1 Explanation of the Linux-Kernel Memory Consist 1 Explanation of the Linux-Kernel Memory Consistency Model
2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3 3
4 :Author: Alan Stern <stern@rowland.harvard.edu> 4 :Author: Alan Stern <stern@rowland.harvard.edu>
5 :Created: October 2017 5 :Created: October 2017
6 6
7 .. Contents 7 .. Contents
8 8
9 1. INTRODUCTION 9 1. INTRODUCTION
10 2. BACKGROUND 10 2. BACKGROUND
11 3. A SIMPLE EXAMPLE 11 3. A SIMPLE EXAMPLE
12 4. A SELECTION OF MEMORY MODELS 12 4. A SELECTION OF MEMORY MODELS
13 5. ORDERING AND CYCLES 13 5. ORDERING AND CYCLES
14 6. EVENTS 14 6. EVENTS
15 7. THE PROGRAM ORDER RELATION: po AND po-loc 15 7. THE PROGRAM ORDER RELATION: po AND po-loc
16 8. A WARNING 16 8. A WARNING
17 9. DEPENDENCY RELATIONS: data, addr, and ctr 17 9. DEPENDENCY RELATIONS: data, addr, and ctrl
18 10. THE READS-FROM RELATION: rf, rfi, and rf 18 10. THE READS-FROM RELATION: rf, rfi, and rfe
19 11. CACHE COHERENCE AND THE COHERENCE ORDER 19 11. CACHE COHERENCE AND THE COHERENCE ORDER RELATION: co, coi, and coe
20 12. THE FROM-READS RELATION: fr, fri, and fr 20 12. THE FROM-READS RELATION: fr, fri, and fre
21 13. AN OPERATIONAL MODEL 21 13. AN OPERATIONAL MODEL
22 14. PROPAGATION ORDER RELATION: cumul-fence 22 14. PROPAGATION ORDER RELATION: cumul-fence
23 15. DERIVATION OF THE LKMM FROM THE OPERATIO 23 15. DERIVATION OF THE LKMM FROM THE OPERATIONAL MODEL
24 16. SEQUENTIAL CONSISTENCY PER VARIABLE 24 16. SEQUENTIAL CONSISTENCY PER VARIABLE
25 17. ATOMIC UPDATES: rmw 25 17. ATOMIC UPDATES: rmw
26 18. THE PRESERVED PROGRAM ORDER RELATION: pp 26 18. THE PRESERVED PROGRAM ORDER RELATION: ppo
27 19. AND THEN THERE WAS ALPHA 27 19. AND THEN THERE WAS ALPHA
28 20. THE HAPPENS-BEFORE RELATION: hb 28 20. THE HAPPENS-BEFORE RELATION: hb
29 21. THE PROPAGATES-BEFORE RELATION: pb 29 21. THE PROPAGATES-BEFORE RELATION: pb
30 22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rsc 30 22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
31 23. SRCU READ-SIDE CRITICAL SECTIONS !! 31 23. LOCKING
32 24. LOCKING !! 32 24. PLAIN ACCESSES AND DATA RACES
33 25. PLAIN ACCESSES AND DATA RACES !! 33 25. ODDS AND ENDS
34 26. ODDS AND ENDS <<
35 34
36 35
37 36
38 INTRODUCTION 37 INTRODUCTION
39 ------------ 38 ------------
40 39
41 The Linux-kernel memory consistency model (LKM 40 The Linux-kernel memory consistency model (LKMM) is rather complex and
42 obscure. This is particularly evident if you 41 obscure. This is particularly evident if you read through the
43 linux-kernel.bell and linux-kernel.cat files t 42 linux-kernel.bell and linux-kernel.cat files that make up the formal
44 version of the model; they are extremely terse 43 version of the model; they are extremely terse and their meanings are
45 far from clear. 44 far from clear.
46 45
47 This document describes the ideas underlying t 46 This document describes the ideas underlying the LKMM. It is meant
48 for people who want to understand how the mode 47 for people who want to understand how the model was designed. It does
49 not go into the details of the code in the .be 48 not go into the details of the code in the .bell and .cat files;
50 rather, it explains in English what the code e 49 rather, it explains in English what the code expresses symbolically.
51 50
52 Sections 2 (BACKGROUND) through 5 (ORDERING AN 51 Sections 2 (BACKGROUND) through 5 (ORDERING AND CYCLES) are aimed
53 toward beginners; they explain what memory con 52 toward beginners; they explain what memory consistency models are and
54 the basic notions shared by all such models. 53 the basic notions shared by all such models. People already familiar
55 with these concepts can skim or skip over them 54 with these concepts can skim or skip over them. Sections 6 (EVENTS)
56 through 12 (THE FROM_READS RELATION) describe 55 through 12 (THE FROM_READS RELATION) describe the fundamental
57 relations used in many models. Starting in Se 56 relations used in many models. Starting in Section 13 (AN OPERATIONAL
58 MODEL), the workings of the LKMM itself are co 57 MODEL), the workings of the LKMM itself are covered.
59 58
60 Warning: The code examples in this document ar 59 Warning: The code examples in this document are not written in the
61 proper format for litmus tests. They don't in 60 proper format for litmus tests. They don't include a header line, the
62 initializations are not enclosed in braces, th 61 initializations are not enclosed in braces, the global variables are
63 not passed by pointers, and they don't have an 62 not passed by pointers, and they don't have an "exists" clause at the
64 end. Converting them to the right format is l 63 end. Converting them to the right format is left as an exercise for
65 the reader. 64 the reader.
66 65
67 66
68 BACKGROUND 67 BACKGROUND
69 ---------- 68 ----------
70 69
71 A memory consistency model (or just memory mod 70 A memory consistency model (or just memory model, for short) is
72 something which predicts, given a piece of com 71 something which predicts, given a piece of computer code running on a
73 particular kind of system, what values may be 72 particular kind of system, what values may be obtained by the code's
74 load instructions. The LKMM makes these predi 73 load instructions. The LKMM makes these predictions for code running
75 as part of the Linux kernel. 74 as part of the Linux kernel.
76 75
77 In practice, people tend to use memory models 76 In practice, people tend to use memory models the other way around.
78 That is, given a piece of code and a collectio 77 That is, given a piece of code and a collection of values specified
79 for the loads, the model will predict whether 78 for the loads, the model will predict whether it is possible for the
80 code to run in such a way that the loads will 79 code to run in such a way that the loads will indeed obtain the
81 specified values. Of course, this is just ano 80 specified values. Of course, this is just another way of expressing
82 the same idea. 81 the same idea.
83 82
84 For code running on a uniprocessor system, the 83 For code running on a uniprocessor system, the predictions are easy:
85 Each load instruction must obtain the value wr 84 Each load instruction must obtain the value written by the most recent
86 store instruction accessing the same location 85 store instruction accessing the same location (we ignore complicating
87 factors such as DMA and mixed-size accesses.) 86 factors such as DMA and mixed-size accesses.) But on multiprocessor
88 systems, with multiple CPUs making concurrent 87 systems, with multiple CPUs making concurrent accesses to shared
89 memory locations, things aren't so simple. 88 memory locations, things aren't so simple.
90 89
91 Different architectures have differing memory 90 Different architectures have differing memory models, and the Linux
92 kernel supports a variety of architectures. T 91 kernel supports a variety of architectures. The LKMM has to be fairly
93 permissive, in the sense that any behavior all 92 permissive, in the sense that any behavior allowed by one of these
94 architectures also has to be allowed by the LK 93 architectures also has to be allowed by the LKMM.
95 94
96 95
97 A SIMPLE EXAMPLE 96 A SIMPLE EXAMPLE
98 ---------------- 97 ----------------
99 98
100 Here is a simple example to illustrate the bas 99 Here is a simple example to illustrate the basic concepts. Consider
101 some code running as part of a device driver f 100 some code running as part of a device driver for an input device. The
102 driver might contain an interrupt handler whic 101 driver might contain an interrupt handler which collects data from the
103 device, stores it in a buffer, and sets a flag 102 device, stores it in a buffer, and sets a flag to indicate the buffer
104 is full. Running concurrently on a different 103 is full. Running concurrently on a different CPU might be a part of
105 the driver code being executed by a process in 104 the driver code being executed by a process in the midst of a read(2)
106 system call. This code tests the flag to see 105 system call. This code tests the flag to see whether the buffer is
107 ready, and if it is, copies the data back to u 106 ready, and if it is, copies the data back to userspace. The buffer
108 and the flag are memory locations shared betwe 107 and the flag are memory locations shared between the two CPUs.
109 108
110 We can abstract out the important pieces of th 109 We can abstract out the important pieces of the driver code as follows
111 (the reason for using WRITE_ONCE() and READ_ON 110 (the reason for using WRITE_ONCE() and READ_ONCE() instead of simple
112 assignment statements is discussed later): 111 assignment statements is discussed later):
113 112
114 int buf = 0, flag = 0; 113 int buf = 0, flag = 0;
115 114
116 P0() 115 P0()
117 { 116 {
118 WRITE_ONCE(buf, 1); 117 WRITE_ONCE(buf, 1);
119 WRITE_ONCE(flag, 1); 118 WRITE_ONCE(flag, 1);
120 } 119 }
121 120
122 P1() 121 P1()
123 { 122 {
124 int r1; 123 int r1;
125 int r2 = 0; 124 int r2 = 0;
126 125
127 r1 = READ_ONCE(flag); 126 r1 = READ_ONCE(flag);
128 if (r1) 127 if (r1)
129 r2 = READ_ONCE(buf); 128 r2 = READ_ONCE(buf);
130 } 129 }
131 130
132 Here the P0() function represents the interrup 131 Here the P0() function represents the interrupt handler running on one
133 CPU and P1() represents the read() routine run 132 CPU and P1() represents the read() routine running on another. The
134 value 1 stored in buf represents input data co 133 value 1 stored in buf represents input data collected from the device.
135 Thus, P0 stores the data in buf and then sets 134 Thus, P0 stores the data in buf and then sets flag. Meanwhile, P1
136 reads flag into the private variable r1, and i 135 reads flag into the private variable r1, and if it is set, reads the
137 data from buf into a second private variable r 136 data from buf into a second private variable r2 for copying to
138 userspace. (Presumably if flag is not set the 137 userspace. (Presumably if flag is not set then the driver will wait a
139 while and try again.) 138 while and try again.)
140 139
141 This pattern of memory accesses, where one CPU 140 This pattern of memory accesses, where one CPU stores values to two
142 shared memory locations and another CPU loads 141 shared memory locations and another CPU loads from those locations in
143 the opposite order, is widely known as the "Me 142 the opposite order, is widely known as the "Message Passing" or MP
144 pattern. It is typical of memory access patte 143 pattern. It is typical of memory access patterns in the kernel.
145 144
146 Please note that this example code is a simpli 145 Please note that this example code is a simplified abstraction. Real
147 buffers are usually larger than a single integ 146 buffers are usually larger than a single integer, real device drivers
148 usually use sleep and wakeup mechanisms rather 147 usually use sleep and wakeup mechanisms rather than polling for I/O
149 completion, and real code generally doesn't bo 148 completion, and real code generally doesn't bother to copy values into
150 private variables before using them. All that 149 private variables before using them. All that is beside the point;
151 the idea here is simply to illustrate the over 150 the idea here is simply to illustrate the overall pattern of memory
152 accesses by the CPUs. 151 accesses by the CPUs.
153 152
154 A memory model will predict what values P1 mig 153 A memory model will predict what values P1 might obtain for its loads
155 from flag and buf, or equivalently, what value 154 from flag and buf, or equivalently, what values r1 and r2 might end up
156 with after the code has finished running. 155 with after the code has finished running.
157 156
158 Some predictions are trivial. For instance, n 157 Some predictions are trivial. For instance, no sane memory model would
159 predict that r1 = 42 or r2 = -7, because neith 158 predict that r1 = 42 or r2 = -7, because neither of those values ever
160 gets stored in flag or buf. 159 gets stored in flag or buf.
161 160
162 Some nontrivial predictions are nonetheless qu 161 Some nontrivial predictions are nonetheless quite simple. For
163 instance, P1 might run entirely before P0 begi 162 instance, P1 might run entirely before P0 begins, in which case r1 and
164 r2 will both be 0 at the end. Or P0 might run 163 r2 will both be 0 at the end. Or P0 might run entirely before P1
165 begins, in which case r1 and r2 will both be 1 164 begins, in which case r1 and r2 will both be 1.
166 165
167 The interesting predictions concern what might 166 The interesting predictions concern what might happen when the two
168 routines run concurrently. One possibility is 167 routines run concurrently. One possibility is that P1 runs after P0's
169 store to buf but before the store to flag. In 168 store to buf but before the store to flag. In this case, r1 and r2
170 will again both be 0. (If P1 had been designe 169 will again both be 0. (If P1 had been designed to read buf
171 unconditionally then we would instead have r1 170 unconditionally then we would instead have r1 = 0 and r2 = 1.)
172 171
173 However, the most interesting possibility is w 172 However, the most interesting possibility is where r1 = 1 and r2 = 0.
174 If this were to occur it would mean the driver 173 If this were to occur it would mean the driver contains a bug, because
175 incorrect data would get sent to the user: 0 i 174 incorrect data would get sent to the user: 0 instead of 1. As it
176 happens, the LKMM does predict this outcome ca 175 happens, the LKMM does predict this outcome can occur, and the example
177 driver code shown above is indeed buggy. 176 driver code shown above is indeed buggy.
178 177
179 178
180 A SELECTION OF MEMORY MODELS 179 A SELECTION OF MEMORY MODELS
181 ---------------------------- 180 ----------------------------
182 181
183 The first widely cited memory model, and the s 182 The first widely cited memory model, and the simplest to understand,
184 is Sequential Consistency. According to this 183 is Sequential Consistency. According to this model, systems behave as
185 if each CPU executed its instructions in order 184 if each CPU executed its instructions in order but with unspecified
186 timing. In other words, the instructions from 185 timing. In other words, the instructions from the various CPUs get
187 interleaved in a nondeterministic way, always 186 interleaved in a nondeterministic way, always according to some single
188 global order that agrees with the order of the 187 global order that agrees with the order of the instructions in the
189 program source for each CPU. The model says t 188 program source for each CPU. The model says that the value obtained
190 by each load is simply the value written by th 189 by each load is simply the value written by the most recently executed
191 store to the same memory location, from any CP 190 store to the same memory location, from any CPU.
192 191
193 For the MP example code shown above, Sequentia 192 For the MP example code shown above, Sequential Consistency predicts
194 that the undesired result r1 = 1, r2 = 0 canno 193 that the undesired result r1 = 1, r2 = 0 cannot occur. The reasoning
195 goes like this: 194 goes like this:
196 195
197 Since r1 = 1, P0 must store 1 to flag 196 Since r1 = 1, P0 must store 1 to flag before P1 loads 1 from
198 it, as loads can obtain values only fr 197 it, as loads can obtain values only from earlier stores.
199 198
200 P1 loads from flag before loading from 199 P1 loads from flag before loading from buf, since CPUs execute
201 their instructions in order. 200 their instructions in order.
202 201
203 P1 must load 0 from buf before P0 stor 202 P1 must load 0 from buf before P0 stores 1 to it; otherwise r2
204 would be 1 since a load obtains its va 203 would be 1 since a load obtains its value from the most recent
205 store to the same address. 204 store to the same address.
206 205
207 P0 stores 1 to buf before storing 1 to 206 P0 stores 1 to buf before storing 1 to flag, since it executes
208 its instructions in order. 207 its instructions in order.
209 208
210 Since an instruction (in this case, P0 209 Since an instruction (in this case, P0's store to flag) cannot
211 execute before itself, the specified o 210 execute before itself, the specified outcome is impossible.
212 211
213 However, real computer hardware almost never f 212 However, real computer hardware almost never follows the Sequential
214 Consistency memory model; doing so would rule 213 Consistency memory model; doing so would rule out too many valuable
215 performance optimizations. On ARM and PowerPC 214 performance optimizations. On ARM and PowerPC architectures, for
216 instance, the MP example code really does some 215 instance, the MP example code really does sometimes yield r1 = 1 and
217 r2 = 0. 216 r2 = 0.
