subsequent instr 1960 data transfer caused by subsequent instructions is initiated. This is
1971 in addition to the ordering done by wmb( 1961 in addition to the ordering done by wmb().
1972 1962
1973 For load from persistent memory, existin 1963 For load from persistent memory, existing read memory barriers are sufficient
1974 to ensure read ordering. 1964 to ensure read ordering.
1975 1965
1976 (*) io_stop_wc(); 1966 (*) io_stop_wc();
1977 1967
1978 For memory accesses with write-combining 1968 For memory accesses with write-combining attributes (e.g. those returned
1979 by ioremap_wc()), the CPU may wait for p 1969 by ioremap_wc()), the CPU may wait for prior accesses to be merged with
1980 subsequent ones. io_stop_wc() can be use 1970 subsequent ones. io_stop_wc() can be used to prevent the merging of
1981 write-combining memory accesses before t 1971 write-combining memory accesses before this macro with those after it when
1982 such wait has performance implications. 1972 such wait has performance implications.
1983 1973
1984 =============================== 1974 ===============================
1985 IMPLICIT KERNEL MEMORY BARRIERS 1975 IMPLICIT KERNEL MEMORY BARRIERS
1986 =============================== 1976 ===============================
1987 1977
1988 Some of the other functions in the linux kern 1978 Some of the other functions in the linux kernel imply memory barriers, amongst
1989 which are locking and scheduling functions. 1979 which are locking and scheduling functions.
1990 1980
1991 This specification is a _minimum_ guarantee; 1981 This specification is a _minimum_ guarantee; any particular architecture may
1992 provide more substantial guarantees, but thes 1982 provide more substantial guarantees, but these may not be relied upon outside
1993 of arch specific code. 1983 of arch specific code.
1994 1984
1995 1985
1996 LOCK ACQUISITION FUNCTIONS 1986 LOCK ACQUISITION FUNCTIONS
1997 -------------------------- 1987 --------------------------
1998 1988
1999 The Linux kernel has a number of locking cons 1989 The Linux kernel has a number of locking constructs:
2000 1990
2001 (*) spin locks 1991 (*) spin locks
2002 (*) R/W spin locks 1992 (*) R/W spin locks
2003 (*) mutexes 1993 (*) mutexes
2004 (*) semaphores 1994 (*) semaphores
2005 (*) R/W semaphores 1995 (*) R/W semaphores
2006 1996
2007 In all cases there are variants on "ACQUIRE" 1997 In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
2008 for each construct. These operations all imp 1998 for each construct. These operations all imply certain barriers:
2009 1999
2010 (1) ACQUIRE operation implication: 2000 (1) ACQUIRE operation implication:
2011 2001
2012 Memory operations issued after the ACQUI 2002 Memory operations issued after the ACQUIRE will be completed after the
2013 ACQUIRE operation has completed. 2003 ACQUIRE operation has completed.
2014 2004
2015 Memory operations issued before the ACQU 2005 Memory operations issued before the ACQUIRE may be completed after
2016 the ACQUIRE operation has completed. 2006 the ACQUIRE operation has completed.
2017 2007
2018 (2) RELEASE operation implication: 2008 (2) RELEASE operation implication:
2019 2009
2020 Memory operations issued before the RELE 2010 Memory operations issued before the RELEASE will be completed before the
2021 RELEASE operation has completed. 2011 RELEASE operation has completed.
2022 2012
2023 Memory operations issued after the RELEA 2013 Memory operations issued after the RELEASE may be completed before the
2024 RELEASE operation has completed. 2014 RELEASE operation has completed.
2025 2015
2026 (3) ACQUIRE vs ACQUIRE implication: 2016 (3) ACQUIRE vs ACQUIRE implication:
2027 2017
2028 All ACQUIRE operations issued before ano 2018 All ACQUIRE operations issued before another ACQUIRE operation will be
2029 completed before that ACQUIRE operation. 2019 completed before that ACQUIRE operation.
2030 2020
2031 (4) ACQUIRE vs RELEASE implication: 2021 (4) ACQUIRE vs RELEASE implication:
2032 2022
2033 All ACQUIRE operations issued before a R 2023 All ACQUIRE operations issued before a RELEASE operation will be
2034 completed before the RELEASE operation. 2024 completed before the RELEASE operation.
2035 2025
2036 (5) Failed conditional ACQUIRE implication: 2026 (5) Failed conditional ACQUIRE implication:
2037 2027
2038 Certain locking variants of the ACQUIRE 2028 Certain locking variants of the ACQUIRE operation may fail, either due to
2039 being unable to get the lock immediately 2029 being unable to get the lock immediately, or due to receiving an unblocked
2040 signal while asleep waiting for the lock 2030 signal while asleep waiting for the lock to become available. Failed
2041 locks do not imply any sort of barrier. 2031 locks do not imply any sort of barrier.
2042 2032
2043 [!] Note: one of the consequences of lock ACQ 2033 [!] Note: one of the consequences of lock ACQUIREs and RELEASEs being only
2044 one-way barriers is that the effects of instr 2034 one-way barriers is that the effects of instructions outside of a critical
2045 section may seep into the inside of the criti 2035 section may seep into the inside of the critical section.
2046 2036
2047 An ACQUIRE followed by a RELEASE may not be a 2037 An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
2048 because it is possible for an access precedin 2038 because it is possible for an access preceding the ACQUIRE to happen after the
2049 ACQUIRE, and an access following the RELEASE 2039 ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
2050 the two accesses can themselves then cross: 2040 the two accesses can themselves then cross:
2051 2041
2052 *A = a; 2042 *A = a;
2053 ACQUIRE M 2043 ACQUIRE M
2054 RELEASE M 2044 RELEASE M
2055 *B = b; 2045 *B = b;
2056 2046
2057 may occur as: 2047 may occur as:
2058 2048
2059 ACQUIRE M, STORE *B, STORE *A, RELEAS 2049 ACQUIRE M, STORE *B, STORE *A, RELEASE M
2060 2050
2061 When the ACQUIRE and RELEASE are a lock acqui 2051 When the ACQUIRE and RELEASE are a lock acquisition and release,
2062 respectively, this same reordering can occur 2052 respectively, this same reordering can occur if the lock's ACQUIRE and
2063 RELEASE are to the same lock variable, but on 2053 RELEASE are to the same lock variable, but only from the perspective of
2064 another CPU not holding that lock. In short, 2054 another CPU not holding that lock. In short, a ACQUIRE followed by an
2065 RELEASE may -not- be assumed to be a full mem 2055 RELEASE may -not- be assumed to be a full memory barrier.
2066 2056
2067 Similarly, the reverse case of a RELEASE foll 2057 Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
2068 not imply a full memory barrier. Therefore, 2058 not imply a full memory barrier. Therefore, the CPU's execution of the
2069 critical sections corresponding to the RELEAS 2059 critical sections corresponding to the RELEASE and the ACQUIRE can cross,
2070 so that: 2060 so that:
2071 2061
2072 *A = a; 2062 *A = a;
2073 RELEASE M 2063 RELEASE M
2074 ACQUIRE N 2064 ACQUIRE N
2075 *B = b; 2065 *B = b;
2076 2066
2077 could occur as: 2067 could occur as:
2078 2068
2079 ACQUIRE N, STORE *B, STORE *A, RELEAS 2069 ACQUIRE N, STORE *B, STORE *A, RELEASE M
2080 2070
2081 It might appear that this reordering could in 2071 It might appear that this reordering could introduce a deadlock.
2082 However, this cannot happen because if such a 2072 However, this cannot happen because if such a deadlock threatened,
2083 the RELEASE would simply complete, thereby av 2073 the RELEASE would simply complete, thereby avoiding the deadlock.
2084 2074
2085 Why does this work? 2075 Why does this work?
2086 2076
2087 One key point is that we are only tal 2077 One key point is that we are only talking about the CPU doing
2088 the reordering, not the compiler. If 2078 the reordering, not the compiler. If the compiler (or, for
2089 that matter, the developer) switched 2079 that matter, the developer) switched the operations, deadlock
2090 -could- occur. 2080 -could- occur.
2091 2081
2092 But suppose the CPU reordered the ope 2082 But suppose the CPU reordered the operations. In this case,
2093 the unlock precedes the lock in the a 2083 the unlock precedes the lock in the assembly code. The CPU
2094 simply elected to try executing the l 2084 simply elected to try executing the later lock operation first.
2095 If there is a deadlock, this lock ope 2085 If there is a deadlock, this lock operation will simply spin (or
2096 try to sleep, but more on that later) 2086 try to sleep, but more on that later). The CPU will eventually
2097 execute the unlock operation (which p 2087 execute the unlock operation (which preceded the lock operation
2098 in the assembly code), which will unr 2088 in the assembly code), which will unravel the potential deadlock,
2099 allowing the lock operation to succee 2089 allowing the lock operation to succeed.
2100 2090
2101 But what if the lock is a sleeplock? 2091 But what if the lock is a sleeplock? In that case, the code will
2102 try to enter the scheduler, where it 2092 try to enter the scheduler, where it will eventually encounter
2103 a memory barrier, which will force th 2093 a memory barrier, which will force the earlier unlock operation
2104 to complete, again unraveling the dea 2094 to complete, again unraveling the deadlock. There might be
2105 a sleep-unlock race, but the locking 2095 a sleep-unlock race, but the locking primitive needs to resolve
2106 such races properly in any case. 2096 such races properly in any case.
2107 2097
2108 Locks and semaphores may not provide any guar 2098 Locks and semaphores may not provide any guarantee of ordering on UP compiled
2109 systems, and so cannot be counted on in such 2099 systems, and so cannot be counted on in such a situation to actually achieve
2110 anything at all - especially with respect to 2100 anything at all - especially with respect to I/O accesses - unless combined
2111 with interrupt disabling operations. 2101 with interrupt disabling operations.
2112 2102
2113 See also the section on "Inter-CPU acquiring 2103 See also the section on "Inter-CPU acquiring barrier effects".
2114 2104
2115 2105
2116 As an example, consider the following: 2106 As an example, consider the following:
2117 2107
2118 *A = a; 2108 *A = a;
2119 *B = b; 2109 *B = b;
2120 ACQUIRE 2110 ACQUIRE
2121 *C = c; 2111 *C = c;
2122 *D = d; 2112 *D = d;
2123 RELEASE 2113 RELEASE
2124 *E = e; 2114 *E = e;
2125 *F = f; 2115 *F = f;
2126 2116
2127 The following sequence of events is acceptabl 2117 The following sequence of events is acceptable:
2128 2118
2129 ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RE 2119 ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
2130 2120
2131 [+] Note that {*F,*A} indicates a com 2121 [+] Note that {*F,*A} indicates a combined access.
2132 2122
2133 But none of the following are: 2123 But none of the following are:
2134 2124
2135 {*F,*A}, *B, ACQUIRE, *C, *D, 2125 {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E
2136 *A, *B, *C, ACQUIRE, *D, 2126 *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F
2137 *A, *B, ACQUIRE, *C, 2127 *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F
2138 *B, ACQUIRE, *C, *D, 2128 *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E
2139 2129
2140 2130
2141 2131
2142 INTERRUPT DISABLING FUNCTIONS 2132 INTERRUPT DISABLING FUNCTIONS
2143 ----------------------------- 2133 -----------------------------
2144 2134
2145 Functions that disable interrupts (ACQUIRE eq 2135 Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
2146 (RELEASE equivalent) will act as compiler bar 2136 (RELEASE equivalent) will act as compiler barriers only. So if memory or I/O
2147 barriers are required in such a situation, th 2137 barriers are required in such a situation, they must be provided from some
2148 other means. 2138 other means.
2149 2139
2150 2140
2151 SLEEP AND WAKE-UP FUNCTIONS 2141 SLEEP AND WAKE-UP FUNCTIONS
2152 --------------------------- 2142 ---------------------------
2153 2143
2154 Sleeping and waking on an event flagged in gl 2144 Sleeping and waking on an event flagged in global data can be viewed as an
2155 interaction between two pieces of data: the t 2145 interaction between two pieces of data: the task state of the task waiting for
2156 the event and the global data used to indicat 2146 the event and the global data used to indicate the event. To make sure that
2157 these appear to happen in the right order, th 2147 these appear to happen in the right order, the primitives to begin the process
2158 of going to sleep, and the primitives to init 2148 of going to sleep, and the primitives to initiate a wake up imply certain
2159 barriers. 2149 barriers.
2160 2150
2161 Firstly, the sleeper normally follows somethi 2151 Firstly, the sleeper normally follows something like this sequence of events:
2162 2152
2163 for (;;) { 2153 for (;;) {
2164 set_current_state(TASK_UNINTE 2154 set_current_state(TASK_UNINTERRUPTIBLE);
2165 if (event_indicated) 2155 if (event_indicated)
2166 break; 2156 break;
2167 schedule(); 2157 schedule();
2168 } 2158 }
2169 2159
2170 A general memory barrier is interpolated auto 2160 A general memory barrier is interpolated automatically by set_current_state()
2171 after it has altered the task state: 2161 after it has altered the task state:
2172 2162
2173 CPU 1 2163 CPU 1
2174 =============================== 2164 ===============================
2175 set_current_state(); 2165 set_current_state();
2176 smp_store_mb(); 2166 smp_store_mb();
2177 STORE current->state 2167 STORE current->state
2178 <general barrier> 2168 <general barrier>
2179 LOAD event_indicated 2169 LOAD event_indicated
2180 2170
2181 set_current_state() may be wrapped by: 2171 set_current_state() may be wrapped by:
2182 2172
2183 prepare_to_wait(); 2173 prepare_to_wait();
2184 prepare_to_wait_exclusive(); 2174 prepare_to_wait_exclusive();
2185 2175
2186 which therefore also imply a general memory b 2176 which therefore also imply a general memory barrier after setting the state.
2187 The whole sequence above is available in vari 2177 The whole sequence above is available in various canned forms, all of which
2188 interpolate the memory barrier in the right p 2178 interpolate the memory barrier in the right place:
2189 2179
2190 wait_event(); 2180 wait_event();
2191 wait_event_interruptible(); 2181 wait_event_interruptible();
2192 wait_event_interruptible_exclusive(); 2182 wait_event_interruptible_exclusive();
2193 wait_event_interruptible_timeout(); 2183 wait_event_interruptible_timeout();
2194 wait_event_killable(); 2184 wait_event_killable();
2195 wait_event_timeout(); 2185 wait_event_timeout();
2196 wait_on_bit(); 2186 wait_on_bit();
2197 wait_on_bit_lock(); 2187 wait_on_bit_lock();
2198 2188
2199 2189
2200 Secondly, code that performs a wake up normal 2190 Secondly, code that performs a wake up normally follows something like this:
2201 2191
2202 event_indicated = 1; 2192 event_indicated = 1;
2203 wake_up(&event_wait_queue); 2193 wake_up(&event_wait_queue);
2204 2194
2205 or: 2195 or:
2206 2196
2207 event_indicated = 1; 2197 event_indicated = 1;
2208 wake_up_process(event_daemon); 2198 wake_up_process(event_daemon);
2209 2199
2210 A general memory barrier is executed by wake_ 2200 A general memory barrier is executed by wake_up() if it wakes something up.
2211 If it doesn't wake anything up then a memory 2201 If it doesn't wake anything up then a memory barrier may or may not be
2212 executed; you must not rely on it. The barri 2202 executed; you must not rely on it. The barrier occurs before the task state
2213 is accessed, in particular, it sits between t 2203 is accessed, in particular, it sits between the STORE to indicate the event
2214 and the STORE to set TASK_RUNNING: 2204 and the STORE to set TASK_RUNNING:
2215 2205
2216 CPU 1 (Sleeper) CPU 2 2206 CPU 1 (Sleeper) CPU 2 (Waker)
2217 =============================== ===== 2207 =============================== ===============================
2218 set_current_state(); STORE 2208 set_current_state(); STORE event_indicated
2219 smp_store_mb(); wake_ 2209 smp_store_mb(); wake_up();
2220 STORE current->state ... 2210 STORE current->state ...
2221 <general barrier> <ge 2211 <general barrier> <general barrier>
2222 LOAD event_indicated if 2212 LOAD event_indicated if ((LOAD task->state) & TASK_NORMAL)
2223 S 2213 STORE task->state
2224 2214
2225 where "task" is the thread being woken up and 2215 where "task" is the thread being woken up and it equals CPU 1's "current".
2226 2216
2227 To repeat, a general memory barrier is guaran 2217 To repeat, a general memory barrier is guaranteed to be executed by wake_up()
2228 if something is actually awakened, but otherw 2218 if something is actually awakened, but otherwise there is no such guarantee.
2229 To see this, consider the following sequence 2219 To see this, consider the following sequence of events, where X and Y are both
2230 initially zero: 2220 initially zero:
2231 2221
2232 CPU 1 CPU 2 2222 CPU 1 CPU 2
2233 =============================== ===== 2223 =============================== ===============================
2234 X = 1; Y = 1 2224 X = 1; Y = 1;
2235 smp_mb(); wake_ 2225 smp_mb(); wake_up();
2236 LOAD Y LOAD 2226 LOAD Y LOAD X
2237 2227
2238 If a wakeup does occur, one (at least) of the 2228 If a wakeup does occur, one (at least) of the two loads must see 1. If, on
2239 the other hand, a wakeup does not occur, both 2229 the other hand, a wakeup does not occur, both loads might see 0.
2240 2230
2241 wake_up_process() always executes a general m 2231 wake_up_process() always executes a general memory barrier. The barrier again
2242 occurs before the task state is accessed. In 2232 occurs before the task state is accessed. In particular, if the wake_up() in
2243 the previous snippet were replaced by a call 2233 the previous snippet were replaced by a call to wake_up_process() then one of
2244 the two loads would be guaranteed to see 1. 2234 the two loads would be guaranteed to see 1.