218 217
219 x86 and SPARC follow yet a different memory mo 218 x86 and SPARC follow yet a different memory model: TSO (Total Store
220 Ordering). This model predicts that the undes 219 Ordering). This model predicts that the undesired outcome for the MP
221 pattern cannot occur, but in other respects it 220 pattern cannot occur, but in other respects it differs from Sequential
222 Consistency. One example is the Store Buffer 221 Consistency. One example is the Store Buffer (SB) pattern, in which
223 each CPU stores to its own shared location and 222 each CPU stores to its own shared location and then loads from the
224 other CPU's location: 223 other CPU's location:
225 224
226 int x = 0, y = 0; 225 int x = 0, y = 0;
227 226
228 P0() 227 P0()
229 { 228 {
230 int r0; 229 int r0;
231 230
232 WRITE_ONCE(x, 1); 231 WRITE_ONCE(x, 1);
233 r0 = READ_ONCE(y); 232 r0 = READ_ONCE(y);
234 } 233 }
235 234
236 P1() 235 P1()
237 { 236 {
238 int r1; 237 int r1;
239 238
240 WRITE_ONCE(y, 1); 239 WRITE_ONCE(y, 1);
241 r1 = READ_ONCE(x); 240 r1 = READ_ONCE(x);
242 } 241 }
243 242
244 Sequential Consistency predicts that the outco 243 Sequential Consistency predicts that the outcome r0 = 0, r1 = 0 is
245 impossible. (Exercise: Figure out the reasoni 244 impossible. (Exercise: Figure out the reasoning.) But TSO allows
246 this outcome to occur, and in fact it does som 245 this outcome to occur, and in fact it does sometimes occur on x86 and
247 SPARC systems. 246 SPARC systems.
248 247
249 The LKMM was inspired by the memory models fol 248 The LKMM was inspired by the memory models followed by PowerPC, ARM,
250 x86, Alpha, and other architectures. However, 249 x86, Alpha, and other architectures. However, it is different in
251 detail from each of them. 250 detail from each of them.
252 251
253 252
254 ORDERING AND CYCLES 253 ORDERING AND CYCLES
255 ------------------- 254 -------------------
256 255
257 Memory models are all about ordering. Often t 256 Memory models are all about ordering. Often this is temporal ordering
258 (i.e., the order in which certain events occur 257 (i.e., the order in which certain events occur) but it doesn't have to
259 be; consider for example the order of instruct 258 be; consider for example the order of instructions in a program's
260 source code. We saw above that Sequential Con 259 source code. We saw above that Sequential Consistency makes an
261 important assumption that CPUs execute instruc 260 important assumption that CPUs execute instructions in the same order
262 as those instructions occur in the code, and t 261 as those instructions occur in the code, and there are many other
263 instances of ordering playing central roles in 262 instances of ordering playing central roles in memory models.
264 263
265 The counterpart to ordering is a cycle. Order 264 The counterpart to ordering is a cycle. Ordering rules out cycles:
266 It's not possible to have X ordered before Y, 265 It's not possible to have X ordered before Y, Y ordered before Z, and
267 Z ordered before X, because this would mean th 266 Z ordered before X, because this would mean that X is ordered before
268 itself. The analysis of the MP example under 267 itself. The analysis of the MP example under Sequential Consistency
269 involved just such an impossible cycle: 268 involved just such an impossible cycle:
270 269
271 W: P0 stores 1 to flag executes befo 270 W: P0 stores 1 to flag executes before
272 X: P1 loads 1 from flag executes befo 271 X: P1 loads 1 from flag executes before
273 Y: P1 loads 0 from buf executes befo 272 Y: P1 loads 0 from buf executes before
274 Z: P0 stores 1 to buf executes befo 273 Z: P0 stores 1 to buf executes before
275 W: P0 stores 1 to flag. 274 W: P0 stores 1 to flag.
276 275
277 In short, if a memory model requires certain a 276 In short, if a memory model requires certain accesses to be ordered,
278 and a certain outcome for the loads in a piece 277 and a certain outcome for the loads in a piece of code can happen only
279 if those accesses would form a cycle, then the 278 if those accesses would form a cycle, then the memory model predicts
280 that outcome cannot occur. 279 that outcome cannot occur.
281 280
282 The LKMM is defined largely in terms of cycles 281 The LKMM is defined largely in terms of cycles, as we will see.
283 282
284 283
285 EVENTS 284 EVENTS
286 ------ 285 ------
287 286
288 The LKMM does not work directly with the C sta 287 The LKMM does not work directly with the C statements that make up
289 kernel source code. Instead it considers the 288 kernel source code. Instead it considers the effects of those
290 statements in a more abstract form, namely, ev 289 statements in a more abstract form, namely, events. The model
291 includes three types of events: 290 includes three types of events:
292 291
293 Read events correspond to loads from s 292 Read events correspond to loads from shared memory, such as
294 calls to READ_ONCE(), smp_load_acquire 293 calls to READ_ONCE(), smp_load_acquire(), or
295 rcu_dereference(). 294 rcu_dereference().
296 295
297 Write events correspond to stores to s 296 Write events correspond to stores to shared memory, such as
298 calls to WRITE_ONCE(), smp_store_relea 297 calls to WRITE_ONCE(), smp_store_release(), or atomic_set().
299 298
300 Fence events correspond to memory barr 299 Fence events correspond to memory barriers (also known as
301 fences), such as calls to smp_rmb() or 300 fences), such as calls to smp_rmb() or rcu_read_lock().
302 301
303 These categories are not exclusive; a read or 302 These categories are not exclusive; a read or write event can also be
304 a fence. This happens with functions like smp 303 a fence. This happens with functions like smp_load_acquire() or
305 spin_lock(). However, no single event can be 304 spin_lock(). However, no single event can be both a read and a write.
306 Atomic read-modify-write accesses, such as ato 305 Atomic read-modify-write accesses, such as atomic_inc() or xchg(),
307 correspond to a pair of events: a read followe 306 correspond to a pair of events: a read followed by a write. (The
308 write event is omitted for executions where it 307 write event is omitted for executions where it doesn't occur, such as
309 a cmpxchg() where the comparison fails.) 308 a cmpxchg() where the comparison fails.)
310 309
311 Other parts of the code, those which do not in 310 Other parts of the code, those which do not involve interaction with
312 shared memory, do not give rise to events. Th 311 shared memory, do not give rise to events. Thus, arithmetic and
313 logical computations, control-flow instruction 312 logical computations, control-flow instructions, or accesses to
314 private memory or CPU registers are not of cen 313 private memory or CPU registers are not of central interest to the
315 memory model. They only affect the model's pr 314 memory model. They only affect the model's predictions indirectly.
316 For example, an arithmetic computation might d 315 For example, an arithmetic computation might determine the value that
317 gets stored to a shared memory location (or in 316 gets stored to a shared memory location (or in the case of an array
318 index, the address where the value gets stored 317 index, the address where the value gets stored), but the memory model
319 is concerned only with the store itself -- its 318 is concerned only with the store itself -- its value and its address
320 -- not the computation leading up to it. 319 -- not the computation leading up to it.
321 320
322 Events in the LKMM can be linked by various re 321 Events in the LKMM can be linked by various relations, which we will
323 describe in the following sections. The memor 322 describe in the following sections. The memory model requires certain
324 of these relations to be orderings, that is, i 323 of these relations to be orderings, that is, it requires them not to
325 have any cycles. 324 have any cycles.
326 325
327 326
328 THE PROGRAM ORDER RELATION: po AND po-loc 327 THE PROGRAM ORDER RELATION: po AND po-loc
329 ----------------------------------------- 328 -----------------------------------------
330 329
331 The most important relation between events is 330 The most important relation between events is program order (po). You
332 can think of it as the order in which statemen 331 can think of it as the order in which statements occur in the source
333 code after branches are taken into account and 332 code after branches are taken into account and loops have been
334 unrolled. A better description might be the o 333 unrolled. A better description might be the order in which
335 instructions are presented to a CPU's executio 334 instructions are presented to a CPU's execution unit. Thus, we say
336 that X is po-before Y (written as "X ->po Y" i 335 that X is po-before Y (written as "X ->po Y" in formulas) if X occurs
337 before Y in the instruction stream. 336 before Y in the instruction stream.
338 337
339 This is inherently a single-CPU relation; two 338 This is inherently a single-CPU relation; two instructions executing
340 on different CPUs are never linked by po. Als 339 on different CPUs are never linked by po. Also, it is by definition
341 an ordering so it cannot have any cycles. 340 an ordering so it cannot have any cycles.
342 341
343 po-loc is a sub-relation of po. It links two 342 po-loc is a sub-relation of po. It links two memory accesses when the
344 first comes before the second in program order 343 first comes before the second in program order and they access the
345 same memory location (the "-loc" suffix). 344 same memory location (the "-loc" suffix).
346 345
347 Although this may seem straightforward, there 346 Although this may seem straightforward, there is one subtle aspect to
348 program order we need to explain. The LKMM wa 347 program order we need to explain. The LKMM was inspired by low-level
349 architectural memory models which describe the 348 architectural memory models which describe the behavior of machine
350 code, and it retains their outlook to a consid 349 code, and it retains their outlook to a considerable extent. The
351 read, write, and fence events used by the mode 350 read, write, and fence events used by the model are close in spirit to
352 individual machine instructions. Nevertheless 351 individual machine instructions. Nevertheless, the LKMM describes
353 kernel code written in C, and the mapping from 352 kernel code written in C, and the mapping from C to machine code can
354 be extremely complex. 353 be extremely complex.
355 354
356 Optimizing compilers have great freedom in the 355 Optimizing compilers have great freedom in the way they translate
357 source code to object code. They are allowed 356 source code to object code. They are allowed to apply transformations
358 that add memory accesses, eliminate accesses, 357 that add memory accesses, eliminate accesses, combine them, split them
359 into pieces, or move them around. The use of 358 into pieces, or move them around. The use of READ_ONCE(), WRITE_ONCE(),
360 or one of the other atomic or synchronization 359 or one of the other atomic or synchronization primitives prevents a
361 large number of compiler optimizations. In pa 360 large number of compiler optimizations. In particular, it is guaranteed
362 that the compiler will not remove such accesse 361 that the compiler will not remove such accesses from the generated code
363 (unless it can prove the accesses will never b 362 (unless it can prove the accesses will never be executed), it will not
364 change the order in which they occur in the co 363 change the order in which they occur in the code (within limits imposed
365 by the C standard), and it will not introduce 364 by the C standard), and it will not introduce extraneous accesses.
366 365
367 The MP and SB examples above used READ_ONCE() 366 The MP and SB examples above used READ_ONCE() and WRITE_ONCE() rather
368 than ordinary memory accesses. Thanks to this 367 than ordinary memory accesses. Thanks to this usage, we can be certain
369 that in the MP example, the compiler won't reo 368 that in the MP example, the compiler won't reorder P0's write event to
370 buf and P0's write event to flag, and similarl 369 buf and P0's write event to flag, and similarly for the other shared
371 memory accesses in the examples. 370 memory accesses in the examples.
372 371
373 Since private variables are not shared between 372 Since private variables are not shared between CPUs, they can be
374 accessed normally without READ_ONCE() or WRITE 373 accessed normally without READ_ONCE() or WRITE_ONCE(). In fact, they
375 need not even be stored in normal memory at al 374 need not even be stored in normal memory at all -- in principle a
376 private variable could be stored in a CPU regi 375 private variable could be stored in a CPU register (hence the convention
377 that these variables have names starting with 376 that these variables have names starting with the letter 'r').
378 377
379 378
380 A WARNING 379 A WARNING
381 --------- 380 ---------
382 381
383 The protections provided by READ_ONCE(), WRITE 382 The protections provided by READ_ONCE(), WRITE_ONCE(), and others are
384 not perfect; and under some circumstances it i 383 not perfect; and under some circumstances it is possible for the
385 compiler to undermine the memory model. Here 384 compiler to undermine the memory model. Here is an example. Suppose
386 both branches of an "if" statement store the s 385 both branches of an "if" statement store the same value to the same
387 location: 386 location:
388 387
389 r1 = READ_ONCE(x); 388 r1 = READ_ONCE(x);
390 if (r1) { 389 if (r1) {
391 WRITE_ONCE(y, 2); 390 WRITE_ONCE(y, 2);
392 ... /* do something */ 391 ... /* do something */
393 } else { 392 } else {
394 WRITE_ONCE(y, 2); 393 WRITE_ONCE(y, 2);
395 ... /* do something else */ 394 ... /* do something else */
396 } 395 }
397 396
398 For this code, the LKMM predicts that the load 397 For this code, the LKMM predicts that the load from x will always be
399 executed before either of the stores to y. Ho 398 executed before either of the stores to y. However, a compiler could
400 lift the stores out of the conditional, transf 399 lift the stores out of the conditional, transforming the code into
401 something resembling: 400 something resembling:
402 401
403 r1 = READ_ONCE(x); 402 r1 = READ_ONCE(x);
404 WRITE_ONCE(y, 2); 403 WRITE_ONCE(y, 2);
405 if (r1) { 404 if (r1) {
406 ... /* do something */ 405 ... /* do something */
407 } else { 406 } else {
408 ... /* do something else */ 407 ... /* do something else */
409 } 408 }
410 409
411 Given this version of the code, the LKMM would 410 Given this version of the code, the LKMM would predict that the load
412 from x could be executed after the store to y. 411 from x could be executed after the store to y. Thus, the memory
413 model's original prediction could be invalidat 412 model's original prediction could be invalidated by the compiler.
414 413
415 Another issue arises from the fact that in C, 414 Another issue arises from the fact that in C, arguments to many
416 operators and function calls can be evaluated 415 operators and function calls can be evaluated in any order. For
417 example: 416 example:
418 417
419 r1 = f(5) + g(6); 418 r1 = f(5) + g(6);
420 419
421 The object code might call f(5) either before 420 The object code might call f(5) either before or after g(6); the
422 memory model cannot assume there is a fixed pr 421 memory model cannot assume there is a fixed program order relation
423 between them. (In fact, if the function calls 422 between them. (In fact, if the function calls are inlined then the
424 compiler might even interleave their object co 423 compiler might even interleave their object code.)
425 424
426 425
427 DEPENDENCY RELATIONS: data, addr, and ctrl 426 DEPENDENCY RELATIONS: data, addr, and ctrl
428 ------------------------------------------ 427 ------------------------------------------
429 428
430 We say that two events are linked by a depende 429 We say that two events are linked by a dependency relation when the
431 execution of the second event depends in some 430 execution of the second event depends in some way on a value obtained
432 from memory by the first. The first event mus 431 from memory by the first. The first event must be a read, and the
433 value it obtains must somehow affect what the 432 value it obtains must somehow affect what the second event does.
434 There are three kinds of dependencies: data, a 433 There are three kinds of dependencies: data, address (addr), and
435 control (ctrl). 434 control (ctrl).
436 435
437 A read and a write event are linked by a data 436 A read and a write event are linked by a data dependency if the value
438 obtained by the read affects the value stored 437 obtained by the read affects the value stored by the write. As a very
439 simple example: 438 simple example:
440 439
441 int x, y; 440 int x, y;
442 441
443 r1 = READ_ONCE(x); 442 r1 = READ_ONCE(x);
444 WRITE_ONCE(y, r1 + 5); 443 WRITE_ONCE(y, r1 + 5);
445 444
446 The value stored by the WRITE_ONCE obviously d 445 The value stored by the WRITE_ONCE obviously depends on the value
447 loaded by the READ_ONCE. Such dependencies ca 446 loaded by the READ_ONCE. Such dependencies can wind through
448 arbitrarily complicated computations, and a wr 447 arbitrarily complicated computations, and a write can depend on the
449 values of multiple reads. 448 values of multiple reads.
450 449
451 A read event and another memory access event a 450 A read event and another memory access event are linked by an address
452 dependency if the value obtained by the read a 451 dependency if the value obtained by the read affects the location
453 accessed by the other event. The second event 452 accessed by the other event. The second event can be either a read or
454 a write. Here's another simple example: 453 a write. Here's another simple example:
455 454
456 int a[20]; 455 int a[20];
457 int i; 456 int i;
458 457
459 r1 = READ_ONCE(i); 458 r1 = READ_ONCE(i);
460 r2 = READ_ONCE(a[r1]); 459 r2 = READ_ONCE(a[r1]);
461 460
462 Here the location accessed by the second READ_ 461 Here the location accessed by the second READ_ONCE() depends on the
463 index value loaded by the first. Pointer indi 462 index value loaded by the first. Pointer indirection also gives rise
464 to address dependencies, since the address of 463 to address dependencies, since the address of a location accessed
465 through a pointer will depend on the value rea 464 through a pointer will depend on the value read earlier from that
466 pointer. 465 pointer.