2245 2235
2246 The available waker functions include: 2236 The available waker functions include:
2247 2237
2248 complete(); 2238 complete();
2249 wake_up(); 2239 wake_up();
2250 wake_up_all(); 2240 wake_up_all();
2251 wake_up_bit(); 2241 wake_up_bit();
2252 wake_up_interruptible(); 2242 wake_up_interruptible();
2253 wake_up_interruptible_all(); 2243 wake_up_interruptible_all();
2254 wake_up_interruptible_nr(); 2244 wake_up_interruptible_nr();
2255 wake_up_interruptible_poll(); 2245 wake_up_interruptible_poll();
2256 wake_up_interruptible_sync(); 2246 wake_up_interruptible_sync();
2257 wake_up_interruptible_sync_poll(); 2247 wake_up_interruptible_sync_poll();
2258 wake_up_locked(); 2248 wake_up_locked();
2259 wake_up_locked_poll(); 2249 wake_up_locked_poll();
2260 wake_up_nr(); 2250 wake_up_nr();
2261 wake_up_poll(); 2251 wake_up_poll();
2262 wake_up_process(); 2252 wake_up_process();
2263 2253
2264 In terms of memory ordering, these functions 2254 In terms of memory ordering, these functions all provide the same guarantees of
2265 a wake_up() (or stronger). 2255 a wake_up() (or stronger).
2266 2256
2267 [!] Note that the memory barriers implied by 2257 [!] Note that the memory barriers implied by the sleeper and the waker do _not_
2268 order multiple stores before the wake-up with 2258 order multiple stores before the wake-up with respect to loads of those stored
2269 values after the sleeper has called set_curre 2259 values after the sleeper has called set_current_state(). For instance, if the
2270 sleeper does: 2260 sleeper does:
2271 2261
2272 set_current_state(TASK_INTERRUPTIBLE) 2262 set_current_state(TASK_INTERRUPTIBLE);
2273 if (event_indicated) 2263 if (event_indicated)
2274 break; 2264 break;
2275 __set_current_state(TASK_RUNNING); 2265 __set_current_state(TASK_RUNNING);
2276 do_something(my_data); 2266 do_something(my_data);
2277 2267
2278 and the waker does: 2268 and the waker does:
2279 2269
2280 my_data = value; 2270 my_data = value;
2281 event_indicated = 1; 2271 event_indicated = 1;
2282 wake_up(&event_wait_queue); 2272 wake_up(&event_wait_queue);
2283 2273
2284 there's no guarantee that the change to event 2274 there's no guarantee that the change to event_indicated will be perceived by
2285 the sleeper as coming after the change to my_ 2275 the sleeper as coming after the change to my_data. In such a circumstance, the
2286 code on both sides must interpolate its own m 2276 code on both sides must interpolate its own memory barriers between the
2287 separate data accesses. Thus the above sleep 2277 separate data accesses. Thus the above sleeper ought to do:
2288 2278
2289 set_current_state(TASK_INTERRUPTIBLE) 2279 set_current_state(TASK_INTERRUPTIBLE);
2290 if (event_indicated) { 2280 if (event_indicated) {
2291 smp_rmb(); 2281 smp_rmb();
2292 do_something(my_data); 2282 do_something(my_data);
2293 } 2283 }
2294 2284
2295 and the waker should do: 2285 and the waker should do:
2296 2286
2297 my_data = value; 2287 my_data = value;
2298 smp_wmb(); 2288 smp_wmb();
2299 event_indicated = 1; 2289 event_indicated = 1;
2300 wake_up(&event_wait_queue); 2290 wake_up(&event_wait_queue);
2301 2291
2302 2292
2303 MISCELLANEOUS FUNCTIONS 2293 MISCELLANEOUS FUNCTIONS
2304 ----------------------- 2294 -----------------------
2305 2295
2306 Other functions that imply barriers: 2296 Other functions that imply barriers:
2307 2297
2308 (*) schedule() and similar imply full memory 2298 (*) schedule() and similar imply full memory barriers.
2309 2299
2310 2300
2311 =================================== 2301 ===================================
2312 INTER-CPU ACQUIRING BARRIER EFFECTS 2302 INTER-CPU ACQUIRING BARRIER EFFECTS
2313 =================================== 2303 ===================================
2314 2304
2315 On SMP systems locking primitives give a more 2305 On SMP systems locking primitives give a more substantial form of barrier: one
2316 that does affect memory access ordering on ot 2306 that does affect memory access ordering on other CPUs, within the context of
2317 conflict on any particular lock. 2307 conflict on any particular lock.
2318 2308
2319 2309
2320 ACQUIRES VS MEMORY ACCESSES 2310 ACQUIRES VS MEMORY ACCESSES
2321 --------------------------- 2311 ---------------------------
2322 2312
2323 Consider the following: the system has a pair 2313 Consider the following: the system has a pair of spinlocks (M) and (Q), and
2324 three CPUs; then should the following sequenc 2314 three CPUs; then should the following sequence of events occur:
2325 2315
2326 CPU 1 CPU 2 2316 CPU 1 CPU 2
2327 =============================== ===== 2317 =============================== ===============================
2328 WRITE_ONCE(*A, a); WRITE 2318 WRITE_ONCE(*A, a); WRITE_ONCE(*E, e);
2329 ACQUIRE M ACQUI 2319 ACQUIRE M ACQUIRE Q
2330 WRITE_ONCE(*B, b); WRITE 2320 WRITE_ONCE(*B, b); WRITE_ONCE(*F, f);
2331 WRITE_ONCE(*C, c); WRITE 2321 WRITE_ONCE(*C, c); WRITE_ONCE(*G, g);
2332 RELEASE M RELEA 2322 RELEASE M RELEASE Q
2333 WRITE_ONCE(*D, d); WRITE 2323 WRITE_ONCE(*D, d); WRITE_ONCE(*H, h);
2334 2324
2335 Then there is no guarantee as to what order C 2325 Then there is no guarantee as to what order CPU 3 will see the accesses to *A
2336 through *H occur in, other than the constrain 2326 through *H occur in, other than the constraints imposed by the separate locks
2337 on the separate CPUs. It might, for example, 2327 on the separate CPUs. It might, for example, see:
2338 2328
2339 *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, 2329 *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
2340 2330
2341 But it won't see any of: 2331 But it won't see any of:
2342 2332
2343 *B, *C or *D preceding ACQUIRE M 2333 *B, *C or *D preceding ACQUIRE M
2344 *A, *B or *C following RELEASE M 2334 *A, *B or *C following RELEASE M
2345 *F, *G or *H preceding ACQUIRE Q 2335 *F, *G or *H preceding ACQUIRE Q
2346 *E, *F or *G following RELEASE Q 2336 *E, *F or *G following RELEASE Q
2347 2337
2348 2338
2349 ================================= 2339 =================================
2350 WHERE ARE MEMORY BARRIERS NEEDED? 2340 WHERE ARE MEMORY BARRIERS NEEDED?
2351 ================================= 2341 =================================
2352 2342
2353 Under normal operation, memory operation reor 2343 Under normal operation, memory operation reordering is generally not going to
2354 be a problem as a single-threaded linear piec 2344 be a problem as a single-threaded linear piece of code will still appear to
2355 work correctly, even if it's in an SMP kernel 2345 work correctly, even if it's in an SMP kernel. There are, however, four
2356 circumstances in which reordering definitely 2346 circumstances in which reordering definitely _could_ be a problem:
2357 2347
2358 (*) Interprocessor interaction. 2348 (*) Interprocessor interaction.
2359 2349
2360 (*) Atomic operations. 2350 (*) Atomic operations.
2361 2351
2362 (*) Accessing devices. 2352 (*) Accessing devices.
2363 2353
2364 (*) Interrupts. 2354 (*) Interrupts.
2365 2355
2366 2356
2367 INTERPROCESSOR INTERACTION 2357 INTERPROCESSOR INTERACTION
2368 -------------------------- 2358 --------------------------
2369 2359
2370 When there's a system with more than one proc 2360 When there's a system with more than one processor, more than one CPU in the
2371 system may be working on the same data set at 2361 system may be working on the same data set at the same time. This can cause
2372 synchronisation problems, and the usual way o 2362 synchronisation problems, and the usual way of dealing with them is to use
2373 locks. Locks, however, are quite expensive, 2363 locks. Locks, however, are quite expensive, and so it may be preferable to
2374 operate without the use of a lock if at all p 2364 operate without the use of a lock if at all possible. In such a case
2375 operations that affect both CPUs may have to 2365 operations that affect both CPUs may have to be carefully ordered to prevent
2376 a malfunction. 2366 a malfunction.
2377 2367
2378 Consider, for example, the R/W semaphore slow 2368 Consider, for example, the R/W semaphore slow path. Here a waiting process is
2379 queued on the semaphore, by virtue of it havi 2369 queued on the semaphore, by virtue of it having a piece of its stack linked to
2380 the semaphore's list of waiting processes: 2370 the semaphore's list of waiting processes:
2381 2371
2382 struct rw_semaphore { 2372 struct rw_semaphore {
2383 ... 2373 ...