467 466
468 Finally, a read event X and a write event Y ar !! 467 Finally, a read event and another memory access event are linked by a
469 dependency if Y syntactically lies within an a !! 468 control dependency if the value obtained by the read affects whether
470 X affects the evaluation of the if condition v !! 469 the second event is executed at all. Simple example:
471 dependency (or similarly for a switch statemen <<
472 470
473 int x, y; 471 int x, y;
474 472
475 r1 = READ_ONCE(x); 473 r1 = READ_ONCE(x);
476 if (r1) 474 if (r1)
477 WRITE_ONCE(y, 1984); 475 WRITE_ONCE(y, 1984);
478 476
479 Execution of the WRITE_ONCE() is controlled by 477 Execution of the WRITE_ONCE() is controlled by a conditional expression
480 which depends on the value obtained by the REA 478 which depends on the value obtained by the READ_ONCE(); hence there is
481 a control dependency from the load to the stor 479 a control dependency from the load to the store.
482 480
483 It should be pretty obvious that events can on 481 It should be pretty obvious that events can only depend on reads that
484 come earlier in program order. Symbolically, 482 come earlier in program order. Symbolically, if we have R ->data X,
485 R ->addr X, or R ->ctrl X (where R is a read e 483 R ->addr X, or R ->ctrl X (where R is a read event), then we must also
486 have R ->po X. It wouldn't make sense for a c 484 have R ->po X. It wouldn't make sense for a computation to depend
487 somehow on a value that doesn't get loaded fro 485 somehow on a value that doesn't get loaded from shared memory until
488 later in the code! 486 later in the code!
489 487
490 Here's a trick question: When is a dependency <<
491 When it is purely syntactic rather than semant <<
492 between two accesses is purely syntactic if th <<
493 actually depend on the result of the first. H <<
494 <<
495 r1 = READ_ONCE(x); <<
496 WRITE_ONCE(y, r1 * 0); <<
497 <<
498 There appears to be a data dependency from the <<
499 of y, since the value to be stored is computed <<
500 loaded. But in fact, the value stored does no <<
501 anything since it will always be 0. Thus the <<
502 syntactic (it appears to exist in the code) bu <<
503 second access will always be the same, regardl <<
504 first access). Given code like this, a compil <<
505 the value returned by the load from x, which w <<
506 any dependency. (The compiler is not permitte <<
507 the load generated for a READ_ONCE() -- that's <<
508 properties of READ_ONCE() -- but it is allowed <<
509 value.) <<
510 <<
511 It's natural to object that no one in their ri <<
512 code like the above. However, macro expansion <<
513 to this sort of thing, in ways that often are <<
514 programmer. <<
515 <<
516 Another mechanism that can lead to purely synt <<
517 related to the notion of "undefined behavior". <<
518 behaviors are called "undefined" in the C lang <<
519 which means that when they occur there are no <<
520 the outcome. Consider the following example: <<
521 <<
522 int a[1]; <<
523 int i; <<
524 <<
525 r1 = READ_ONCE(i); <<
526 r2 = READ_ONCE(a[r1]); <<
527 <<
528 Access beyond the end or before the beginning <<
529 of undefined behavior. Therefore the compiler <<
530 about what will happen if r1 is nonzero, and i <<
531 will always be zero regardless of the value ac <<
532 (If the assumption turns out to be wrong the r <<
533 be undefined anyway, so the compiler doesn't c <<
534 from the load can be discarded, breaking the a <<
535 <<
536 The LKMM is unaware that purely syntactic depe <<
537 from semantic dependencies and therefore mista <<
538 accesses in the two examples above will be ord <<
539 example of how the compiler can undermine the <<
540 <<
541 488
542 THE READS-FROM RELATION: rf, rfi, and rfe 489 THE READS-FROM RELATION: rf, rfi, and rfe
543 ----------------------------------------- 490 -----------------------------------------
544 491
545 The reads-from relation (rf) links a write eve 492 The reads-from relation (rf) links a write event to a read event when
546 the value loaded by the read is the value that 493 the value loaded by the read is the value that was stored by the
547 write. In colloquial terms, the load "reads f 494 write. In colloquial terms, the load "reads from" the store. We
548 write W ->rf R to indicate that the load R rea 495 write W ->rf R to indicate that the load R reads from the store W. We
549 further distinguish the cases where the load a 496 further distinguish the cases where the load and the store occur on
550 the same CPU (internal reads-from, or rfi) and 497 the same CPU (internal reads-from, or rfi) and where they occur on
551 different CPUs (external reads-from, or rfe). 498 different CPUs (external reads-from, or rfe).
552 499
553 For our purposes, a memory location's initial 500 For our purposes, a memory location's initial value is treated as
554 though it had been written there by an imagina 501 though it had been written there by an imaginary initial store that
555 executes on a separate CPU before the main pro 502 executes on a separate CPU before the main program runs.
556 503
557 Usage of the rf relation implicitly assumes th 504 Usage of the rf relation implicitly assumes that loads will always
558 read from a single store. It doesn't apply pr 505 read from a single store. It doesn't apply properly in the presence
559 of load-tearing, where a load obtains some of 506 of load-tearing, where a load obtains some of its bits from one store
560 and some of them from another store. Fortunat 507 and some of them from another store. Fortunately, use of READ_ONCE()
561 and WRITE_ONCE() will prevent load-tearing; it 508 and WRITE_ONCE() will prevent load-tearing; it's not possible to have:
562 509
563 int x = 0; 510 int x = 0;
564 511
565 P0() 512 P0()
566 { 513 {
567 WRITE_ONCE(x, 0x1234); 514 WRITE_ONCE(x, 0x1234);
568 } 515 }
569 516
570 P1() 517 P1()
571 { 518 {
572 int r1; 519 int r1;
573 520
574 r1 = READ_ONCE(x); 521 r1 = READ_ONCE(x);
575 } 522 }
576 523
577 and end up with r1 = 0x1200 (partly from x's i 524 and end up with r1 = 0x1200 (partly from x's initial value and partly
578 from the value stored by P0). 525 from the value stored by P0).
579 526
580 On the other hand, load-tearing is unavoidable 527 On the other hand, load-tearing is unavoidable when mixed-size
581 accesses are used. Consider this example: 528 accesses are used. Consider this example:
582 529
583 union { 530 union {
584 u32 w; 531 u32 w;
585 u16 h[2]; 532 u16 h[2];
586 } x; 533 } x;
587 534
588 P0() 535 P0()
589 { 536 {
590 WRITE_ONCE(x.h[0], 0x1234); 537 WRITE_ONCE(x.h[0], 0x1234);
591 WRITE_ONCE(x.h[1], 0x5678); 538 WRITE_ONCE(x.h[1], 0x5678);
592 } 539 }
593 540
594 P1() 541 P1()
595 { 542 {
596 int r1; 543 int r1;
597 544
598 r1 = READ_ONCE(x.w); 545 r1 = READ_ONCE(x.w);
599 } 546 }
600 547
601 If r1 = 0x56781234 (little-endian!) at the end 548 If r1 = 0x56781234 (little-endian!) at the end, then P1 must have read
602 from both of P0's stores. It is possible to h 549 from both of P0's stores. It is possible to handle mixed-size and
603 unaligned accesses in a memory model, but the 550 unaligned accesses in a memory model, but the LKMM currently does not
604 attempt to do so. It requires all accesses to 551 attempt to do so. It requires all accesses to be properly aligned and
605 of the location's actual size. 552 of the location's actual size.
606 553
607 554
608 CACHE COHERENCE AND THE COHERENCE ORDER RELATI 555 CACHE COHERENCE AND THE COHERENCE ORDER RELATION: co, coi, and coe
609 ---------------------------------------------- 556 ------------------------------------------------------------------
610 557
611 Cache coherence is a general principle requiri 558 Cache coherence is a general principle requiring that in a
612 multi-processor system, the CPUs must share a 559 multi-processor system, the CPUs must share a consistent view of the
613 memory contents. Specifically, it requires th 560 memory contents. Specifically, it requires that for each location in
614 shared memory, the stores to that location mus 561 shared memory, the stores to that location must form a single global
615 ordering which all the CPUs agree on (the cohe 562 ordering which all the CPUs agree on (the coherence order), and this
616 ordering must be consistent with the program o 563 ordering must be consistent with the program order for accesses to
617 that location. 564 that location.
618 565
619 To put it another way, for any variable x, the 566 To put it another way, for any variable x, the coherence order (co) of
620 the stores to x is simply the order in which t 567 the stores to x is simply the order in which the stores overwrite one
621 another. The imaginary store which establishe 568 another. The imaginary store which establishes x's initial value
622 comes first in the coherence order; the store 569 comes first in the coherence order; the store which directly
623 overwrites the initial value comes second; the 570 overwrites the initial value comes second; the store which overwrites
624 that value comes third, and so on. 571 that value comes third, and so on.
625 572
626 You can think of the coherence order as being 573 You can think of the coherence order as being the order in which the
627 stores reach x's location in memory (or if you 574 stores reach x's location in memory (or if you prefer a more
628 hardware-centric view, the order in which the 575 hardware-centric view, the order in which the stores get written to
629 x's cache line). We write W ->co W' if W come 576 x's cache line). We write W ->co W' if W comes before W' in the
630 coherence order, that is, if the value stored 577 coherence order, that is, if the value stored by W gets overwritten,
631 directly or indirectly, by the value stored by 578 directly or indirectly, by the value stored by W'.
632 579
633 Coherence order is required to be consistent w 580 Coherence order is required to be consistent with program order. This
634 requirement takes the form of four coherency r 581 requirement takes the form of four coherency rules:
635 582
636 Write-write coherence: If W ->po-loc W 583 Write-write coherence: If W ->po-loc W' (i.e., W comes before
637 W' in program order and they access th 584 W' in program order and they access the same location), where W
638 and W' are two stores, then W ->co W'. 585 and W' are two stores, then W ->co W'.
639 586
640 Write-read coherence: If W ->po-loc R, 587 Write-read coherence: If W ->po-loc R, where W is a store and R
641 is a load, then R must read from W or 588 is a load, then R must read from W or from some other store
642 which comes after W in the coherence o 589 which comes after W in the coherence order.
643 590
644 Read-write coherence: If R ->po-loc W, 591 Read-write coherence: If R ->po-loc W, where R is a load and W
645 is a store, then the store which R rea 592 is a store, then the store which R reads from must come before
646 W in the coherence order. 593 W in the coherence order.
647 594
648 Read-read coherence: If R ->po-loc R', 595 Read-read coherence: If R ->po-loc R', where R and R' are two
649 loads, then either they read from the 596 loads, then either they read from the same store or else the
650 store read by R comes before the store 597 store read by R comes before the store read by R' in the
651 coherence order. 598 coherence order.
652 599
653 This is sometimes referred to as sequential co 600 This is sometimes referred to as sequential consistency per variable,
654 because it means that the accesses to any sing 601 because it means that the accesses to any single memory location obey
655 the rules of the Sequential Consistency memory 602 the rules of the Sequential Consistency memory model. (According to
656 Wikipedia, sequential consistency per variable 603 Wikipedia, sequential consistency per variable and cache coherence
657 mean the same thing except that cache coherenc 604 mean the same thing except that cache coherence includes an extra
658 requirement that every store eventually become 605 requirement that every store eventually becomes visible to every CPU.)
659 606
660 Any reasonable memory model will include cache 607 Any reasonable memory model will include cache coherence. Indeed, our
661 expectation of cache coherence is so deeply in 608 expectation of cache coherence is so deeply ingrained that violations
662 of its requirements look more like hardware bu 609 of its requirements look more like hardware bugs than programming
663 errors: 610 errors:
664 611
665 int x; 612 int x;
666 613
667 P0() 614 P0()
668 { 615 {
669 WRITE_ONCE(x, 17); 616 WRITE_ONCE(x, 17);
670 WRITE_ONCE(x, 23); 617 WRITE_ONCE(x, 23);
671 } 618 }
672 619
673 If the final value stored in x after this code 620 If the final value stored in x after this code ran was 17, you would
674 think your computer was broken. It would be a 621 think your computer was broken. It would be a violation of the
675 write-write coherence rule: Since the store of 622 write-write coherence rule: Since the store of 23 comes later in
676 program order, it must also come later in x's 623 program order, it must also come later in x's coherence order and
677 thus must overwrite the store of 17. 624 thus must overwrite the store of 17.
678 625
679 int x = 0; 626 int x = 0;
680 627
681 P0() 628 P0()
682 { 629 {
683 int r1; 630 int r1;
684 631
685 r1 = READ_ONCE(x); 632 r1 = READ_ONCE(x);
686 WRITE_ONCE(x, 666); 633 WRITE_ONCE(x, 666);
687 } 634 }
688 635
689 If r1 = 666 at the end, this would violate the 636 If r1 = 666 at the end, this would violate the read-write coherence
690 rule: The READ_ONCE() load comes before the WR 637 rule: The READ_ONCE() load comes before the WRITE_ONCE() store in
691 program order, so it must not read from that s 638 program order, so it must not read from that store but rather from one
692 coming earlier in the coherence order (in this 639 coming earlier in the coherence order (in this case, x's initial
693 value). 640 value).
694 641
695 int x = 0; 642 int x = 0;
696 643
697 P0() 644 P0()
698 { 645 {
699 WRITE_ONCE(x, 5); 646 WRITE_ONCE(x, 5);
700 } 647 }
701 648
702 P1() 649 P1()
703 { 650 {
704 int r1, r2; 651 int r1, r2;
705 652
706 r1 = READ_ONCE(x); 653 r1 = READ_ONCE(x);
707 r2 = READ_ONCE(x); 654 r2 = READ_ONCE(x);
708 } 655 }
709 656
710 If r1 = 5 (reading from P0's store) and r2 = 0 657 If r1 = 5 (reading from P0's store) and r2 = 0 (reading from the
711 imaginary store which establishes x's initial 658 imaginary store which establishes x's initial value) at the end, this
712 would violate the read-read coherence rule: Th 659 would violate the read-read coherence rule: The r1 load comes before
713 the r2 load in program order, so it must not r 660 the r2 load in program order, so it must not read from a store that
714 comes later in the coherence order. 661 comes later in the coherence order.
715 662
716 (As a minor curiosity, if this code had used n 663 (As a minor curiosity, if this code had used normal loads instead of
717 READ_ONCE() in P1, on Itanium it sometimes cou 664 READ_ONCE() in P1, on Itanium it sometimes could end up with r1 = 5
718 and r2 = 0! This results from parallel execut 665 and r2 = 0! This results from parallel execution of the operations
719 encoded in Itanium's Very-Long-Instruction-Wor 666 encoded in Itanium's Very-Long-Instruction-Word format, and it is yet
720 another motivation for using READ_ONCE() when 667 another motivation for using READ_ONCE() when accessing shared memory
721 locations.) 668 locations.)
722 669
723 Just like the po relation, co is inherently an 670 Just like the po relation, co is inherently an ordering -- it is not
724 possible for a store to directly or indirectly 671 possible for a store to directly or indirectly overwrite itself! And
725 just like with the rf relation, we distinguish 672 just like with the rf relation, we distinguish between stores that
726 occur on the same CPU (internal coherence orde 673 occur on the same CPU (internal coherence order, or coi) and stores
727 that occur on different CPUs (external coheren 674 that occur on different CPUs (external coherence order, or coe).
728 675
729 On the other hand, stores to different memory 676 On the other hand, stores to different memory locations are never
730 related by co, just as instructions on differe 677 related by co, just as instructions on different CPUs are never
731 related by po. Coherence order is strictly pe 678 related by po. Coherence order is strictly per-location, or if you
732 prefer, each location has its own independent 679 prefer, each location has its own independent coherence order.
733 680
734 681
735 THE FROM-READS RELATION: fr, fri, and fre 682 THE FROM-READS RELATION: fr, fri, and fre
736 ----------------------------------------- 683 -----------------------------------------
737 684
738 The from-reads relation (fr) can be a little d 685 The from-reads relation (fr) can be a little difficult for people to
739 grok. It describes the situation where a load 686 grok. It describes the situation where a load reads a value that gets
740 overwritten by a store. In other words, we ha 687 overwritten by a store. In other words, we have R ->fr W when the
741 value that R reads is overwritten (directly or 688 value that R reads is overwritten (directly or indirectly) by W, or
742 equivalently, when R reads from a store which 689 equivalently, when R reads from a store which comes earlier than W in
743 the coherence order. 690 the coherence order.
744 691
745 For example: 692 For example:
746 693
747 int x = 0; 694 int x = 0;
748 695
749 P0() 696 P0()
750 { 697 {
751 int r1; 698 int r1;
752 699
753 r1 = READ_ONCE(x); 700 r1 = READ_ONCE(x);
754 WRITE_ONCE(x, 2); 701 WRITE_ONCE(x, 2);
755 } 702 }
756 703
757 The value loaded from x will be 0 (assuming ca 704 The value loaded from x will be 0 (assuming cache coherence!), and it
758 gets overwritten by the value 2. Thus there i 705 gets overwritten by the value 2. Thus there is an fr link from the
759 READ_ONCE() to the WRITE_ONCE(). If the code 706 READ_ONCE() to the WRITE_ONCE(). If the code contained any later
760 stores to x, there would also be fr links from 707 stores to x, there would also be fr links from the READ_ONCE() to
761 them. 708 them.
762 709
763 As with rf, rfi, and rfe, we subdivide the fr 710 As with rf, rfi, and rfe, we subdivide the fr relation into fri (when
764 the load and the store are on the same CPU) an 711 the load and the store are on the same CPU) and fre (when they are on
765 different CPUs). 712 different CPUs).
766 713
767 Note that the fr relation is determined entire 714 Note that the fr relation is determined entirely by the rf and co
768 relations; it is not independent. Given a rea 715 relations; it is not independent. Given a read event R and a write
769 event W for the same location, we will have R 716 event W for the same location, we will have R ->fr W if and only if
770 the write which R reads from is co-before W. 717 the write which R reads from is co-before W. In symbols,
771 718
772 (R ->fr W) := (there exists W' with W' 719 (R ->fr W) := (there exists W' with W' ->rf R and W' ->co W).
773 720
774 721
775 AN OPERATIONAL MODEL 722 AN OPERATIONAL MODEL
776 -------------------- 723 --------------------
777 724
778 The LKMM is based on various operational memor 725 The LKMM is based on various operational memory models, meaning that
779 the models arise from an abstract view of how 726 the models arise from an abstract view of how a computer system
780 operates. Here are the main ideas, as incorpo 727 operates. Here are the main ideas, as incorporated into the LKMM.
781 728
782 The system as a whole is divided into the CPUs 729 The system as a whole is divided into the CPUs and a memory subsystem.
783 The CPUs are responsible for executing instruc 730 The CPUs are responsible for executing instructions (not necessarily
784 in program order), and they communicate with t 731 in program order), and they communicate with the memory subsystem.
785 For the most part, executing an instruction re 732 For the most part, executing an instruction requires a CPU to perform
786 only internal operations. However, loads, sto 733 only internal operations. However, loads, stores, and fences involve
787 more. 734 more.
788 735
789 When CPU C executes a store instruction, it te 736 When CPU C executes a store instruction, it tells the memory subsystem
790 to store a certain value at a certain location 737 to store a certain value at a certain location. The memory subsystem
791 propagates the store to all the other CPUs as 738 propagates the store to all the other CPUs as well as to RAM. (As a
792 special case, we say that the store propagates 739 special case, we say that the store propagates to its own CPU at the
793 time it is executed.) The memory subsystem al 740 time it is executed.) The memory subsystem also determines where the
794 store falls in the location's coherence order. 741 store falls in the location's coherence order. In particular, it must
795 arrange for the store to be co-later than (i.e 742 arrange for the store to be co-later than (i.e., to overwrite) any
796 other store to the same location which has alr 743 other store to the same location which has already propagated to CPU C.
797 744
798 When a CPU executes a load instruction R, it f 745 When a CPU executes a load instruction R, it first checks to see
799 whether there are any as-yet unexecuted store 746 whether there are any as-yet unexecuted store instructions, for the
800 same location, that come before R in program o 747 same location, that come before R in program order. If there are, it
801 uses the value of the po-latest such store as 748 uses the value of the po-latest such store as the value obtained by R,
802 and we say that the store's value is forwarded 749 and we say that the store's value is forwarded to R. Otherwise, the
803 CPU asks the memory subsystem for the value to 750 CPU asks the memory subsystem for the value to load and we say that R
804 is satisfied from memory. The memory subsyste 751 is satisfied from memory. The memory subsystem hands back the value
805 of the co-latest store to the location in ques 752 of the co-latest store to the location in question which has already
806 propagated to that CPU. 753 propagated to that CPU.
807 754
808 (In fact, the picture needs to be a little mor 755 (In fact, the picture needs to be a little more complicated than this.
809 CPUs have local caches, and propagating a stor 756 CPUs have local caches, and propagating a store to a CPU really means
810 propagating it to the CPU's local cache. A lo 757 propagating it to the CPU's local cache. A local cache can take some
811 time to process the stores that it receives, a 758 time to process the stores that it receives, and a store can't be used
812 to satisfy one of the CPU's loads until it has 759 to satisfy one of the CPU's loads until it has been processed. On
813 most architectures, the local caches process s 760 most architectures, the local caches process stores in
814 First-In-First-Out order, and consequently the 761 First-In-First-Out order, and consequently the processing delay
815 doesn't matter for the memory model. But on A 762 doesn't matter for the memory model. But on Alpha, the local caches
816 have a partitioned design that results in non- 763 have a partitioned design that results in non-FIFO behavior. We will
817 discuss this in more detail later.) 764 discuss this in more detail later.)
818 765
819 Note that load instructions may be executed sp 766 Note that load instructions may be executed speculatively and may be
820 restarted under certain circumstances. The me 767 restarted under certain circumstances. The memory model ignores these
821 premature executions; we simply say that the l 768 premature executions; we simply say that the load executes at the
822 final time it is forwarded or satisfied. 769 final time it is forwarded or satisfied.
823 770
824 Executing a fence (or memory barrier) instruct 771 Executing a fence (or memory barrier) instruction doesn't require a
825 CPU to do anything special other than informin 772 CPU to do anything special other than informing the memory subsystem
826 about the fence. However, fences do constrain 773 about the fence. However, fences do constrain the way CPUs and the
827 memory subsystem handle other instructions, in 774 memory subsystem handle other instructions, in two respects.
828 775
829 First, a fence forces the CPU to execute vario 776 First, a fence forces the CPU to execute various instructions in
830 program order. Exactly which instructions are 777 program order. Exactly which instructions are ordered depends on the
831 type of fence: 778 type of fence:
832 779
833 Strong fences, including smp_mb() and 780 Strong fences, including smp_mb() and synchronize_rcu(), force
834 the CPU to execute all po-earlier inst 781 the CPU to execute all po-earlier instructions before any
835 po-later instructions; 782 po-later instructions;
836 783
837 smp_rmb() forces the CPU to execute al 784 smp_rmb() forces the CPU to execute all po-earlier loads
838 before any po-later loads; 785 before any po-later loads;
839 786
840 smp_wmb() forces the CPU to execute al 787 smp_wmb() forces the CPU to execute all po-earlier stores
841 before any po-later stores; 788 before any po-later stores;
842 789
843 Acquire fences, such as smp_load_acqui 790 Acquire fences, such as smp_load_acquire(), force the CPU to
844 execute the load associated with the f 791 execute the load associated with the fence (e.g., the load
845 part of an smp_load_acquire()) before 792 part of an smp_load_acquire()) before any po-later
846 instructions; 793 instructions;
847 794
848 Release fences, such as smp_store_rele 795 Release fences, such as smp_store_release(), force the CPU to
849 execute all po-earlier instructions be 796 execute all po-earlier instructions before the store
850 associated with the fence (e.g., the s 797 associated with the fence (e.g., the store part of an
851 smp_store_release()). 798 smp_store_release()).
852 799
853 Second, some types of fence affect the way the 800 Second, some types of fence affect the way the memory subsystem
854 propagates stores. When a fence instruction i 801 propagates stores. When a fence instruction is executed on CPU C:
855 802
856 For each other CPU C', smp_wmb() force 803 For each other CPU C', smp_wmb() forces all po-earlier stores
857 on C to propagate to C' before any po- 804 on C to propagate to C' before any po-later stores do.
858 805
859 For each other CPU C', any store which 806 For each other CPU C', any store which propagates to C before
860 a release fence is executed (including 807 a release fence is executed (including all po-earlier
861 stores executed on C) is forced to pro 808 stores executed on C) is forced to propagate to C' before the
862 store associated with the release fenc 809 store associated with the release fence does.
863 810
864 Any store which propagates to C before 811 Any store which propagates to C before a strong fence is
865 executed (including all po-earlier sto 812 executed (including all po-earlier stores on C) is forced to
866 propagate to all other CPUs before any 813 propagate to all other CPUs before any instructions po-after
867 the strong fence are executed on C. 814 the strong fence are executed on C.
868 815
869 The propagation ordering enforced by release f 816 The propagation ordering enforced by release fences and strong fences
870 affects stores from other CPUs that propagate 817 affects stores from other CPUs that propagate to CPU C before the
871 fence is executed, as well as stores that are 818 fence is executed, as well as stores that are executed on C before the
872 fence. We describe this property by saying th 819 fence. We describe this property by saying that release fences and
873 strong fences are A-cumulative. By contrast, 820 strong fences are A-cumulative. By contrast, smp_wmb() fences are not
874 A-cumulative; they only affect the propagation 821 A-cumulative; they only affect the propagation of stores that are
875 executed on C before the fence (i.e., those wh 822 executed on C before the fence (i.e., those which precede the fence in
876 program order). 823 program order).
877 824
878 rcu_read_lock(), rcu_read_unlock(), and synchr 825 rcu_read_lock(), rcu_read_unlock(), and synchronize_rcu() fences have
879 other properties which we discuss later. 826 other properties which we discuss later.
880 827
881 828
882 PROPAGATION ORDER RELATION: cumul-fence 829 PROPAGATION ORDER RELATION: cumul-fence
883 --------------------------------------- 830 ---------------------------------------
884 831
885 The fences which affect propagation order (i.e 832 The fences which affect propagation order (i.e., strong, release, and
886 smp_wmb() fences) are collectively referred to 833 smp_wmb() fences) are collectively referred to as cumul-fences, even
887 though smp_wmb() isn't A-cumulative. The cumu 834 though smp_wmb() isn't A-cumulative. The cumul-fence relation is
888 defined to link memory access events E and F w 835 defined to link memory access events E and F whenever:
889 836
890 E and F are both stores on the same CP 837 E and F are both stores on the same CPU and an smp_wmb() fence
891 event occurs between them in program o 838 event occurs between them in program order; or
892 839
893 F is a release fence and some X comes 840 F is a release fence and some X comes before F in program order,
894 where either X = E or else E ->rf X; o 841 where either X = E or else E ->rf X; or
895 842
896 A strong fence event occurs between so 843 A strong fence event occurs between some X and F in program
897 order, where either X = E or else E -> 844 order, where either X = E or else E ->rf X.
898 845
899 The operational model requires that whenever W 846 The operational model requires that whenever W and W' are both stores
900 and W ->cumul-fence W', then W must propagate 847 and W ->cumul-fence W', then W must propagate to any given CPU
901 before W' does. However, for different CPUs C 848 before W' does. However, for different CPUs C and C', it does not
902 require W to propagate to C before W' propagat 849 require W to propagate to C before W' propagates to C'.
903 850
904 851
905 DERIVATION OF THE LKMM FROM THE OPERATIONAL MO 852 DERIVATION OF THE LKMM FROM THE OPERATIONAL MODEL
906 ---------------------------------------------- 853 -------------------------------------------------
907 854
908 The LKMM is derived from the restrictions impo 855 The LKMM is derived from the restrictions imposed by the design
909 outlined above. These restrictions involve th 856 outlined above. These restrictions involve the necessity of
910 maintaining cache coherence and the fact that 857 maintaining cache coherence and the fact that a CPU can't operate on a
911 value before it knows what that value is, amon 858 value before it knows what that value is, among other things.
912 859
913 The formal version of the LKMM is defined by s 860 The formal version of the LKMM is defined by six requirements, or
914 axioms: 861 axioms:
915 862
916 Sequential consistency per variable: T 863 Sequential consistency per variable: This requires that the
917 system obey the four coherency rules. 864 system obey the four coherency rules.
918 865
919 Atomicity: This requires that atomic r 866 Atomicity: This requires that atomic read-modify-write
920 operations really are atomic, that is, 867 operations really are atomic, that is, no other stores can
921 sneak into the middle of such an updat 868 sneak into the middle of such an update.
922 869
923 Happens-before: This requires that cer 870 Happens-before: This requires that certain instructions are
924 executed in a specific order. 871 executed in a specific order.
925 872
926 Propagation: This requires that certai 873 Propagation: This requires that certain stores propagate to
927 CPUs and to RAM in a specific order. 874 CPUs and to RAM in a specific order.
928 875
929 Rcu: This requires that RCU read-side 876 Rcu: This requires that RCU read-side critical sections and
930 grace periods obey the rules of RCU, i 877 grace periods obey the rules of RCU, in particular, the
931 Grace-Period Guarantee. 878 Grace-Period Guarantee.
932 879
933 Plain-coherence: This requires that pl 880 Plain-coherence: This requires that plain memory accesses
934 (those not using READ_ONCE(), WRITE_ON 881 (those not using READ_ONCE(), WRITE_ONCE(), etc.) must obey
935 the operational model's rules regardin 882 the operational model's rules regarding cache coherence.
936 883
937 The first and second are quite common; they ca 884 The first and second are quite common; they can be found in many
938 memory models (such as those for C11/C++11). 885 memory models (such as those for C11/C++11). The "happens-before" and
939 "propagation" axioms have analogs in other mem 886 "propagation" axioms have analogs in other memory models as well. The
940 "rcu" and "plain-coherence" axioms are specifi 887 "rcu" and "plain-coherence" axioms are specific to the LKMM.
941 888
942 Each of these axioms is discussed below. 889 Each of these axioms is discussed below.
943 890
944 891
945 SEQUENTIAL CONSISTENCY PER VARIABLE 892 SEQUENTIAL CONSISTENCY PER VARIABLE
946 ----------------------------------- 893 -----------------------------------
947 894
948 According to the principle of cache coherence, 895 According to the principle of cache coherence, the stores to any fixed
949 shared location in memory form a global orderi 896 shared location in memory form a global ordering. We can imagine
950 inserting the loads from that location into th 897 inserting the loads from that location into this ordering, by placing
951 each load between the store that it reads from 898 each load between the store that it reads from and the following
952 store. This leaves the relative positions of 899 store. This leaves the relative positions of loads that read from the
953 same store unspecified; let's say they are ins 900 same store unspecified; let's say they are inserted in program order,
954 first for CPU 0, then CPU 1, etc. 901 first for CPU 0, then CPU 1, etc.
955 902
956 You can check that the four coherency rules im 903 You can check that the four coherency rules imply that the rf, co, fr,
957 and po-loc relations agree with this global or 904 and po-loc relations agree with this global ordering; in other words,
958 whenever we have X ->rf Y or X ->co Y or X ->f 905 whenever we have X ->rf Y or X ->co Y or X ->fr Y or X ->po-loc Y, the
959 X event comes before the Y event in the global 906 X event comes before the Y event in the global ordering. The LKMM's
960 "coherence" axiom expresses this by requiring 907 "coherence" axiom expresses this by requiring the union of these
961 relations not to have any cycles. This means 908 relations not to have any cycles. This means it must not be possible
962 to find events 909 to find events
963 910
964 X0 -> X1 -> X2 -> ... -> Xn -> X0, 911 X0 -> X1 -> X2 -> ... -> Xn -> X0,
965 912
966 where each of the links is either rf, co, fr, 913 where each of the links is either rf, co, fr, or po-loc. This has to
967 hold if the accesses to the fixed memory locat 914 hold if the accesses to the fixed memory location can be ordered as
968 cache coherence demands. 915 cache coherence demands.