2384 spinlock_t lock; 2374 spinlock_t lock;
2385 struct list_head waiters; 2375 struct list_head waiters;
2386 }; 2376 };
2387 2377
2388 struct rwsem_waiter { 2378 struct rwsem_waiter {
2389 struct list_head list; 2379 struct list_head list;
2390 struct task_struct *task; 2380 struct task_struct *task;
2391 }; 2381 };
2392 2382
2393 To wake up a particular waiter, the up_read() 2383 To wake up a particular waiter, the up_read() or up_write() functions have to:
2394 2384
2395 (1) read the next pointer from this waiter's 2385 (1) read the next pointer from this waiter's record to know as to where the
2396 next waiter record is; 2386 next waiter record is;
2397 2387
2398 (2) read the pointer to the waiter's task st 2388 (2) read the pointer to the waiter's task structure;
2399 2389
2400 (3) clear the task pointer to tell the waite 2390 (3) clear the task pointer to tell the waiter it has been given the semaphore;
2401 2391
2402 (4) call wake_up_process() on the task; and 2392 (4) call wake_up_process() on the task; and
2403 2393
2404 (5) release the reference held on the waiter 2394 (5) release the reference held on the waiter's task struct.
2405 2395
2406 In other words, it has to perform this sequen 2396 In other words, it has to perform this sequence of events:
2407 2397
2408 LOAD waiter->list.next; 2398 LOAD waiter->list.next;
2409 LOAD waiter->task; 2399 LOAD waiter->task;
2410 STORE waiter->task; 2400 STORE waiter->task;
2411 CALL wakeup 2401 CALL wakeup
2412 RELEASE task 2402 RELEASE task
2413 2403
2414 and if any of these steps occur out of order, 2404 and if any of these steps occur out of order, then the whole thing may
2415 malfunction. 2405 malfunction.
2416 2406
2417 Once it has queued itself and dropped the sem 2407 Once it has queued itself and dropped the semaphore lock, the waiter does not
2418 get the lock again; it instead just waits for 2408 get the lock again; it instead just waits for its task pointer to be cleared
2419 before proceeding. Since the record is on th 2409 before proceeding. Since the record is on the waiter's stack, this means that
2420 if the task pointer is cleared _before_ the n 2410 if the task pointer is cleared _before_ the next pointer in the list is read,
2421 another CPU might start processing the waiter 2411 another CPU might start processing the waiter and might clobber the waiter's
2422 stack before the up*() function has a chance 2412 stack before the up*() function has a chance to read the next pointer.
2423 2413
2424 Consider then what might happen to the above 2414 Consider then what might happen to the above sequence of events:
2425 2415
2426 CPU 1 CPU 2 2416 CPU 1 CPU 2
2427 =============================== ===== 2417 =============================== ===============================
2428 down_ 2418 down_xxx()
2429 Queue 2419 Queue waiter
2430 Sleep 2420 Sleep
2431 up_yyy() 2421 up_yyy()
2432 LOAD waiter->task; 2422 LOAD waiter->task;
2433 STORE waiter->task; 2423 STORE waiter->task;
2434 Woken 2424 Woken up by other event
2435 <preempt> 2425 <preempt>
2436 Resum 2426 Resume processing
2437 down_ 2427 down_xxx() returns
2438 call 2428 call foo()
2439 foo() 2429 foo() clobbers *waiter
2440 </preempt> 2430 </preempt>
2441 LOAD waiter->list.next; 2431 LOAD waiter->list.next;
2442 --- OOPS --- 2432 --- OOPS ---
2443 2433
2444 This could be dealt with using the semaphore 2434 This could be dealt with using the semaphore lock, but then the down_xxx()
2445 function has to needlessly get the spinlock a 2435 function has to needlessly get the spinlock again after being woken up.
2446 2436
2447 The way to deal with this is to insert a gene 2437 The way to deal with this is to insert a general SMP memory barrier:
2448 2438
2449 LOAD waiter->list.next; 2439 LOAD waiter->list.next;
2450 LOAD waiter->task; 2440 LOAD waiter->task;
2451 smp_mb(); 2441 smp_mb();
2452 STORE waiter->task; 2442 STORE waiter->task;
2453 CALL wakeup 2443 CALL wakeup
2454 RELEASE task 2444 RELEASE task
2455 2445
2456 In this case, the barrier makes a guarantee t 2446 In this case, the barrier makes a guarantee that all memory accesses before the
2457 barrier will appear to happen before all the 2447 barrier will appear to happen before all the memory accesses after the barrier
2458 with respect to the other CPUs on the system. 2448 with respect to the other CPUs on the system. It does _not_ guarantee that all
2459 the memory accesses before the barrier will b 2449 the memory accesses before the barrier will be complete by the time the barrier
2460 instruction itself is complete. 2450 instruction itself is complete.
2461 2451
2462 On a UP system - where this wouldn't be a pro 2452 On a UP system - where this wouldn't be a problem - the smp_mb() is just a
2463 compiler barrier, thus making sure the compil 2453 compiler barrier, thus making sure the compiler emits the instructions in the
2464 right order without actually intervening in t 2454 right order without actually intervening in the CPU. Since there's only one
2465 CPU, that CPU's dependency ordering logic wil 2455 CPU, that CPU's dependency ordering logic will take care of everything else.
2466 2456
2467 2457
2468 ATOMIC OPERATIONS 2458 ATOMIC OPERATIONS
2469 ----------------- 2459 -----------------
2470 2460
2471 While they are technically interprocessor int 2461 While they are technically interprocessor interaction considerations, atomic
2472 operations are noted specially as some of the 2462 operations are noted specially as some of them imply full memory barriers and
2473 some don't, but they're very heavily relied o 2463 some don't, but they're very heavily relied on as a group throughout the
2474 kernel. 2464 kernel.
2475 2465
2476 See Documentation/atomic_t.txt for more infor 2466 See Documentation/atomic_t.txt for more information.
2477 2467
2478 2468
2479 ACCESSING DEVICES 2469 ACCESSING DEVICES
2480 ----------------- 2470 -----------------
2481 2471
2482 Many devices can be memory mapped, and so app 2472 Many devices can be memory mapped, and so appear to the CPU as if they're just
2483 a set of memory locations. To control such a 2473 a set of memory locations. To control such a device, the driver usually has to
2484 make the right memory accesses in exactly the 2474 make the right memory accesses in exactly the right order.
2485 2475
2486 However, having a clever CPU or a clever comp 2476 However, having a clever CPU or a clever compiler creates a potential problem
2487 in that the carefully sequenced accesses in t 2477 in that the carefully sequenced accesses in the driver code won't reach the
2488 device in the requisite order if the CPU or t 2478 device in the requisite order if the CPU or the compiler thinks it is more
2489 efficient to reorder, combine or merge access 2479 efficient to reorder, combine or merge accesses - something that would cause
2490 the device to malfunction. 2480 the device to malfunction.
2491 2481
2492 Inside of the Linux kernel, I/O should be don 2482 Inside of the Linux kernel, I/O should be done through the appropriate accessor
2493 routines - such as inb() or writel() - which 2483 routines - such as inb() or writel() - which know how to make such accesses
2494 appropriately sequential. While this, for th 2484 appropriately sequential. While this, for the most part, renders the explicit
2495 use of memory barriers unnecessary, if the ac 2485 use of memory barriers unnecessary, if the accessor functions are used to refer
2496 to an I/O memory window with relaxed memory a 2486 to an I/O memory window with relaxed memory access properties, then _mandatory_
2497 memory barriers are required to enforce order 2487 memory barriers are required to enforce ordering.
2498 2488
2499 See Documentation/driver-api/device-io.rst fo 2489 See Documentation/driver-api/device-io.rst for more information.
2500 2490
2501 2491
2502 INTERRUPTS 2492 INTERRUPTS
2503 ---------- 2493 ----------
2504 2494
2505 A driver may be interrupted by its own interr 2495 A driver may be interrupted by its own interrupt service routine, and thus the
2506 two parts of the driver may interfere with ea 2496 two parts of the driver may interfere with each other's attempts to control or
2507 access the device. 2497 access the device.
2508 2498
2509 This may be alleviated - at least in part - b 2499 This may be alleviated - at least in part - by disabling local interrupts (a
2510 form of locking), such that the critical oper 2500 form of locking), such that the critical operations are all contained within
2511 the interrupt-disabled section in the driver. 2501 the interrupt-disabled section in the driver. While the driver's interrupt
2512 routine is executing, the driver's core may n 2502 routine is executing, the driver's core may not run on the same CPU, and its
2513 interrupt is not permitted to happen again un 2503 interrupt is not permitted to happen again until the current interrupt has been
2514 handled, thus the interrupt handler does not 2504 handled, thus the interrupt handler does not need to lock against that.