969 916
970 Although it is not obvious, it can be shown th 917 Although it is not obvious, it can be shown that the converse is also
971 true: This LKMM axiom implies that the four co 918 true: This LKMM axiom implies that the four coherency rules are
972 obeyed. 919 obeyed.
973 920
974 921
975 ATOMIC UPDATES: rmw 922 ATOMIC UPDATES: rmw
976 ------------------- 923 -------------------
977 924
978 What does it mean to say that a read-modify-wr 925 What does it mean to say that a read-modify-write (rmw) update, such
979 as atomic_inc(&x), is atomic? It means that t 926 as atomic_inc(&x), is atomic? It means that the memory location (x in
980 this case) does not get altered between the re 927 this case) does not get altered between the read and the write events
981 making up the atomic operation. In particular 928 making up the atomic operation. In particular, if two CPUs perform
982 atomic_inc(&x) concurrently, it must be guaran 929 atomic_inc(&x) concurrently, it must be guaranteed that the final
983 value of x will be the initial value plus two. 930 value of x will be the initial value plus two. We should never have
984 the following sequence of events: 931 the following sequence of events:
985 932
986 CPU 0 loads x obtaining 13; 933 CPU 0 loads x obtaining 13;
987 CPU 1 934 CPU 1 loads x obtaining 13;
988 CPU 0 stores 14 to x; 935 CPU 0 stores 14 to x;
989 CPU 1 936 CPU 1 stores 14 to x;
990 937
991 where the final value of x is wrong (14 rather 938 where the final value of x is wrong (14 rather than 15).
992 939
993 In this example, CPU 0's increment effectively 940 In this example, CPU 0's increment effectively gets lost because it
994 occurs in between CPU 1's load and store. To 941 occurs in between CPU 1's load and store. To put it another way, the
995 problem is that the position of CPU 0's store 942 problem is that the position of CPU 0's store in x's coherence order
996 is between the store that CPU 1 reads from and 943 is between the store that CPU 1 reads from and the store that CPU 1
997 performs. 944 performs.
998 945
999 The same analysis applies to all atomic update 946 The same analysis applies to all atomic update operations. Therefore,
1000 to enforce atomicity the LKMM requires that a 947 to enforce atomicity the LKMM requires that atomic updates follow this
1001 rule: Whenever R and W are the read and write 948 rule: Whenever R and W are the read and write events composing an
1002 atomic read-modify-write and W' is the write 949 atomic read-modify-write and W' is the write event which R reads from,
1003 there must not be any stores coming between W 950 there must not be any stores coming between W' and W in the coherence
1004 order. Equivalently, 951 order. Equivalently,
1005 952
1006 (R ->rmw W) implies (there is no X wi 953 (R ->rmw W) implies (there is no X with R ->fr X and X ->co W),
1007 954
1008 where the rmw relation links the read and wri 955 where the rmw relation links the read and write events making up each
1009 atomic update. This is what the LKMM's "atom 956 atomic update. This is what the LKMM's "atomic" axiom says.
1010 957
1011 Atomic rmw updates play one more role in the <<
1012 sequences". An rmw sequence is simply a bunc <<
1013 each update reads from the previous one. Wri <<
1014 looks like this: <<
1015 <<
1016 Z0 ->rf Y1 ->rmw Z1 ->rf ... ->rf Yn <<
1017 <<
1018 where Z0 is some store event and n can be any <<
1019 degenerate case). We write this relation as: <<
1020 Note that this implies Z0 and Zn are stores t <<
1021 <<
1022 Rmw sequences have a special property in the <<
1023 cumul-fence relation. That is, if we have: <<
1024 <<
1025 U ->cumul-fence X -> rmw-sequence Y <<
1026 <<
1027 then also U ->cumul-fence Y. Thinking about <<
1028 operational model, U ->cumul-fence X says tha <<
1029 to each CPU before the store X does. Then th <<
1030 linked by an rmw sequence means that U also p <<
1031 before Y does. In an analogous way, rmw sequ <<
1032 the w-post-bounded relation defined below in <<
1033 DATA RACES section. <<
1034 <<
1035 (The notion of rmw sequences in the LKMM is s <<
1036 the same as, that of release sequences in the <<
1037 were added to the LKMM to fix an obscure bug; <<
1038 updates with full-barrier semantics did not a <<
1039 at least as strong as atomic updates with rel <<
1040 <<
1041 958
1042 THE PRESERVED PROGRAM ORDER RELATION: ppo 959 THE PRESERVED PROGRAM ORDER RELATION: ppo
1043 ----------------------------------------- 960 -----------------------------------------
1044 961
1045 There are many situations where a CPU is obli 962 There are many situations where a CPU is obliged to execute two
1046 instructions in program order. We amalgamate 963 instructions in program order. We amalgamate them into the ppo (for
1047 "preserved program order") relation, which li 964 "preserved program order") relation, which links the po-earlier
1048 instruction to the po-later instruction and i 965 instruction to the po-later instruction and is thus a sub-relation of
1049 po. 966 po.
1050 967
1051 The operational model already includes a desc 968 The operational model already includes a description of one such
1052 situation: Fences are a source of ppo links. 969 situation: Fences are a source of ppo links. Suppose X and Y are
1053 memory accesses with X ->po Y; then the CPU m 970 memory accesses with X ->po Y; then the CPU must execute X before Y if
1054 any of the following hold: 971 any of the following hold:
1055 972
1056 A strong (smp_mb() or synchronize_rcu 973 A strong (smp_mb() or synchronize_rcu()) fence occurs between
1057 X and Y; 974 X and Y;
1058 975
1059 X and Y are both stores and an smp_wm 976 X and Y are both stores and an smp_wmb() fence occurs between
1060 them; 977 them;
1061 978
1062 X and Y are both loads and an smp_rmb 979 X and Y are both loads and an smp_rmb() fence occurs between
1063 them; 980 them;
1064 981
1065 X is also an acquire fence, such as s 982 X is also an acquire fence, such as smp_load_acquire();
1066 983
1067 Y is also a release fence, such as sm 984 Y is also a release fence, such as smp_store_release().
1068 985
1069 Another possibility, not mentioned earlier bu 986 Another possibility, not mentioned earlier but discussed in the next
1070 section, is: 987 section, is:
1071 988
1072 X and Y are both loads, X ->addr Y (i 989 X and Y are both loads, X ->addr Y (i.e., there is an address
1073 dependency from X to Y), and X is a R 990 dependency from X to Y), and X is a READ_ONCE() or an atomic
1074 access. 991 access.
1075 992
1076 Dependencies can also cause instructions to b 993 Dependencies can also cause instructions to be executed in program
1077 order. This is uncontroversial when the seco 994 order. This is uncontroversial when the second instruction is a
1078 store; either a data, address, or control dep 995 store; either a data, address, or control dependency from a load R to
1079 a store W will force the CPU to execute R bef 996 a store W will force the CPU to execute R before W. This is very
1080 simply because the CPU cannot tell the memory 997 simply because the CPU cannot tell the memory subsystem about W's
1081 store before it knows what value should be st 998 store before it knows what value should be stored (in the case of a
1082 data dependency), what location it should be 999 data dependency), what location it should be stored into (in the case
1083 of an address dependency), or whether the sto 1000 of an address dependency), or whether the store should actually take
1084 place (in the case of a control dependency). 1001 place (in the case of a control dependency).
1085 1002
1086 Dependencies to load instructions are more pr 1003 Dependencies to load instructions are more problematic. To begin with,
1087 there is no such thing as a data dependency t 1004 there is no such thing as a data dependency to a load. Next, a CPU
1088 has no reason to respect a control dependency 1005 has no reason to respect a control dependency to a load, because it
1089 can always satisfy the second load speculativ 1006 can always satisfy the second load speculatively before the first, and
1090 then ignore the result if it turns out that t 1007 then ignore the result if it turns out that the second load shouldn't
1091 be executed after all. And lastly, the real 1008 be executed after all. And lastly, the real difficulties begin when
1092 we consider address dependencies to loads. 1009 we consider address dependencies to loads.
1093 1010
1094 To be fair about it, all Linux-supported arch 1011 To be fair about it, all Linux-supported architectures do execute
1095 loads in program order if there is an address 1012 loads in program order if there is an address dependency between them.
1096 After all, a CPU cannot ask the memory subsys 1013 After all, a CPU cannot ask the memory subsystem to load a value from
1097 a particular location before it knows what th 1014 a particular location before it knows what that location is. However,
1098 the split-cache design used by Alpha can caus 1015 the split-cache design used by Alpha can cause it to behave in a way
1099 that looks as if the loads were executed out 1016 that looks as if the loads were executed out of order (see the next
1100 section for more details). The kernel includ 1017 section for more details). The kernel includes a workaround for this
1101 problem when the loads come from READ_ONCE(), 1018 problem when the loads come from READ_ONCE(), and therefore the LKMM
1102 includes address dependencies to loads in the 1019 includes address dependencies to loads in the ppo relation.
1103 1020
1104 On the other hand, dependencies can indirectl 1021 On the other hand, dependencies can indirectly affect the ordering of
1105 two loads. This happens when there is a depe 1022 two loads. This happens when there is a dependency from a load to a
1106 store and a second, po-later load reads from 1023 store and a second, po-later load reads from that store:
1107 1024
1108 R ->dep W ->rfi R', 1025 R ->dep W ->rfi R',
1109 1026
1110 where the dep link can be either an address o 1027 where the dep link can be either an address or a data dependency. In
1111 this situation we know it is possible for the 1028 this situation we know it is possible for the CPU to execute R' before
1112 W, because it can forward the value that W wi 1029 W, because it can forward the value that W will store to R'. But it
1113 cannot execute R' before R, because it cannot 1030 cannot execute R' before R, because it cannot forward the value before
1114 it knows what that value is, or that W and R' 1031 it knows what that value is, or that W and R' do access the same
1115 location. However, if there is merely a cont 1032 location. However, if there is merely a control dependency between R
1116 and W then the CPU can speculatively forward 1033 and W then the CPU can speculatively forward W to R' before executing
1117 R; if the speculation turns out to be wrong t 1034 R; if the speculation turns out to be wrong then the CPU merely has to
1118 restart or abandon R'. 1035 restart or abandon R'.
1119 1036
1120 (In theory, a CPU might forward a store to a 1037 (In theory, a CPU might forward a store to a load when it runs across
1121 an address dependency like this: 1038 an address dependency like this:
1122 1039
1123 r1 = READ_ONCE(ptr); 1040 r1 = READ_ONCE(ptr);
1124 WRITE_ONCE(*r1, 17); 1041 WRITE_ONCE(*r1, 17);
1125 r2 = READ_ONCE(*r1); 1042 r2 = READ_ONCE(*r1);
1126 1043
1127 because it could tell that the store and the 1044 because it could tell that the store and the second load access the
1128 same location even before it knows what the l 1045 same location even before it knows what the location's address is.
1129 However, none of the architectures supported 1046 However, none of the architectures supported by the Linux kernel do
1130 this.) 1047 this.)
1131 1048
1132 Two memory accesses of the same location must 1049 Two memory accesses of the same location must always be executed in
1133 program order if the second access is a store 1050 program order if the second access is a store. Thus, if we have
1134 1051
1135 R ->po-loc W 1052 R ->po-loc W
1136 1053
1137 (the po-loc link says that R comes before W i 1054 (the po-loc link says that R comes before W in program order and they
1138 access the same location), the CPU is obliged 1055 access the same location), the CPU is obliged to execute W after R.
1139 If it executed W first then the memory subsys 1056 If it executed W first then the memory subsystem would respond to R's
1140 read request with the value stored by W (or a 1057 read request with the value stored by W (or an even later store), in
1141 violation of the read-write coherence rule. 1058 violation of the read-write coherence rule. Similarly, if we had
1142 1059
1143 W ->po-loc W' 1060 W ->po-loc W'
1144 1061
1145 and the CPU executed W' before W, then the me 1062 and the CPU executed W' before W, then the memory subsystem would put
1146 W' before W in the coherence order. It would 1063 W' before W in the coherence order. It would effectively cause W to
1147 overwrite W', in violation of the write-write 1064 overwrite W', in violation of the write-write coherence rule.
1148 (Interestingly, an early ARMv8 memory model, 1065 (Interestingly, an early ARMv8 memory model, now obsolete, proposed
1149 allowing out-of-order writes like this to occ 1066 allowing out-of-order writes like this to occur. The model avoided
1150 violating the write-write coherence rule by r 1067 violating the write-write coherence rule by requiring the CPU not to
1151 send the W write to the memory subsystem at a 1068 send the W write to the memory subsystem at all!)
1152 1069
1153 1070
1154 AND THEN THERE WAS ALPHA 1071 AND THEN THERE WAS ALPHA
1155 ------------------------ 1072 ------------------------
1156 1073
1157 As mentioned above, the Alpha architecture is 1074 As mentioned above, the Alpha architecture is unique in that it does
1158 not appear to respect address dependencies to 1075 not appear to respect address dependencies to loads. This means that
1159 code such as the following: 1076 code such as the following:
1160 1077
1161 int x = 0; 1078 int x = 0;
1162 int y = -1; 1079 int y = -1;
1163 int *ptr = &y; 1080 int *ptr = &y;
1164 1081
1165 P0() 1082 P0()
1166 { 1083 {
1167 WRITE_ONCE(x, 1); 1084 WRITE_ONCE(x, 1);
1168 smp_wmb(); 1085 smp_wmb();
1169 WRITE_ONCE(ptr, &x); 1086 WRITE_ONCE(ptr, &x);
1170 } 1087 }
1171 1088
1172 P1() 1089 P1()
1173 { 1090 {
1174 int *r1; 1091 int *r1;
1175 int r2; 1092 int r2;
1176 1093
1177 r1 = ptr; 1094 r1 = ptr;
1178 r2 = READ_ONCE(*r1); 1095 r2 = READ_ONCE(*r1);
1179 } 1096 }
1180 1097
1181 can malfunction on Alpha systems (notice that 1098 can malfunction on Alpha systems (notice that P1 uses an ordinary load
1182 to read ptr instead of READ_ONCE()). It is q 1099 to read ptr instead of READ_ONCE()). It is quite possible that r1 = &x
1183 and r2 = 0 at the end, in spite of the addres 1100 and r2 = 0 at the end, in spite of the address dependency.
1184 1101
1185 At first glance this doesn't seem to make sen 1102 At first glance this doesn't seem to make sense. We know that the
1186 smp_wmb() forces P0's store to x to propagate 1103 smp_wmb() forces P0's store to x to propagate to P1 before the store
1187 to ptr does. And since P1 can't execute its 1104 to ptr does. And since P1 can't execute its second load
1188 until it knows what location to load from, i. 1105 until it knows what location to load from, i.e., after executing its
1189 first load, the value x = 1 must have propaga 1106 first load, the value x = 1 must have propagated to P1 before the
1190 second load executed. So why doesn't r2 end 1107 second load executed. So why doesn't r2 end up equal to 1?
1191 1108
1192 The answer lies in the Alpha's split local ca 1109 The answer lies in the Alpha's split local caches. Although the two
1193 stores do reach P1's local cache in the prope 1110 stores do reach P1's local cache in the proper order, it can happen
1194 that the first store is processed by a busy p 1111 that the first store is processed by a busy part of the cache while
1195 the second store is processed by an idle part 1112 the second store is processed by an idle part. As a result, the x = 1
1196 value may not become available for P1's CPU t 1113 value may not become available for P1's CPU to read until after the
1197 ptr = &x value does, leading to the undesirab 1114 ptr = &x value does, leading to the undesirable result above. The
1198 final effect is that even though the two load 1115 final effect is that even though the two loads really are executed in
1199 program order, it appears that they aren't. 1116 program order, it appears that they aren't.
1200 1117
1201 This could not have happened if the local cac 1118 This could not have happened if the local cache had processed the
1202 incoming stores in FIFO order. By contrast, 1119 incoming stores in FIFO order. By contrast, other architectures
1203 maintain at least the appearance of FIFO orde 1120 maintain at least the appearance of FIFO order.
1204 1121
1205 In practice, this difficulty is solved by ins 1122 In practice, this difficulty is solved by inserting a special fence
1206 between P1's two loads when the kernel is com 1123 between P1's two loads when the kernel is compiled for the Alpha
1207 architecture. In fact, as of version 4.15, t 1124 architecture. In fact, as of version 4.15, the kernel automatically
1208 adds this fence after every READ_ONCE() and a !! 1125 adds this fence (called smp_read_barrier_depends() and defined as
1209 effect of the fence is to cause the CPU not t !! 1126 nothing at all on non-Alpha builds) after every READ_ONCE() and atomic
1210 instructions until after the local cache has !! 1127 load. The effect of the fence is to cause the CPU not to execute any
1211 the stores it has already received. Thus, if !! 1128 po-later instructions until after the local cache has finished
>> 1129 processing all the stores it has already received. Thus, if the code
>> 1130 was changed to:
1212 1131
1213 P1() 1132 P1()
1214 { 1133 {
1215 int *r1; 1134 int *r1;
1216 int r2; 1135 int r2;
1217 1136
1218 r1 = READ_ONCE(ptr); 1137 r1 = READ_ONCE(ptr);
1219 r2 = READ_ONCE(*r1); 1138 r2 = READ_ONCE(*r1);
1220 } 1139 }
1221 1140
1222 then we would never get r1 = &x and r2 = 0. 1141 then we would never get r1 = &x and r2 = 0. By the time P1 executed
1223 its second load, the x = 1 store would alread 1142 its second load, the x = 1 store would already be fully processed by
1224 the local cache and available for satisfying 1143 the local cache and available for satisfying the read request. Thus
1225 we have yet another reason why shared data sh 1144 we have yet another reason why shared data should always be read with
1226 READ_ONCE() or another synchronization primit 1145 READ_ONCE() or another synchronization primitive rather than accessed
1227 directly. 1146 directly.
1228 1147
1229 The LKMM requires that smp_rmb(), acquire fen 1148 The LKMM requires that smp_rmb(), acquire fences, and strong fences
1230 share this property: They do not allow the CP !! 1149 share this property with smp_read_barrier_depends(): They do not allow
1231 instructions (or po-later loads in the case o !! 1150 the CPU to execute any po-later instructions (or po-later loads in the
1232 outstanding stores have been processed by the !! 1151 case of smp_rmb()) until all outstanding stores have been processed by
1233 case of a strong fence, the CPU first has to !! 1152 the local cache. In the case of a strong fence, the CPU first has to
1234 po-earlier stores to propagate to every other !! 1153 wait for all of its po-earlier stores to propagate to every other CPU
1235 it has to wait for the local cache to process !! 1154 in the system; then it has to wait for the local cache to process all
1236 as of that time -- not just the stores receiv !! 1155 the stores received as of that time -- not just the stores received
1237 began. !! 1156 when the strong fence began.
1238 1157
1239 And of course, none of this matters for any a 1158 And of course, none of this matters for any architecture other than
1240 Alpha. 1159 Alpha.
1241 1160
1242 1161
1243 THE HAPPENS-BEFORE RELATION: hb 1162 THE HAPPENS-BEFORE RELATION: hb
1244 ------------------------------- 1163 -------------------------------
1245 1164
1246 The happens-before relation (hb) links memory 1165 The happens-before relation (hb) links memory accesses that have to
1247 execute in a certain order. hb includes the 1166 execute in a certain order. hb includes the ppo relation and two
1248 others, one of which is rfe. 1167 others, one of which is rfe.
1249 1168
1250 W ->rfe R implies that W and R are on differe 1169 W ->rfe R implies that W and R are on different CPUs. It also means
1251 that W's store must have propagated to R's CP 1170 that W's store must have propagated to R's CPU before R executed;
1252 otherwise R could not have read the value sto 1171 otherwise R could not have read the value stored by W. Therefore W
1253 must have executed before R, and so we have W 1172 must have executed before R, and so we have W ->hb R.
1254 1173
1255 The equivalent fact need not hold if W ->rfi 1174 The equivalent fact need not hold if W ->rfi R (i.e., W and R are on
1256 the same CPU). As we have already seen, the 1175 the same CPU). As we have already seen, the operational model allows
1257 W's value to be forwarded to R in such cases, 1176 W's value to be forwarded to R in such cases, meaning that R may well
1258 execute before W does. 1177 execute before W does.
1259 1178
1260 It's important to understand that neither coe 1179 It's important to understand that neither coe nor fre is included in
1261 hb, despite their similarities to rfe. For e 1180 hb, despite their similarities to rfe. For example, suppose we have
1262 W ->coe W'. This means that W and W' are sto 1181 W ->coe W'. This means that W and W' are stores to the same location,
1263 they execute on different CPUs, and W comes b 1182 they execute on different CPUs, and W comes before W' in the coherence
1264 order (i.e., W' overwrites W). Nevertheless, 1183 order (i.e., W' overwrites W). Nevertheless, it is possible for W' to
1265 execute before W, because the decision as to 1184 execute before W, because the decision as to which store overwrites
1266 the other is made later by the memory subsyst 1185 the other is made later by the memory subsystem. When the stores are
1267 nearly simultaneous, either one can come out 1186 nearly simultaneous, either one can come out on top. Similarly,
1268 R ->fre W means that W overwrites the value w 1187 R ->fre W means that W overwrites the value which R reads, but it
1269 doesn't mean that W has to execute after R. 1188 doesn't mean that W has to execute after R. All that's necessary is
1270 for the memory subsystem not to propagate W t 1189 for the memory subsystem not to propagate W to R's CPU until after R
1271 has executed, which is possible if W executes 1190 has executed, which is possible if W executes shortly before R.
1272 1191
1273 The third relation included in hb is like ppo 1192 The third relation included in hb is like ppo, in that it only links
1274 events that are on the same CPU. However it 1193 events that are on the same CPU. However it is more difficult to
1275 explain, because it arises only indirectly fr 1194 explain, because it arises only indirectly from the requirement of
1276 cache coherence. The relation is called prop 1195 cache coherence. The relation is called prop, and it links two events
1277 on CPU C in situations where a store from som 1196 on CPU C in situations where a store from some other CPU comes after
1278 the first event in the coherence order and pr 1197 the first event in the coherence order and propagates to C before the
1279 second event executes. 1198 second event executes.
1280 1199
1281 This is best explained with some examples. T 1200 This is best explained with some examples. The simplest case looks
1282 like this: 1201 like this:
1283 1202
1284 int x; 1203 int x;
1285 1204
1286 P0() 1205 P0()
1287 { 1206 {
1288 int r1; 1207 int r1;
1289 1208
1290 WRITE_ONCE(x, 1); 1209 WRITE_ONCE(x, 1);
1291 r1 = READ_ONCE(x); 1210 r1 = READ_ONCE(x);
1292 } 1211 }
1293 1212
1294 P1() 1213 P1()
1295 { 1214 {
1296 WRITE_ONCE(x, 8); 1215 WRITE_ONCE(x, 8);
1297 } 1216 }
1298 1217
1299 If r1 = 8 at the end then P0's accesses must 1218 If r1 = 8 at the end then P0's accesses must have executed in program
1300 order. We can deduce this from the operation 1219 order. We can deduce this from the operational model; if P0's load
1301 had executed before its store then the value 1220 had executed before its store then the value of the store would have
1302 been forwarded to the load, so r1 would have 1221 been forwarded to the load, so r1 would have ended up equal to 1, not
1303 8. In this case there is a prop link from P0 1222 8. In this case there is a prop link from P0's write event to its read
1304 event, because P1's store came after P0's sto 1223 event, because P1's store came after P0's store in x's coherence
1305 order, and P1's store propagated to P0 before 1224 order, and P1's store propagated to P0 before P0's load executed.
1306 1225
1307 An equally simple case involves two loads of 1226 An equally simple case involves two loads of the same location that
1308 read from different stores: 1227 read from different stores:
1309 1228
1310 int x = 0; 1229 int x = 0;
1311 1230
1312 P0() 1231 P0()
1313 { 1232 {
1314 int r1, r2; 1233 int r1, r2;
1315 1234
1316 r1 = READ_ONCE(x); 1235 r1 = READ_ONCE(x);
1317 r2 = READ_ONCE(x); 1236 r2 = READ_ONCE(x);
1318 } 1237 }
1319 1238
1320 P1() 1239 P1()
1321 { 1240 {
1322 WRITE_ONCE(x, 9); 1241 WRITE_ONCE(x, 9);
1323 } 1242 }
1324 1243
1325 If r1 = 0 and r2 = 9 at the end then P0's acc 1244 If r1 = 0 and r2 = 9 at the end then P0's accesses must have executed
1326 in program order. If the second load had exe 1245 in program order. If the second load had executed before the first
1327 then the x = 9 store must have been propagate 1246 then the x = 9 store must have been propagated to P0 before the first
1328 load executed, and so r1 would have been 9 ra 1247 load executed, and so r1 would have been 9 rather than 0. In this
1329 case there is a prop link from P0's first rea 1248 case there is a prop link from P0's first read event to its second,
1330 because P1's store overwrote the value read b 1249 because P1's store overwrote the value read by P0's first load, and
1331 P1's store propagated to P0 before P0's secon 1250 P1's store propagated to P0 before P0's second load executed.
1332 1251
1333 Less trivial examples of prop all involve fen 1252 Less trivial examples of prop all involve fences. Unlike the simple
1334 examples above, they can require that some in 1253 examples above, they can require that some instructions are executed
1335 out of program order. This next one should l 1254 out of program order. This next one should look familiar:
1336 1255
1337 int buf = 0, flag = 0; 1256 int buf = 0, flag = 0;
1338 1257
1339 P0() 1258 P0()
1340 { 1259 {
1341 WRITE_ONCE(buf, 1); 1260 WRITE_ONCE(buf, 1);
1342 smp_wmb(); 1261 smp_wmb();
1343 WRITE_ONCE(flag, 1); 1262 WRITE_ONCE(flag, 1);
1344 } 1263 }
1345 1264
1346 P1() 1265 P1()
1347 { 1266 {
1348 int r1; 1267 int r1;
1349 int r2; 1268 int r2;
1350 1269
1351 r1 = READ_ONCE(flag); 1270 r1 = READ_ONCE(flag);
1352 r2 = READ_ONCE(buf); 1271 r2 = READ_ONCE(buf);
1353 } 1272 }
1354 1273
1355 This is the MP pattern again, with an smp_wmb 1274 This is the MP pattern again, with an smp_wmb() fence between the two
1356 stores. If r1 = 1 and r2 = 0 at the end then 1275 stores. If r1 = 1 and r2 = 0 at the end then there is a prop link
1357 from P1's second load to its first (backwards 1276 from P1's second load to its first (backwards!). The reason is
1358 similar to the previous examples: The value P 1277 similar to the previous examples: The value P1 loads from buf gets
1359 overwritten by P0's store to buf, the fence g 1278 overwritten by P0's store to buf, the fence guarantees that the store
1360 to buf will propagate to P1 before the store 1279 to buf will propagate to P1 before the store to flag does, and the
1361 store to flag propagates to P1 before P1 read 1280 store to flag propagates to P1 before P1 reads flag.
1362 1281
1363 The prop link says that in order to obtain th 1282 The prop link says that in order to obtain the r1 = 1, r2 = 0 result,
1364 P1 must execute its second load before the fi 1283 P1 must execute its second load before the first. Indeed, if the load
1365 from flag were executed first, then the buf = 1284 from flag were executed first, then the buf = 1 store would already
1366 have propagated to P1 by the time P1's load f 1285 have propagated to P1 by the time P1's load from buf executed, so r2
1367 would have been 1 at the end, not 0. (The re 1286 would have been 1 at the end, not 0. (The reasoning holds even for
1368 Alpha, although the details are more complica 1287 Alpha, although the details are more complicated and we will not go
1369 into them.) 1288 into them.)
1370 1289
1371 But what if we put an smp_rmb() fence between 1290 But what if we put an smp_rmb() fence between P1's loads? The fence
1372 would force the two loads to be executed in p 1291 would force the two loads to be executed in program order, and it
1373 would generate a cycle in the hb relation: Th 1292 would generate a cycle in the hb relation: The fence would create a ppo
1374 link (hence an hb link) from the first load t 1293 link (hence an hb link) from the first load to the second, and the
1375 prop relation would give an hb link from the 1294 prop relation would give an hb link from the second load to the first.
1376 Since an instruction can't execute before its 1295 Since an instruction can't execute before itself, we are forced to
1377 conclude that if an smp_rmb() fence is added, 1296 conclude that if an smp_rmb() fence is added, the r1 = 1, r2 = 0
1378 outcome is impossible -- as it should be. 1297 outcome is impossible -- as it should be.
1379 1298
1380 The formal definition of the prop relation in 1299 The formal definition of the prop relation involves a coe or fre link,
1381 followed by an arbitrary number of cumul-fenc 1300 followed by an arbitrary number of cumul-fence links, ending with an
1382 rfe link. You can concoct more exotic exampl 1301 rfe link. You can concoct more exotic examples, containing more than
1383 one fence, although this quickly leads to dim 1302 one fence, although this quickly leads to diminishing returns in terms
1384 of complexity. For instance, here's an examp 1303 of complexity. For instance, here's an example containing a coe link
1385 followed by two cumul-fences and an rfe link, 1304 followed by two cumul-fences and an rfe link, utilizing the fact that
1386 release fences are A-cumulative: 1305 release fences are A-cumulative:
1387 1306
1388 int x, y, z; 1307 int x, y, z;
1389 1308
1390 P0() 1309 P0()
1391 { 1310 {
1392 int r0; 1311 int r0;
1393 1312
1394 WRITE_ONCE(x, 1); 1313 WRITE_ONCE(x, 1);
1395 r0 = READ_ONCE(z); 1314 r0 = READ_ONCE(z);
1396 } 1315 }
1397 1316
1398 P1() 1317 P1()
1399 { 1318 {
1400 WRITE_ONCE(x, 2); 1319 WRITE_ONCE(x, 2);
1401 smp_wmb(); 1320 smp_wmb();
1402 WRITE_ONCE(y, 1); 1321 WRITE_ONCE(y, 1);
1403 } 1322 }
1404 1323
1405 P2() 1324 P2()
1406 { 1325 {
1407 int r2; 1326 int r2;
1408 1327
1409 r2 = READ_ONCE(y); 1328 r2 = READ_ONCE(y);
1410 smp_store_release(&z, 1); 1329 smp_store_release(&z, 1);
1411 } 1330 }
1412 1331
1413 If x = 2, r0 = 1, and r2 = 1 after this code 1332 If x = 2, r0 = 1, and r2 = 1 after this code runs then there is a prop
1414 link from P0's store to its load. This is be 1333 link from P0's store to its load. This is because P0's store gets
1415 overwritten by P1's store since x = 2 at the 1334 overwritten by P1's store since x = 2 at the end (a coe link), the
1416 smp_wmb() ensures that P1's store to x propag 1335 smp_wmb() ensures that P1's store to x propagates to P2 before the
1417 store to y does (the first cumul-fence), the 1336 store to y does (the first cumul-fence), the store to y propagates to P2
1418 before P2's load and store execute, P2's smp_ 1337 before P2's load and store execute, P2's smp_store_release()
1419 guarantees that the stores to x and y both pr 1338 guarantees that the stores to x and y both propagate to P0 before the
1420 store to z does (the second cumul-fence), and 1339 store to z does (the second cumul-fence), and P0's load executes after the
1421 store to z has propagated to P0 (an rfe link) 1340 store to z has propagated to P0 (an rfe link).
1422 1341
1423 In summary, the fact that the hb relation lin 1342 In summary, the fact that the hb relation links memory access events
1424 in the order they execute means that it must 1343 in the order they execute means that it must not have cycles. This
1425 requirement is the content of the LKMM's "hap 1344 requirement is the content of the LKMM's "happens-before" axiom.
1426 1345
1427 The LKMM defines yet another relation connect 1346 The LKMM defines yet another relation connected to times of
1428 instruction execution, but it is not included 1347 instruction execution, but it is not included in hb. It relies on the
1429 particular properties of strong fences, which 1348 particular properties of strong fences, which we cover in the next
1430 section. 1349 section.