2515 2505
2516 However, consider a driver that was talking t 2506 However, consider a driver that was talking to an ethernet card that sports an
2517 address register and a data register. If tha 2507 address register and a data register. If that driver's core talks to the card
2518 under interrupt-disablement and then the driv 2508 under interrupt-disablement and then the driver's interrupt handler is invoked:
2519 2509
2520 LOCAL IRQ DISABLE 2510 LOCAL IRQ DISABLE
2521 writew(ADDR, 3); 2511 writew(ADDR, 3);
2522 writew(DATA, y); 2512 writew(DATA, y);
2523 LOCAL IRQ ENABLE 2513 LOCAL IRQ ENABLE
2524 <interrupt> 2514 <interrupt>
2525 writew(ADDR, 4); 2515 writew(ADDR, 4);
2526 q = readw(DATA); 2516 q = readw(DATA);
2527 </interrupt> 2517 </interrupt>
2528 2518
2529 The store to the data register might happen a 2519 The store to the data register might happen after the second store to the
2530 address register if ordering rules are suffic 2520 address register if ordering rules are sufficiently relaxed:
2531 2521
2532 STORE *ADDR = 3, STORE *ADDR = 4, STO 2522 STORE *ADDR = 3, STORE *ADDR = 4, STORE *DATA = y, q = LOAD *DATA
2533 2523
2534 2524
2535 If ordering rules are relaxed, it must be ass 2525 If ordering rules are relaxed, it must be assumed that accesses done inside an
2536 interrupt disabled section may leak outside o 2526 interrupt disabled section may leak outside of it and may interleave with
2537 accesses performed in an interrupt - and vice 2527 accesses performed in an interrupt - and vice versa - unless implicit or
2538 explicit barriers are used. 2528 explicit barriers are used.
2539 2529
2540 Normally this won't be a problem because the 2530 Normally this won't be a problem because the I/O accesses done inside such
2541 sections will include synchronous load operat 2531 sections will include synchronous load operations on strictly ordered I/O
2542 registers that form implicit I/O barriers. 2532 registers that form implicit I/O barriers.
2543 2533
2544 2534
2545 A similar situation may occur between an inte 2535 A similar situation may occur between an interrupt routine and two routines
2546 running on separate CPUs that communicate wit 2536 running on separate CPUs that communicate with each other. If such a case is
2547 likely, then interrupt-disabling locks should 2537 likely, then interrupt-disabling locks should be used to guarantee ordering.
2548 2538
2549 2539
2550 ========================== 2540 ==========================
2551 KERNEL I/O BARRIER EFFECTS 2541 KERNEL I/O BARRIER EFFECTS
2552 ========================== 2542 ==========================
2553 2543
2554 Interfacing with peripherals via I/O accesses 2544 Interfacing with peripherals via I/O accesses is deeply architecture and device
2555 specific. Therefore, drivers which are inhere 2545 specific. Therefore, drivers which are inherently non-portable may rely on
2556 specific behaviours of their target systems i 2546 specific behaviours of their target systems in order to achieve synchronization
2557 in the most lightweight manner possible. For 2547 in the most lightweight manner possible. For drivers intending to be portable
2558 between multiple architectures and bus implem 2548 between multiple architectures and bus implementations, the kernel offers a
2559 series of accessor functions that provide var 2549 series of accessor functions that provide various degrees of ordering
2560 guarantees: 2550 guarantees:
2561 2551
2562 (*) readX(), writeX(): 2552 (*) readX(), writeX():
2563 2553
2564 The readX() and writeX() MMIO accesso 2554 The readX() and writeX() MMIO accessors take a pointer to the
2565 peripheral being accessed as an __iom 2555 peripheral being accessed as an __iomem * parameter. For pointers
2566 mapped with the default I/O attribute 2556 mapped with the default I/O attributes (e.g. those returned by
2567 ioremap()), the ordering guarantees a 2557 ioremap()), the ordering guarantees are as follows:
2568 2558
2569 1. All readX() and writeX() accesses 2559 1. All readX() and writeX() accesses to the same peripheral are ordered
2570 with respect to each other. This e 2560 with respect to each other. This ensures that MMIO register accesses
2571 by the same CPU thread to a partic 2561 by the same CPU thread to a particular device will arrive in program
2572 order. 2562 order.
2573 2563
2574 2. A writeX() issued by a CPU thread 2564 2. A writeX() issued by a CPU thread holding a spinlock is ordered
2575 before a writeX() to the same peri 2565 before a writeX() to the same peripheral from another CPU thread
2576 issued after a later acquisition o 2566 issued after a later acquisition of the same spinlock. This ensures
2577 that MMIO register writes to a par 2567 that MMIO register writes to a particular device issued while holding
2578 a spinlock will arrive in an order 2568 a spinlock will arrive in an order consistent with acquisitions of
2579 the lock. 2569 the lock.
2580 2570
2581 3. A writeX() by a CPU thread to the 2571 3. A writeX() by a CPU thread to the peripheral will first wait for the
2582 completion of all prior writes to 2572 completion of all prior writes to memory either issued by, or
2583 propagated to, the same thread. Th 2573 propagated to, the same thread. This ensures that writes by the CPU
2584 to an outbound DMA buffer allocate 2574 to an outbound DMA buffer allocated by dma_alloc_coherent() will be
2585 visible to a DMA engine when the C 2575 visible to a DMA engine when the CPU writes to its MMIO control
2586 register to trigger the transfer. 2576 register to trigger the transfer.
2587 2577
2588 4. A readX() by a CPU thread from the 2578 4. A readX() by a CPU thread from the peripheral will complete before
2589 any subsequent reads from memory b 2579 any subsequent reads from memory by the same thread can begin. This
2590 ensures that reads by the CPU from 2580 ensures that reads by the CPU from an incoming DMA buffer allocated
2591 by dma_alloc_coherent() will not s 2581 by dma_alloc_coherent() will not see stale data after reading from
2592 the DMA engine's MMIO status regis 2582 the DMA engine's MMIO status register to establish that the DMA
2593 transfer has completed. 2583 transfer has completed.
2594 2584
2595 5. A readX() by a CPU thread from the 2585 5. A readX() by a CPU thread from the peripheral will complete before
2596 any subsequent delay() loop can be 2586 any subsequent delay() loop can begin execution on the same thread.
2597 This ensures that two MMIO registe 2587 This ensures that two MMIO register writes by the CPU to a peripheral
2598 will arrive at least 1us apart if 2588 will arrive at least 1us apart if the first write is immediately read
2599 back with readX() and udelay(1) is 2589 back with readX() and udelay(1) is called prior to the second
2600 writeX(): 2590 writeX():
2601 2591
2602 writel(42, DEVICE_REGISTER_0) 2592 writel(42, DEVICE_REGISTER_0); // Arrives at the device...
2603 readl(DEVICE_REGISTER_0); 2593 readl(DEVICE_REGISTER_0);
2604 udelay(1); 2594 udelay(1);
2605 writel(42, DEVICE_REGISTER_1) 2595 writel(42, DEVICE_REGISTER_1); // ...at least 1us before this.
2606 2596
2607 The ordering properties of __iomem po 2597 The ordering properties of __iomem pointers obtained with non-default
2608 attributes (e.g. those returned by io 2598 attributes (e.g. those returned by ioremap_wc()) are specific to the
2609 underlying architecture and therefore 2599 underlying architecture and therefore the guarantees listed above cannot
2610 generally be relied upon for accesses 2600 generally be relied upon for accesses to these types of mappings.
2611 2601
2612 (*) readX_relaxed(), writeX_relaxed(): 2602 (*) readX_relaxed(), writeX_relaxed():
2613 2603
2614 These are similar to readX() and writ 2604 These are similar to readX() and writeX(), but provide weaker memory
2615 ordering guarantees. Specifically, th 2605 ordering guarantees. Specifically, they do not guarantee ordering with
2616 respect to locking, normal memory acc 2606 respect to locking, normal memory accesses or delay() loops (i.e.
2617 bullets 2-5 above) but they are still 2607 bullets 2-5 above) but they are still guaranteed to be ordered with
2618 respect to other accesses from the sa 2608 respect to other accesses from the same CPU thread to the same
2619 peripheral when operating on __iomem 2609 peripheral when operating on __iomem pointers mapped with the default
2620 I/O attributes. 2610 I/O attributes.
2621 2611
2622 (*) readsX(), writesX(): 2612 (*) readsX(), writesX():
2623 2613
2624 The readsX() and writesX() MMIO acces 2614 The readsX() and writesX() MMIO accessors are designed for accessing
2625 register-based, memory-mapped FIFOs r 2615 register-based, memory-mapped FIFOs residing on peripherals that are not
2626 capable of performing DMA. Consequent 2616 capable of performing DMA. Consequently, they provide only the ordering
2627 guarantees of readX_relaxed() and wri 2617 guarantees of readX_relaxed() and writeX_relaxed(), as documented above.
2628 2618
2629 (*) inX(), outX(): 2619 (*) inX(), outX():
2630 2620
2631 The inX() and outX() accessors are in 2621 The inX() and outX() accessors are intended to access legacy port-mapped
2632 I/O peripherals, which may require sp 2622 I/O peripherals, which may require special instructions on some
2633 architectures (notably x86). The port 2623 architectures (notably x86). The port number of the peripheral being
2634 accessed is passed as an argument. 2624 accessed is passed as an argument.
2635 2625
2636 Since many CPU architectures ultimate 2626 Since many CPU architectures ultimately access these peripherals via an
2637 internal virtual memory mapping, the 2627 internal virtual memory mapping, the portable ordering guarantees
2638 provided by inX() and outX() are the 2628 provided by inX() and outX() are the same as those provided by readX()
2639 and writeX() respectively when access 2629 and writeX() respectively when accessing a mapping with the default I/O
2640 attributes. 2630 attributes.