1431 1350
1432 1351
1433 THE PROPAGATES-BEFORE RELATION: pb 1352 THE PROPAGATES-BEFORE RELATION: pb
1434 ---------------------------------- 1353 ----------------------------------
1435 1354
1436 The propagates-before (pb) relation capitaliz 1355 The propagates-before (pb) relation capitalizes on the special
1437 features of strong fences. It links two even 1356 features of strong fences. It links two events E and F whenever some
1438 store is coherence-later than E and propagate 1357 store is coherence-later than E and propagates to every CPU and to RAM
1439 before F executes. The formal definition req 1358 before F executes. The formal definition requires that E be linked to
1440 F via a coe or fre link, an arbitrary number 1359 F via a coe or fre link, an arbitrary number of cumul-fences, an
1441 optional rfe link, a strong fence, and an arb 1360 optional rfe link, a strong fence, and an arbitrary number of hb
1442 links. Let's see how this definition works o 1361 links. Let's see how this definition works out.
1443 1362
1444 Consider first the case where E is a store (i 1363 Consider first the case where E is a store (implying that the sequence
1445 of links begins with coe). Then there are ev 1364 of links begins with coe). Then there are events W, X, Y, and Z such
1446 that: 1365 that:
1447 1366
1448 E ->coe W ->cumul-fence* X ->rfe? Y - 1367 E ->coe W ->cumul-fence* X ->rfe? Y ->strong-fence Z ->hb* F,
1449 1368
1450 where the * suffix indicates an arbitrary num 1369 where the * suffix indicates an arbitrary number of links of the
1451 specified type, and the ? suffix indicates th 1370 specified type, and the ? suffix indicates the link is optional (Y may
1452 be equal to X). Because of the cumul-fence l 1371 be equal to X). Because of the cumul-fence links, we know that W will
1453 propagate to Y's CPU before X does, hence bef 1372 propagate to Y's CPU before X does, hence before Y executes and hence
1454 before the strong fence executes. Because th 1373 before the strong fence executes. Because this fence is strong, we
1455 know that W will propagate to every CPU and t 1374 know that W will propagate to every CPU and to RAM before Z executes.
1456 And because of the hb links, we know that Z w 1375 And because of the hb links, we know that Z will execute before F.
1457 Thus W, which comes later than E in the coher 1376 Thus W, which comes later than E in the coherence order, will
1458 propagate to every CPU and to RAM before F ex 1377 propagate to every CPU and to RAM before F executes.
1459 1378
1460 The case where E is a load is exactly the sam 1379 The case where E is a load is exactly the same, except that the first
1461 link in the sequence is fre instead of coe. 1380 link in the sequence is fre instead of coe.
1462 1381
1463 The existence of a pb link from E to F implie 1382 The existence of a pb link from E to F implies that E must execute
1464 before F. To see why, suppose that F execute 1383 before F. To see why, suppose that F executed first. Then W would
1465 have propagated to E's CPU before E executed. 1384 have propagated to E's CPU before E executed. If E was a store, the
1466 memory subsystem would then be forced to make 1385 memory subsystem would then be forced to make E come after W in the
1467 coherence order, contradicting the fact that 1386 coherence order, contradicting the fact that E ->coe W. If E was a
1468 load, the memory subsystem would then be forc 1387 load, the memory subsystem would then be forced to satisfy E's read
1469 request with the value stored by W or an even 1388 request with the value stored by W or an even later store,
1470 contradicting the fact that E ->fre W. 1389 contradicting the fact that E ->fre W.
1471 1390
1472 A good example illustrating how pb works is t 1391 A good example illustrating how pb works is the SB pattern with strong
1473 fences: 1392 fences:
1474 1393
1475 int x = 0, y = 0; 1394 int x = 0, y = 0;
1476 1395
1477 P0() 1396 P0()
1478 { 1397 {
1479 int r0; 1398 int r0;
1480 1399
1481 WRITE_ONCE(x, 1); 1400 WRITE_ONCE(x, 1);
1482 smp_mb(); 1401 smp_mb();
1483 r0 = READ_ONCE(y); 1402 r0 = READ_ONCE(y);
1484 } 1403 }
1485 1404
1486 P1() 1405 P1()
1487 { 1406 {
1488 int r1; 1407 int r1;
1489 1408
1490 WRITE_ONCE(y, 1); 1409 WRITE_ONCE(y, 1);
1491 smp_mb(); 1410 smp_mb();
1492 r1 = READ_ONCE(x); 1411 r1 = READ_ONCE(x);
1493 } 1412 }
1494 1413
1495 If r0 = 0 at the end then there is a pb link 1414 If r0 = 0 at the end then there is a pb link from P0's load to P1's
1496 load: an fre link from P0's load to P1's stor 1415 load: an fre link from P0's load to P1's store (which overwrites the
1497 value read by P0), and a strong fence between 1416 value read by P0), and a strong fence between P1's store and its load.
1498 In this example, the sequences of cumul-fence 1417 In this example, the sequences of cumul-fence and hb links are empty.
1499 Note that this pb link is not included in hb 1418 Note that this pb link is not included in hb as an instance of prop,
1500 because it does not start and end on the same 1419 because it does not start and end on the same CPU.
1501 1420
1502 Similarly, if r1 = 0 at the end then there is 1421 Similarly, if r1 = 0 at the end then there is a pb link from P1's load
1503 to P0's. This means that if both r1 and r2 w 1422 to P0's. This means that if both r1 and r2 were 0 there would be a
1504 cycle in pb, which is not possible since an i 1423 cycle in pb, which is not possible since an instruction cannot execute
1505 before itself. Thus, adding smp_mb() fences 1424 before itself. Thus, adding smp_mb() fences to the SB pattern
1506 prevents the r0 = 0, r1 = 0 outcome. 1425 prevents the r0 = 0, r1 = 0 outcome.
1507 1426
1508 In summary, the fact that the pb relation lin 1427 In summary, the fact that the pb relation links events in the order
1509 they execute means that it cannot have cycles 1428 they execute means that it cannot have cycles. This requirement is
1510 the content of the LKMM's "propagation" axiom 1429 the content of the LKMM's "propagation" axiom.
1511 1430
1512 1431
1513 RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, r 1432 RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
1514 --------------------------------------------- 1433 ------------------------------------------------------------------------
1515 1434
1516 RCU (Read-Copy-Update) is a powerful synchron 1435 RCU (Read-Copy-Update) is a powerful synchronization mechanism. It
1517 rests on two concepts: grace periods and read 1436 rests on two concepts: grace periods and read-side critical sections.
1518 1437
1519 A grace period is the span of time occupied b 1438 A grace period is the span of time occupied by a call to
1520 synchronize_rcu(). A read-side critical sect 1439 synchronize_rcu(). A read-side critical section (or just critical
1521 section, for short) is a region of code delim 1440 section, for short) is a region of code delimited by rcu_read_lock()
1522 at the start and rcu_read_unlock() at the end 1441 at the start and rcu_read_unlock() at the end. Critical sections can
1523 be nested, although we won't make use of this 1442 be nested, although we won't make use of this fact.
1524 1443
1525 As far as memory models are concerned, RCU's 1444 As far as memory models are concerned, RCU's main feature is its
1526 Grace-Period Guarantee, which states that a c 1445 Grace-Period Guarantee, which states that a critical section can never
1527 span a full grace period. In more detail, th 1446 span a full grace period. In more detail, the Guarantee says:
1528 1447
1529 For any critical section C and any gr 1448 For any critical section C and any grace period G, at least
1530 one of the following statements must 1449 one of the following statements must hold:
1531 1450
1532 (1) C ends before G does, and in addition 1451 (1) C ends before G does, and in addition, every store that
1533 propagates to C's CPU before the end 1452 propagates to C's CPU before the end of C must propagate to
1534 every CPU before G ends. 1453 every CPU before G ends.
1535 1454
1536 (2) G starts before C does, and in additi 1455 (2) G starts before C does, and in addition, every store that
1537 propagates to G's CPU before the star 1456 propagates to G's CPU before the start of G must propagate
1538 to every CPU before C starts. 1457 to every CPU before C starts.
1539 1458
1540 In particular, it is not possible for a criti 1459 In particular, it is not possible for a critical section to both start
1541 before and end after a grace period. 1460 before and end after a grace period.
1542 1461
1543 Here is a simple example of RCU in action: 1462 Here is a simple example of RCU in action:
1544 1463
1545 int x, y; 1464 int x, y;
1546 1465
1547 P0() 1466 P0()
1548 { 1467 {
1549 rcu_read_lock(); 1468 rcu_read_lock();
1550 WRITE_ONCE(x, 1); 1469 WRITE_ONCE(x, 1);
1551 WRITE_ONCE(y, 1); 1470 WRITE_ONCE(y, 1);
1552 rcu_read_unlock(); 1471 rcu_read_unlock();
1553 } 1472 }
1554 1473
1555 P1() 1474 P1()
1556 { 1475 {
1557 int r1, r2; 1476 int r1, r2;
1558 1477
1559 r1 = READ_ONCE(x); 1478 r1 = READ_ONCE(x);
1560 synchronize_rcu(); 1479 synchronize_rcu();
1561 r2 = READ_ONCE(y); 1480 r2 = READ_ONCE(y);
1562 } 1481 }
1563 1482
1564 The Grace Period Guarantee tells us that when 1483 The Grace Period Guarantee tells us that when this code runs, it will
1565 never end with r1 = 1 and r2 = 0. The reason 1484 never end with r1 = 1 and r2 = 0. The reasoning is as follows. r1 = 1
1566 means that P0's store to x propagated to P1 b 1485 means that P0's store to x propagated to P1 before P1 called
1567 synchronize_rcu(), so P0's critical section m 1486 synchronize_rcu(), so P0's critical section must have started before
1568 P1's grace period, contrary to part (2) of th 1487 P1's grace period, contrary to part (2) of the Guarantee. On the
1569 other hand, r2 = 0 means that P0's store to y 1488 other hand, r2 = 0 means that P0's store to y, which occurs before the
1570 end of the critical section, did not propagat 1489 end of the critical section, did not propagate to P1 before the end of
1571 the grace period, contrary to part (1). Toge 1490 the grace period, contrary to part (1). Together the results violate
1572 the Guarantee. 1491 the Guarantee.
1573 1492
1574 In the kernel's implementations of RCU, the r 1493 In the kernel's implementations of RCU, the requirements for stores
1575 to propagate to every CPU are fulfilled by pl 1494 to propagate to every CPU are fulfilled by placing strong fences at
1576 suitable places in the RCU-related code. Thu 1495 suitable places in the RCU-related code. Thus, if a critical section
1577 starts before a grace period does then the cr 1496 starts before a grace period does then the critical section's CPU will
1578 execute an smp_mb() fence after the end of th 1497 execute an smp_mb() fence after the end of the critical section and
1579 some time before the grace period's synchroni 1498 some time before the grace period's synchronize_rcu() call returns.
1580 And if a critical section ends after a grace 1499 And if a critical section ends after a grace period does then the
1581 synchronize_rcu() routine will execute an smp 1500 synchronize_rcu() routine will execute an smp_mb() fence at its start
1582 and some time before the critical section's o 1501 and some time before the critical section's opening rcu_read_lock()
1583 executes. 1502 executes.
1584 1503
1585 What exactly do we mean by saying that a crit 1504 What exactly do we mean by saying that a critical section "starts
1586 before" or "ends after" a grace period? Some 1505 before" or "ends after" a grace period? Some aspects of the meaning
1587 are pretty obvious, as in the example above, 1506 are pretty obvious, as in the example above, but the details aren't
1588 entirely clear. The LKMM formalizes this not 1507 entirely clear. The LKMM formalizes this notion by means of the
1589 rcu-link relation. rcu-link encompasses a ve 1508 rcu-link relation. rcu-link encompasses a very general notion of
1590 "before": If E and F are RCU fence events (i. 1509 "before": If E and F are RCU fence events (i.e., rcu_read_lock(),
1591 rcu_read_unlock(), or synchronize_rcu()) then 1510 rcu_read_unlock(), or synchronize_rcu()) then among other things,
1592 E ->rcu-link F includes cases where E is po-b 1511 E ->rcu-link F includes cases where E is po-before some memory-access
1593 event X, F is po-after some memory-access eve 1512 event X, F is po-after some memory-access event Y, and we have any of
1594 X ->rfe Y, X ->co Y, or X ->fr Y. 1513 X ->rfe Y, X ->co Y, or X ->fr Y.
1595 1514
1596 The formal definition of the rcu-link relatio 1515 The formal definition of the rcu-link relation is more than a little
1597 obscure, and we won't give it here. It is cl 1516 obscure, and we won't give it here. It is closely related to the pb
1598 relation, and the details don't matter unless 1517 relation, and the details don't matter unless you want to comb through
1599 a somewhat lengthy formal proof. Pretty much 1518 a somewhat lengthy formal proof. Pretty much all you need to know
1600 about rcu-link is the information in the prec 1519 about rcu-link is the information in the preceding paragraph.
1601 1520
1602 The LKMM also defines the rcu-gp and rcu-rscs 1521 The LKMM also defines the rcu-gp and rcu-rscsi relations. They bring
1603 grace periods and read-side critical sections 1522 grace periods and read-side critical sections into the picture, in the
1604 following way: 1523 following way:
1605 1524
1606 E ->rcu-gp F means that E and F are i 1525 E ->rcu-gp F means that E and F are in fact the same event,
1607 and that event is a synchronize_rcu() 1526 and that event is a synchronize_rcu() fence (i.e., a grace
1608 period). 1527 period).
1609 1528
1610 E ->rcu-rscsi F means that E and F ar 1529 E ->rcu-rscsi F means that E and F are the rcu_read_unlock()
1611 and rcu_read_lock() fence events deli 1530 and rcu_read_lock() fence events delimiting some read-side
1612 critical section. (The 'i' at the en 1531 critical section. (The 'i' at the end of the name emphasizes
1613 that this relation is "inverted": It 1532 that this relation is "inverted": It links the end of the
1614 critical section to the start.) 1533 critical section to the start.)
1615 1534
1616 If we think of the rcu-link relation as stand 1535 If we think of the rcu-link relation as standing for an extended
1617 "before", then X ->rcu-gp Y ->rcu-link Z roug 1536 "before", then X ->rcu-gp Y ->rcu-link Z roughly says that X is a
1618 grace period which ends before Z begins. (In 1537 grace period which ends before Z begins. (In fact it covers more than
1619 this, because it also includes cases where so 1538 this, because it also includes cases where some store propagates to
1620 Z's CPU before Z begins but doesn't propagate 1539 Z's CPU before Z begins but doesn't propagate to some other CPU until
1621 after X ends.) Similarly, X ->rcu-rscsi Y -> 1540 after X ends.) Similarly, X ->rcu-rscsi Y ->rcu-link Z says that X is
1622 the end of a critical section which starts be 1541 the end of a critical section which starts before Z begins.
1623 1542
1624 The LKMM goes on to define the rcu-order rela 1543 The LKMM goes on to define the rcu-order relation as a sequence of
1625 rcu-gp and rcu-rscsi links separated by rcu-l 1544 rcu-gp and rcu-rscsi links separated by rcu-link links, in which the
1626 number of rcu-gp links is >= the number of rc 1545 number of rcu-gp links is >= the number of rcu-rscsi links. For
1627 example: 1546 example:
1628 1547
1629 X ->rcu-gp Y ->rcu-link Z ->rcu-rscsi 1548 X ->rcu-gp Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V
1630 1549
1631 would imply that X ->rcu-order V, because thi 1550 would imply that X ->rcu-order V, because this sequence contains two
1632 rcu-gp links and one rcu-rscsi link. (It als 1551 rcu-gp links and one rcu-rscsi link. (It also implies that
1633 X ->rcu-order T and Z ->rcu-order V.) On the 1552 X ->rcu-order T and Z ->rcu-order V.) On the other hand:
1634 1553
1635 X ->rcu-rscsi Y ->rcu-link Z ->rcu-rs 1554 X ->rcu-rscsi Y ->rcu-link Z ->rcu-rscsi T ->rcu-link U ->rcu-gp V
1636 1555
1637 does not imply X ->rcu-order V, because the s 1556 does not imply X ->rcu-order V, because the sequence contains only
1638 one rcu-gp link but two rcu-rscsi links. 1557 one rcu-gp link but two rcu-rscsi links.