2641 2631
2642 Device drivers may expect outX() to e 2632 Device drivers may expect outX() to emit a non-posted write transaction
2643 that waits for a completion response 2633 that waits for a completion response from the I/O peripheral before
2644 returning. This is not guaranteed by 2634 returning. This is not guaranteed by all architectures and is therefore
2645 not part of the portable ordering sem 2635 not part of the portable ordering semantics.
2646 2636
2647 (*) insX(), outsX(): 2637 (*) insX(), outsX():
2648 2638
2649 As above, the insX() and outsX() acce 2639 As above, the insX() and outsX() accessors provide the same ordering
2650 guarantees as readsX() and writesX() 2640 guarantees as readsX() and writesX() respectively when accessing a
2651 mapping with the default I/O attribut 2641 mapping with the default I/O attributes.
2652 2642
2653 (*) ioreadX(), iowriteX(): 2643 (*) ioreadX(), iowriteX():
2654 2644
2655 These will perform appropriately for 2645 These will perform appropriately for the type of access they're actually
2656 doing, be it inX()/outX() or readX()/ 2646 doing, be it inX()/outX() or readX()/writeX().
2657 2647
2658 With the exception of the string accessors (i 2648 With the exception of the string accessors (insX(), outsX(), readsX() and
2659 writesX()), all of the above assume that the 2649 writesX()), all of the above assume that the underlying peripheral is
2660 little-endian and will therefore perform byte 2650 little-endian and will therefore perform byte-swapping operations on big-endian
2661 architectures. 2651 architectures.
2662 2652
2663 2653
2664 ======================================== 2654 ========================================
2665 ASSUMED MINIMUM EXECUTION ORDERING MODEL 2655 ASSUMED MINIMUM EXECUTION ORDERING MODEL
2666 ======================================== 2656 ========================================
2667 2657
2668 It has to be assumed that the conceptual CPU 2658 It has to be assumed that the conceptual CPU is weakly-ordered but that it will
2669 maintain the appearance of program causality 2659 maintain the appearance of program causality with respect to itself. Some CPUs
2670 (such as i386 or x86_64) are more constrained 2660 (such as i386 or x86_64) are more constrained than others (such as powerpc or
2671 frv), and so the most relaxed case (namely DE 2661 frv), and so the most relaxed case (namely DEC Alpha) must be assumed outside
2672 of arch-specific code. 2662 of arch-specific code.
2673 2663
2674 This means that it must be considered that th 2664 This means that it must be considered that the CPU will execute its instruction
2675 stream in any order it feels like - or even i 2665 stream in any order it feels like - or even in parallel - provided that if an
2676 instruction in the stream depends on an earli 2666 instruction in the stream depends on an earlier instruction, then that
2677 earlier instruction must be sufficiently comp 2667 earlier instruction must be sufficiently complete[*] before the later
2678 instruction may proceed; in other words: prov 2668 instruction may proceed; in other words: provided that the appearance of
2679 causality is maintained. 2669 causality is maintained.
2680 2670
2681 [*] Some instructions have more than one eff 2671 [*] Some instructions have more than one effect - such as changing the
2682 condition codes, changing registers or c 2672 condition codes, changing registers or changing memory - and different
2683 instructions may depend on different eff 2673 instructions may depend on different effects.
2684 2674
2685 A CPU may also discard any instruction sequen 2675 A CPU may also discard any instruction sequence that winds up having no
2686 ultimate effect. For example, if two adjacen 2676 ultimate effect. For example, if two adjacent instructions both load an
2687 immediate value into the same register, the f 2677 immediate value into the same register, the first may be discarded.
2688 2678
2689 2679
2690 Similarly, it has to be assumed that compiler 2680 Similarly, it has to be assumed that compiler might reorder the instruction
2691 stream in any way it sees fit, again provided 2681 stream in any way it sees fit, again provided the appearance of causality is
2692 maintained. 2682 maintained.
2693 2683
2694 2684
2695 ============================ 2685 ============================
2696 THE EFFECTS OF THE CPU CACHE 2686 THE EFFECTS OF THE CPU CACHE
2697 ============================ 2687 ============================
2698 2688
2699 The way cached memory operations are perceive 2689 The way cached memory operations are perceived across the system is affected to
2700 a certain extent by the caches that lie betwe 2690 a certain extent by the caches that lie between CPUs and memory, and by the
2701 memory coherence system that maintains the co 2691 memory coherence system that maintains the consistency of state in the system.
2702 2692
2703 As far as the way a CPU interacts with anothe 2693 As far as the way a CPU interacts with another part of the system through the
2704 caches goes, the memory system has to include 2694 caches goes, the memory system has to include the CPU's caches, and memory
2705 barriers for the most part act at the interfa 2695 barriers for the most part act at the interface between the CPU and its cache
2706 (memory barriers logically act on the dotted 2696 (memory barriers logically act on the dotted line in the following diagram):
2707 2697
2708 <--- CPU ---> : <-- 2698 <--- CPU ---> : <----------- Memory ----------->
2709 : 2699 :
2710 +--------+ +--------+ : +------ 2700 +--------+ +--------+ : +--------+ +-----------+
2711 | | | | : | 2701 | | | | : | | | | +--------+
2712 | CPU | | Memory | : | CPU 2702 | CPU | | Memory | : | CPU | | | | |
2713 | Core |--->| Access |----->| Cache 2703 | Core |--->| Access |----->| Cache |<-->| | | |
2714 | | | Queue | : | 2704 | | | Queue | : | | | |--->| Memory |
2715 | | | | : | 2705 | | | | : | | | | | |
2716 +--------+ +--------+ : +------ 2706 +--------+ +--------+ : +--------+ | | | |
2717 : 2707 : | Cache | +--------+
2718 : 2708 : | Coherency |
2719 : 2709 : | Mechanism | +--------+
2720 +--------+ +--------+ : +------ 2710 +--------+ +--------+ : +--------+ | | | |
2721 | | | | : | 2711 | | | | : | | | | | |
2722 | CPU | | Memory | : | CPU 2712 | CPU | | Memory | : | CPU | | |--->| Device |
2723 | Core |--->| Access |----->| Cache 2713 | Core |--->| Access |----->| Cache |<-->| | | |
2724 | | | Queue | : | 2714 | | | Queue | : | | | | | |
2725 | | | | : | 2715 | | | | : | | | | +--------+
2726 +--------+ +--------+ : +------ 2716 +--------+ +--------+ : +--------+ +-----------+
2727 : 2717 :
2728 : 2718 :
2729 2719
2730 Although any particular load or store may not 2720 Although any particular load or store may not actually appear outside of the
2731 CPU that issued it since it may have been sat 2721 CPU that issued it since it may have been satisfied within the CPU's own cache,
2732 it will still appear as if the full memory ac 2722 it will still appear as if the full memory access had taken place as far as the
2733 other CPUs are concerned since the cache cohe 2723 other CPUs are concerned since the cache coherency mechanisms will migrate the
2734 cacheline over to the accessing CPU and propa 2724 cacheline over to the accessing CPU and propagate the effects upon conflict.
2735 2725
2736 The CPU core may execute instructions in any 2726 The CPU core may execute instructions in any order it deems fit, provided the
2737 expected program causality appears to be main 2727 expected program causality appears to be maintained. Some of the instructions
2738 generate load and store operations which then 2728 generate load and store operations which then go into the queue of memory
2739 accesses to be performed. The core may place 2729 accesses to be performed. The core may place these in the queue in any order
2740 it wishes, and continue execution until it is 2730 it wishes, and continue execution until it is forced to wait for an instruction
2741 to complete. 2731 to complete.
2742 2732
2743 What memory barriers are concerned with is co 2733 What memory barriers are concerned with is controlling the order in which
2744 accesses cross from the CPU side of things to 2734 accesses cross from the CPU side of things to the memory side of things, and
2745 the order in which the effects are perceived 2735 the order in which the effects are perceived to happen by the other observers
2746 in the system. 2736 in the system.
2747 2737
2748 [!] Memory barriers are _not_ needed within a 2738 [!] Memory barriers are _not_ needed within a given CPU, as CPUs always see
2749 their own loads and stores as if they had hap 2739 their own loads and stores as if they had happened in program order.
2750 2740
2751 [!] MMIO or other device accesses may bypass 2741 [!] MMIO or other device accesses may bypass the cache system. This depends on
2752 the properties of the memory window through w 2742 the properties of the memory window through which devices are accessed and/or
2753 the use of any special device communication i 2743 the use of any special device communication instructions the CPU may have.
2754 2744
2755 2745
2756 CACHE COHERENCY VS DMA 2746 CACHE COHERENCY VS DMA
2757 ---------------------- 2747 ----------------------
2758 2748
2759 Not all systems maintain cache coherency with 2749 Not all systems maintain cache coherency with respect to devices doing DMA. In
2760 such cases, a device attempting DMA may obtai 2750 such cases, a device attempting DMA may obtain stale data from RAM because
2761 dirty cache lines may be resident in the cach 2751 dirty cache lines may be resident in the caches of various CPUs, and may not
2762 have been written back to RAM yet. To deal w 2752 have been written back to RAM yet. To deal with this, the appropriate part of
2763 the kernel must flush the overlapping bits of 2753 the kernel must flush the overlapping bits of cache on each CPU (and maybe
2764 invalidate them as well). 2754 invalidate them as well).
2765 2755
2766 In addition, the data DMA'd to RAM by a devic 2756 In addition, the data DMA'd to RAM by a device may be overwritten by dirty
2767 cache lines being written back to RAM from a 2757 cache lines being written back to RAM from a CPU's cache after the device has
2768 installed its own data, or cache lines presen 2758 installed its own data, or cache lines present in the CPU's cache may simply
2769 obscure the fact that RAM has been updated, u 2759 obscure the fact that RAM has been updated, until at such time as the cacheline
2770 is discarded from the CPU's cache and reloade 2760 is discarded from the CPU's cache and reloaded. To deal with this, the
2771 appropriate part of the kernel must invalidat 2761 appropriate part of the kernel must invalidate the overlapping bits of the
2772 cache on each CPU. 2762 cache on each CPU.
2773 2763
2774 See Documentation/core-api/cachetlb.rst for m 2764 See Documentation/core-api/cachetlb.rst for more information on cache
2775 management. 2765 management.
2776 2766
2777 2767
2778 CACHE COHERENCY VS MMIO 2768 CACHE COHERENCY VS MMIO
2779 ----------------------- 2769 -----------------------
2780 2770
2781 Memory mapped I/O usually takes place through 2771 Memory mapped I/O usually takes place through memory locations that are part of
2782 a window in the CPU's memory space that has d 2772 a window in the CPU's memory space that has different properties assigned than
2783 the usual RAM directed window. 2773 the usual RAM directed window.
2784 2774
2785 Amongst these properties is usually the fact 2775 Amongst these properties is usually the fact that such accesses bypass the
2786 caching entirely and go directly to the devic 2776 caching entirely and go directly to the device buses. This means MMIO accesses
2787 may, in effect, overtake accesses to cached m 2777 may, in effect, overtake accesses to cached memory that were emitted earlier.
2788 A memory barrier isn't sufficient in such a c 2778 A memory barrier isn't sufficient in such a case, but rather the cache must be
2789 flushed between the cached memory write and t 2779 flushed between the cached memory write and the MMIO access if the two are in
2790 any way dependent. 2780 any way dependent.
2791 2781
2792 2782
2793 ========================= 2783 =========================
2794 THE THINGS CPUS GET UP TO 2784 THE THINGS CPUS GET UP TO
2795 ========================= 2785 =========================
2796 2786
2797 A programmer might take it for granted that t 2787 A programmer might take it for granted that the CPU will perform memory
2798 operations in exactly the order specified, so 2788 operations in exactly the order specified, so that if the CPU is, for example,
2799 given the following piece of code to execute: 2789 given the following piece of code to execute:
2800 2790
2801 a = READ_ONCE(*A); 2791 a = READ_ONCE(*A);
2802 WRITE_ONCE(*B, b); 2792 WRITE_ONCE(*B, b);
2803 c = READ_ONCE(*C); 2793 c = READ_ONCE(*C);
2804 d = READ_ONCE(*D); 2794 d = READ_ONCE(*D);
2805 WRITE_ONCE(*E, e); 2795 WRITE_ONCE(*E, e);
2806 2796
2807 they would then expect that the CPU will comp 2797 they would then expect that the CPU will complete the memory operation for each
2808 instruction before moving on to the next one, 2798 instruction before moving on to the next one, leading to a definite sequence of
2809 operations as seen by external observers in t 2799 operations as seen by external observers in the system:
2810 2800
2811 LOAD *A, STORE *B, LOAD *C, LOAD *D, 2801 LOAD *A, STORE *B, LOAD *C, LOAD *D, STORE *E.
2812 2802
2813 2803
2814 Reality is, of course, much messier. With ma 2804 Reality is, of course, much messier. With many CPUs and compilers, the above
2815 assumption doesn't hold because: 2805 assumption doesn't hold because:
2816 2806
2817 (*) loads are more likely to need to be comp 2807 (*) loads are more likely to need to be completed immediately to permit
2818 execution progress, whereas stores can o 2808 execution progress, whereas stores can often be deferred without a
2819 problem; 2809 problem;
2820 2810
2821 (*) loads may be done speculatively, and the 2811 (*) loads may be done speculatively, and the result discarded should it prove
2822 to have been unnecessary; 2812 to have been unnecessary;
2823 2813
2824 (*) loads may be done speculatively, leading 2814 (*) loads may be done speculatively, leading to the result having been fetched
2825 at the wrong time in the expected sequen 2815 at the wrong time in the expected sequence of events;
2826 2816
2827 (*) the order of the memory accesses may be 2817 (*) the order of the memory accesses may be rearranged to promote better use
2828 of the CPU buses and caches; 2818 of the CPU buses and caches;
2829 2819
2830 (*) loads and stores may be combined to impr 2820 (*) loads and stores may be combined to improve performance when talking to
2831 memory or I/O hardware that can do batch 2821 memory or I/O hardware that can do batched accesses of adjacent locations,
2832 thus cutting down on transaction setup c 2822 thus cutting down on transaction setup costs (memory and PCI devices may
2833 both be able to do this); and 2823 both be able to do this); and
2834 2824
2835 (*) the CPU's data cache may affect the orde 2825 (*) the CPU's data cache may affect the ordering, and while cache-coherency
2836 mechanisms may alleviate this - once the 2826 mechanisms may alleviate this - once the store has actually hit the cache
2837 - there's no guarantee that the coherenc 2827 - there's no guarantee that the coherency management will be propagated in
2838 order to other CPUs. 2828 order to other CPUs.
2839 2829
2840 So what another CPU, say, might actually obse 2830 So what another CPU, say, might actually observe from the above piece of code
2841 is: 2831 is:
2842 2832
2843 LOAD *A, ..., LOAD {*C,*D}, STORE *E, 2833 LOAD *A, ..., LOAD {*C,*D}, STORE *E, STORE *B
2844 2834
2845 (Where "LOAD {*C,*D}" is a combined l 2835 (Where "LOAD {*C,*D}" is a combined load)
2846 2836
2847 2837
2848 However, it is guaranteed that a CPU will be 2838 However, it is guaranteed that a CPU will be self-consistent: it will see its
2849 _own_ accesses appear to be correctly ordered 2839 _own_ accesses appear to be correctly ordered, without the need for a memory
2850 barrier. For instance with the following cod 2840 barrier. For instance with the following code:
2851 2841
2852 U = READ_ONCE(*A); 2842 U = READ_ONCE(*A);
2853 WRITE_ONCE(*A, V); 2843 WRITE_ONCE(*A, V);
2854 WRITE_ONCE(*A, W); 2844 WRITE_ONCE(*A, W);
2855 X = READ_ONCE(*A); 2845 X = READ_ONCE(*A);
2856 WRITE_ONCE(*A, Y); 2846 WRITE_ONCE(*A, Y);
2857 Z = READ_ONCE(*A); 2847 Z = READ_ONCE(*A);
2858 2848
2859 and assuming no intervention by an external i 2849 and assuming no intervention by an external influence, it can be assumed that
2860 the final result will appear to be: 2850 the final result will appear to be:
2861 2851
2862 U == the original value of *A 2852 U == the original value of *A
2863 X == W 2853 X == W
2864 Z == Y 2854 Z == Y
2865 *A == Y 2855 *A == Y
2866 2856
2867 The code above may cause the CPU to generate 2857 The code above may cause the CPU to generate the full sequence of memory
2868 accesses: 2858 accesses:
2869 2859
2870 U=LOAD *A, STORE *A=V, STORE *A=W, X= 2860 U=LOAD *A, STORE *A=V, STORE *A=W, X=LOAD *A, STORE *A=Y, Z=LOAD *A
2871 2861
2872 in that order, but, without intervention, the 2862 in that order, but, without intervention, the sequence may have almost any
2873 combination of elements combined or discarded 2863 combination of elements combined or discarded, provided the program's view
2874 of the world remains consistent. Note that R 2864 of the world remains consistent. Note that READ_ONCE() and WRITE_ONCE()
2875 are -not- optional in the above example, as t 2865 are -not- optional in the above example, as there are architectures
2876 where a given CPU might reorder successive lo 2866 where a given CPU might reorder successive loads to the same location.
2877 On such architectures, READ_ONCE() and WRITE_ 2867 On such architectures, READ_ONCE() and WRITE_ONCE() do whatever is
2878 necessary to prevent this, for example, on It 2868 necessary to prevent this, for example, on Itanium the volatile casts
2879 used by READ_ONCE() and WRITE_ONCE() cause GC 2869 used by READ_ONCE() and WRITE_ONCE() cause GCC to emit the special ld.acq
2880 and st.rel instructions (respectively) that p 2870 and st.rel instructions (respectively) that prevent such reordering.
2881 2871
2882 The compiler may also combine, discard or def 2872 The compiler may also combine, discard or defer elements of the sequence before
2883 the CPU even sees them. 2873 the CPU even sees them.
2884 2874
2885 For instance: 2875 For instance:
2886 2876
2887 *A = V; 2877 *A = V;
2888 *A = W; 2878 *A = W;
2889 2879
2890 may be reduced to: 2880 may be reduced to:
2891 2881
2892 *A = W; 2882 *A = W;
2893 2883
2894 since, without either a write barrier or an W 2884 since, without either a write barrier or an WRITE_ONCE(), it can be
2895 assumed that the effect of the storage of V t 2885 assumed that the effect of the storage of V to *A is lost. Similarly:
2896 2886
2897 *A = Y; 2887 *A = Y;
2898 Z = *A; 2888 Z = *A;
2899 2889
2900 may, without a memory barrier or an READ_ONCE 2890 may, without a memory barrier or an READ_ONCE() and WRITE_ONCE(), be
2901 reduced to: 2891 reduced to:
2902 2892
2903 *A = Y; 2893 *A = Y;
2904 Z = Y; 2894 Z = Y;
2905 2895
2906 and the LOAD operation never appear outside o 2896 and the LOAD operation never appear outside of the CPU.
2907 2897
2908 2898
2909 AND THEN THERE'S THE ALPHA 2899 AND THEN THERE'S THE ALPHA
2910 -------------------------- 2900 --------------------------
2911 2901
2912 The DEC Alpha CPU is one of the most relaxed 2902 The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that,
2913 some versions of the Alpha CPU have a split d 2903 some versions of the Alpha CPU have a split data cache, permitting them to have
2914 two semantically-related cache lines updated 2904 two semantically-related cache lines updated at separate times. This is where
2915 the address-dependency barrier really becomes 2905 the address-dependency barrier really becomes necessary as this synchronises
2916 both caches with the memory coherence system, 2906 both caches with the memory coherence system, thus making it seem like pointer
2917 changes vs new data occur in the right order. 2907 changes vs new data occur in the right order.
2918 2908
2919 The Alpha defines the Linux kernel's memory m 2909 The Alpha defines the Linux kernel's memory model, although as of v4.15
2920 the Linux kernel's addition of smp_mb() to RE 2910 the Linux kernel's addition of smp_mb() to READ_ONCE() on Alpha greatly
2921 reduced its impact on the memory model. 2911 reduced its impact on the memory model.
2922 2912
2923 2913
2924 VIRTUAL MACHINE GUESTS 2914 VIRTUAL MACHINE GUESTS
2925 ---------------------- 2915 ----------------------
2926 2916
2927 Guests running within virtual machines might 2917 Guests running within virtual machines might be affected by SMP effects even if
2928 the guest itself is compiled without SMP supp 2918 the guest itself is compiled without SMP support. This is an artifact of
2929 interfacing with an SMP host while running an 2919 interfacing with an SMP host while running an UP kernel. Using mandatory
2930 barriers for this use-case would be possible 2920 barriers for this use-case would be possible but is often suboptimal.
2931 2921
2932 To handle this case optimally, low-level virt 2922 To handle this case optimally, low-level virt_mb() etc macros are available.
2933 These have the same effect as smp_mb() etc wh 2923 These have the same effect as smp_mb() etc when SMP is enabled, but generate
2934 identical code for SMP and non-SMP systems. 2924 identical code for SMP and non-SMP systems. For example, virtual machine guests
2935 should use virt_mb() rather than smp_mb() whe 2925 should use virt_mb() rather than smp_mb() when synchronizing against a
2936 (possibly SMP) host. 2926 (possibly SMP) host.
2937 2927
2938 These are equivalent to smp_mb() etc counterp 2928 These are equivalent to smp_mb() etc counterparts in all other respects,
2939 in particular, they do not control MMIO effec 2929 in particular, they do not control MMIO effects: to control
2940 MMIO effects, use mandatory barriers. 2930 MMIO effects, use mandatory barriers.
2941 2931
2942 2932
2943 ============ 2933 ============
2944 EXAMPLE USES 2934 EXAMPLE USES
2945 ============ 2935 ============
2946 2936
2947 CIRCULAR BUFFERS 2937 CIRCULAR BUFFERS
2948 ---------------- 2938 ----------------
2949 2939
2950 Memory barriers can be used to implement circ 2940 Memory barriers can be used to implement circular buffering without the need
2951 of a lock to serialise the producer with the 2941 of a lock to serialise the producer with the consumer. See:
2952 2942
2953 Documentation/core-api/circular-buffe 2943 Documentation/core-api/circular-buffers.rst
2954 2944
2955 for details. 2945 for details.
2956 2946
2957 2947
2958 ========== 2948 ==========
2959 REFERENCES 2949 REFERENCES
2960 ========== 2950 ==========
2961 2951
2962 Alpha AXP Architecture Reference Manual, Seco 2952 Alpha AXP Architecture Reference Manual, Second Edition (Sites & Witek,
2963 Digital Press) 2953 Digital Press)
2964 Chapter 5.2: Physical Address Space C 2954 Chapter 5.2: Physical Address Space Characteristics
2965 Chapter 5.4: Caches and Write Buffers 2955 Chapter 5.4: Caches and Write Buffers
2966 Chapter 5.5: Data Sharing 2956 Chapter 5.5: Data Sharing
2967 Chapter 5.6: Read/Write Ordering 2957 Chapter 5.6: Read/Write Ordering
2968 2958
2969 AMD64 Architecture Programmer's Manual Volume 2959 AMD64 Architecture Programmer's Manual Volume 2: System Programming
2970 Chapter 7.1: Memory-Access Ordering 2960 Chapter 7.1: Memory-Access Ordering
2971 Chapter 7.4: Buffering and Combining 2961 Chapter 7.4: Buffering and Combining Memory Writes
2972 2962
2973 ARM Architecture Reference Manual (ARMv8, for 2963 ARM Architecture Reference Manual (ARMv8, for ARMv8-A architecture profile)
2974 Chapter B2: The AArch64 Application L 2964 Chapter B2: The AArch64 Application Level Memory Model
2975 2965
2976 IA-32 Intel Architecture Software Developer's 2966 IA-32 Intel Architecture Software Developer's Manual, Volume 3:
2977 System Programming Guide 2967 System Programming Guide
2978 Chapter 7.1: Locked Atomic Operations 2968 Chapter 7.1: Locked Atomic Operations
2979 Chapter 7.2: Memory Ordering 2969 Chapter 7.2: Memory Ordering
2980 Chapter 7.4: Serializing Instructions 2970 Chapter 7.4: Serializing Instructions
2981 2971
2982 The SPARC Architecture Manual, Version 9 2972 The SPARC Architecture Manual, Version 9
2983 Chapter 8: Memory Models 2973 Chapter 8: Memory Models
2984 Appendix D: Formal Specification of t 2974 Appendix D: Formal Specification of the Memory Models
2985 Appendix J: Programming with the Memo 2975 Appendix J: Programming with the Memory Models
2986 2976
2987 Storage in the PowerPC (Stone and Fitzgerald) 2977 Storage in the PowerPC (Stone and Fitzgerald)
2988 2978
2989 UltraSPARC Programmer Reference Manual 2979 UltraSPARC Programmer Reference Manual
2990 Chapter 5: Memory Accesses and Cachea 2980 Chapter 5: Memory Accesses and Cacheability
2991 Chapter 15: Sparc-V9 Memory Models 2981 Chapter 15: Sparc-V9 Memory Models
2992 2982
2993 UltraSPARC III Cu User's Manual 2983 UltraSPARC III Cu User's Manual
2994 Chapter 9: Memory Models 2984 Chapter 9: Memory Models
2995 2985
2996 UltraSPARC IIIi Processor User's Manual 2986 UltraSPARC IIIi Processor User's Manual
2997 Chapter 8: Memory Models 2987 Chapter 8: Memory Models
2998 2988
2999 UltraSPARC Architecture 2005 2989 UltraSPARC Architecture 2005
3000 Chapter 9: Memory 2990 Chapter 9: Memory
3001 Appendix D: Formal Specifications of 2991 Appendix D: Formal Specifications of the Memory Models
3002 2992
3003 UltraSPARC T1 Supplement to the UltraSPARC Ar 2993 UltraSPARC T1 Supplement to the UltraSPARC Architecture 2005
3004 Chapter 8: Memory Models 2994 Chapter 8: Memory Models
3005 Appendix F: Caches and Cache Coherenc 2995 Appendix F: Caches and Cache Coherency
3006 2996
3007 Solaris Internals, Core Kernel Architecture, 2997 Solaris Internals, Core Kernel Architecture, p63-68:
3008 Chapter 3.3: Hardware Considerations 2998 Chapter 3.3: Hardware Considerations for Locks and
3009 Synchronization 2999 Synchronization
3010 3000
3011 Unix Systems for Modern Architectures, Symmet 3001 Unix Systems for Modern Architectures, Symmetric Multiprocessing and Caching
3012 for Kernel Programmers: 3002 for Kernel Programmers:
3013 Chapter 13: Other Memory Models 3003 Chapter 13: Other Memory Models
3014 3004
3015 Intel Itanium Architecture Software Developer 3005 Intel Itanium Architecture Software Developer's Manual: Volume 1:
3016 Section 2.6: Speculation 3006 Section 2.6: Speculation
3017 Section 4.4: Memory Access 3007 Section 4.4: Memory Access
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