1639 1558
1640 The rcu-order relation is important because t 1559 The rcu-order relation is important because the Grace Period Guarantee
1641 means that rcu-order links act kind of like s 1560 means that rcu-order links act kind of like strong fences. In
1642 particular, E ->rcu-order F implies not only 1561 particular, E ->rcu-order F implies not only that E begins before F
1643 ends, but also that any write po-before E wil 1562 ends, but also that any write po-before E will propagate to every CPU
1644 before any instruction po-after F can execute 1563 before any instruction po-after F can execute. (However, it does not
1645 imply that E must execute before F; in fact, 1564 imply that E must execute before F; in fact, each synchronize_rcu()
1646 fence event is linked to itself by rcu-order 1565 fence event is linked to itself by rcu-order as a degenerate case.)
1647 1566
1648 To prove this in full generality requires som 1567 To prove this in full generality requires some intellectual effort.
1649 We'll consider just a very simple case: 1568 We'll consider just a very simple case:
1650 1569
1651 G ->rcu-gp W ->rcu-link Z ->rcu-rscsi 1570 G ->rcu-gp W ->rcu-link Z ->rcu-rscsi F.
1652 1571
1653 This formula means that G and W are the same 1572 This formula means that G and W are the same event (a grace period),
1654 and there are events X, Y and a read-side cri 1573 and there are events X, Y and a read-side critical section C such that:
1655 1574
1656 1. G = W is po-before or equal to X; 1575 1. G = W is po-before or equal to X;
1657 1576
1658 2. X comes "before" Y in some sense ( 1577 2. X comes "before" Y in some sense (including rfe, co and fr);
1659 1578
1660 3. Y is po-before Z; 1579 3. Y is po-before Z;
1661 1580
1662 4. Z is the rcu_read_unlock() event m 1581 4. Z is the rcu_read_unlock() event marking the end of C;
1663 1582
1664 5. F is the rcu_read_lock() event mar 1583 5. F is the rcu_read_lock() event marking the start of C.
1665 1584
1666 From 1 - 4 we deduce that the grace period G 1585 From 1 - 4 we deduce that the grace period G ends before the critical
1667 section C. Then part (2) of the Grace Period 1586 section C. Then part (2) of the Grace Period Guarantee says not only
1668 that G starts before C does, but also that an 1587 that G starts before C does, but also that any write which executes on
1669 G's CPU before G starts must propagate to eve 1588 G's CPU before G starts must propagate to every CPU before C starts.
1670 In particular, the write propagates to every 1589 In particular, the write propagates to every CPU before F finishes
1671 executing and hence before any instruction po 1590 executing and hence before any instruction po-after F can execute.
1672 This sort of reasoning can be extended to han 1591 This sort of reasoning can be extended to handle all the situations
1673 covered by rcu-order. 1592 covered by rcu-order.
1674 1593
1675 The rcu-fence relation is a simple extension 1594 The rcu-fence relation is a simple extension of rcu-order. While
1676 rcu-order only links certain fence events (ca 1595 rcu-order only links certain fence events (calls to synchronize_rcu(),
1677 rcu_read_lock(), or rcu_read_unlock()), rcu-f 1596 rcu_read_lock(), or rcu_read_unlock()), rcu-fence links any events
1678 that are separated by an rcu-order link. Thi 1597 that are separated by an rcu-order link. This is analogous to the way
1679 the strong-fence relation links events that a 1598 the strong-fence relation links events that are separated by an
1680 smp_mb() fence event (as mentioned above, rcu 1599 smp_mb() fence event (as mentioned above, rcu-order links act kind of
1681 like strong fences). Written symbolically, X 1600 like strong fences). Written symbolically, X ->rcu-fence Y means
1682 there are fence events E and F such that: 1601 there are fence events E and F such that:
1683 1602
1684 X ->po E ->rcu-order F ->po Y. 1603 X ->po E ->rcu-order F ->po Y.
1685 1604
1686 From the discussion above, we see this implie 1605 From the discussion above, we see this implies not only that X
1687 executes before Y, but also (if X is a store) 1606 executes before Y, but also (if X is a store) that X propagates to
1688 every CPU before Y executes. Thus rcu-fence 1607 every CPU before Y executes. Thus rcu-fence is sort of a
1689 "super-strong" fence: Unlike the original str 1608 "super-strong" fence: Unlike the original strong fences (smp_mb() and
1690 synchronize_rcu()), rcu-fence is able to link 1609 synchronize_rcu()), rcu-fence is able to link events on different
1691 CPUs. (Perhaps this fact should lead us to s 1610 CPUs. (Perhaps this fact should lead us to say that rcu-fence isn't
1692 really a fence at all!) 1611 really a fence at all!)
1693 1612
1694 Finally, the LKMM defines the RCU-before (rb) 1613 Finally, the LKMM defines the RCU-before (rb) relation in terms of
1695 rcu-fence. This is done in essentially the s 1614 rcu-fence. This is done in essentially the same way as the pb
1696 relation was defined in terms of strong-fence 1615 relation was defined in terms of strong-fence. We will omit the
1697 details; the end result is that E ->rb F impl 1616 details; the end result is that E ->rb F implies E must execute
1698 before F, just as E ->pb F does (and for much 1617 before F, just as E ->pb F does (and for much the same reasons).
1699 1618
1700 Putting this all together, the LKMM expresses 1619 Putting this all together, the LKMM expresses the Grace Period
1701 Guarantee by requiring that the rb relation d 1620 Guarantee by requiring that the rb relation does not contain a cycle.
1702 Equivalently, this "rcu" axiom requires that 1621 Equivalently, this "rcu" axiom requires that there are no events E
1703 and F with E ->rcu-link F ->rcu-order E. Or 1622 and F with E ->rcu-link F ->rcu-order E. Or to put it a third way,
1704 the axiom requires that there are no cycles c 1623 the axiom requires that there are no cycles consisting of rcu-gp and
1705 rcu-rscsi alternating with rcu-link, where th 1624 rcu-rscsi alternating with rcu-link, where the number of rcu-gp links
1706 is >= the number of rcu-rscsi links. 1625 is >= the number of rcu-rscsi links.
1707 1626
1708 Justifying the axiom isn't easy, but it is in 1627 Justifying the axiom isn't easy, but it is in fact a valid
1709 formalization of the Grace Period Guarantee. 1628 formalization of the Grace Period Guarantee. We won't attempt to go
1710 through the detailed argument, but the follow 1629 through the detailed argument, but the following analysis gives a
1711 taste of what is involved. Suppose both part 1630 taste of what is involved. Suppose both parts of the Guarantee are
1712 violated: A critical section starts before a 1631 violated: A critical section starts before a grace period, and some
1713 store propagates to the critical section's CP 1632 store propagates to the critical section's CPU before the end of the
1714 critical section but doesn't propagate to som 1633 critical section but doesn't propagate to some other CPU until after
1715 the end of the grace period. 1634 the end of the grace period.
1716 1635
1717 Putting symbols to these ideas, let L and U b 1636 Putting symbols to these ideas, let L and U be the rcu_read_lock() and
1718 rcu_read_unlock() fence events delimiting the 1637 rcu_read_unlock() fence events delimiting the critical section in
1719 question, and let S be the synchronize_rcu() 1638 question, and let S be the synchronize_rcu() fence event for the grace
1720 period. Saying that the critical section sta 1639 period. Saying that the critical section starts before S means there
1721 are events Q and R where Q is po-after L (whi 1640 are events Q and R where Q is po-after L (which marks the start of the
1722 critical section), Q is "before" R in the sen 1641 critical section), Q is "before" R in the sense used by the rcu-link
1723 relation, and R is po-before the grace period 1642 relation, and R is po-before the grace period S. Thus we have:
1724 1643
1725 L ->rcu-link S. 1644 L ->rcu-link S.
1726 1645
1727 Let W be the store mentioned above, let Y com 1646 Let W be the store mentioned above, let Y come before the end of the
1728 critical section and witness that W propagate 1647 critical section and witness that W propagates to the critical
1729 section's CPU by reading from W, and let Z on 1648 section's CPU by reading from W, and let Z on some arbitrary CPU be a
1730 witness that W has not propagated to that CPU 1649 witness that W has not propagated to that CPU, where Z happens after
1731 some event X which is po-after S. Symbolical 1650 some event X which is po-after S. Symbolically, this amounts to:
1732 1651
1733 S ->po X ->hb* Z ->fr W ->rf Y ->po U 1652 S ->po X ->hb* Z ->fr W ->rf Y ->po U.
1734 1653
1735 The fr link from Z to W indicates that W has 1654 The fr link from Z to W indicates that W has not propagated to Z's CPU
1736 at the time that Z executes. From this, it c 1655 at the time that Z executes. From this, it can be shown (see the
1737 discussion of the rcu-link relation earlier) 1656 discussion of the rcu-link relation earlier) that S and U are related
1738 by rcu-link: 1657 by rcu-link:
1739 1658
1740 S ->rcu-link U. 1659 S ->rcu-link U.
1741 1660
1742 Since S is a grace period we have S ->rcu-gp 1661 Since S is a grace period we have S ->rcu-gp S, and since L and U are
1743 the start and end of the critical section C w 1662 the start and end of the critical section C we have U ->rcu-rscsi L.
1744 From this we obtain: 1663 From this we obtain:
1745 1664
1746 S ->rcu-gp S ->rcu-link U ->rcu-rscsi 1665 S ->rcu-gp S ->rcu-link U ->rcu-rscsi L ->rcu-link S,
1747 1666
1748 a forbidden cycle. Thus the "rcu" axiom rule 1667 a forbidden cycle. Thus the "rcu" axiom rules out this violation of
1749 the Grace Period Guarantee. 1668 the Grace Period Guarantee.
1750 1669
1751 For something a little more down-to-earth, le 1670 For something a little more down-to-earth, let's see how the axiom
1752 works out in practice. Consider the RCU code 1671 works out in practice. Consider the RCU code example from above, this
1753 time with statement labels added: 1672 time with statement labels added:
1754 1673
1755 int x, y; 1674 int x, y;
1756 1675
1757 P0() 1676 P0()
1758 { 1677 {
1759 L: rcu_read_lock(); 1678 L: rcu_read_lock();
1760 X: WRITE_ONCE(x, 1); 1679 X: WRITE_ONCE(x, 1);
1761 Y: WRITE_ONCE(y, 1); 1680 Y: WRITE_ONCE(y, 1);
1762 U: rcu_read_unlock(); 1681 U: rcu_read_unlock();
1763 } 1682 }
1764 1683
1765 P1() 1684 P1()
1766 { 1685 {
1767 int r1, r2; 1686 int r1, r2;
1768 1687
1769 Z: r1 = READ_ONCE(x); 1688 Z: r1 = READ_ONCE(x);
1770 S: synchronize_rcu(); 1689 S: synchronize_rcu();
1771 W: r2 = READ_ONCE(y); 1690 W: r2 = READ_ONCE(y);
1772 } 1691 }
1773 1692
1774 1693
1775 If r2 = 0 at the end then P0's store at Y ove 1694 If r2 = 0 at the end then P0's store at Y overwrites the value that
1776 P1's load at W reads from, so we have W ->fre 1695 P1's load at W reads from, so we have W ->fre Y. Since S ->po W and
1777 also Y ->po U, we get S ->rcu-link U. In add 1696 also Y ->po U, we get S ->rcu-link U. In addition, S ->rcu-gp S
1778 because S is a grace period. 1697 because S is a grace period.
1779 1698
1780 If r1 = 1 at the end then P1's load at Z read 1699 If r1 = 1 at the end then P1's load at Z reads from P0's store at X,
1781 so we have X ->rfe Z. Together with L ->po X 1700 so we have X ->rfe Z. Together with L ->po X and Z ->po S, this
1782 yields L ->rcu-link S. And since L and U are 1701 yields L ->rcu-link S. And since L and U are the start and end of a
1783 critical section, we have U ->rcu-rscsi L. 1702 critical section, we have U ->rcu-rscsi L.
1784 1703
1785 Then U ->rcu-rscsi L ->rcu-link S ->rcu-gp S 1704 Then U ->rcu-rscsi L ->rcu-link S ->rcu-gp S ->rcu-link U is a
1786 forbidden cycle, violating the "rcu" axiom. 1705 forbidden cycle, violating the "rcu" axiom. Hence the outcome is not
1787 allowed by the LKMM, as we would expect. 1706 allowed by the LKMM, as we would expect.
1788 1707
1789 For contrast, let's see what can happen in a 1708 For contrast, let's see what can happen in a more complicated example:
1790 1709
1791 int x, y, z; 1710 int x, y, z;
1792 1711
1793 P0() 1712 P0()
1794 { 1713 {
1795 int r0; 1714 int r0;
1796 1715
1797 L0: rcu_read_lock(); 1716 L0: rcu_read_lock();
1798 r0 = READ_ONCE(x); 1717 r0 = READ_ONCE(x);
1799 WRITE_ONCE(y, 1); 1718 WRITE_ONCE(y, 1);
1800 U0: rcu_read_unlock(); 1719 U0: rcu_read_unlock();
1801 } 1720 }
1802 1721
1803 P1() 1722 P1()
1804 { 1723 {
1805 int r1; 1724 int r1;
1806 1725
1807 r1 = READ_ONCE(y); 1726 r1 = READ_ONCE(y);
1808 S1: synchronize_rcu(); 1727 S1: synchronize_rcu();
1809 WRITE_ONCE(z, 1); 1728 WRITE_ONCE(z, 1);
1810 } 1729 }
1811 1730
1812 P2() 1731 P2()
1813 { 1732 {
1814 int r2; 1733 int r2;
1815 1734
1816 L2: rcu_read_lock(); 1735 L2: rcu_read_lock();
1817 r2 = READ_ONCE(z); 1736 r2 = READ_ONCE(z);
1818 WRITE_ONCE(x, 1); 1737 WRITE_ONCE(x, 1);
1819 U2: rcu_read_unlock(); 1738 U2: rcu_read_unlock();
1820 } 1739 }
1821 1740
1822 If r0 = r1 = r2 = 1 at the end, then similar 1741 If r0 = r1 = r2 = 1 at the end, then similar reasoning to before shows
1823 that U0 ->rcu-rscsi L0 ->rcu-link S1 ->rcu-gp 1742 that U0 ->rcu-rscsi L0 ->rcu-link S1 ->rcu-gp S1 ->rcu-link U2 ->rcu-rscsi
1824 L2 ->rcu-link U0. However this cycle is not 1743 L2 ->rcu-link U0. However this cycle is not forbidden, because the
1825 sequence of relations contains fewer instance 1744 sequence of relations contains fewer instances of rcu-gp (one) than of
1826 rcu-rscsi (two). Consequently the outcome is 1745 rcu-rscsi (two). Consequently the outcome is allowed by the LKMM.
1827 The following instruction timing diagram show 1746 The following instruction timing diagram shows how it might actually
1828 occur: 1747 occur:
1829 1748
1830 P0 P1 1749 P0 P1 P2
1831 -------------------- -------------------- 1750 -------------------- -------------------- --------------------
1832 rcu_read_lock() 1751 rcu_read_lock()
1833 WRITE_ONCE(y, 1) 1752 WRITE_ONCE(y, 1)
1834 r1 = READ_ONCE(y) 1753 r1 = READ_ONCE(y)
1835 synchronize_rcu() sta 1754 synchronize_rcu() starts
1836 . 1755 . rcu_read_lock()
1837 . 1756 . WRITE_ONCE(x, 1)
1838 r0 = READ_ONCE(x) . 1757 r0 = READ_ONCE(x) .
1839 rcu_read_unlock() . 1758 rcu_read_unlock() .
1840 synchronize_rcu() end 1759 synchronize_rcu() ends
1841 WRITE_ONCE(z, 1) 1760 WRITE_ONCE(z, 1)
1842 1761 r2 = READ_ONCE(z)
1843 1762 rcu_read_unlock()
1844 1763
1845 This requires P0 and P2 to execute their load 1764 This requires P0 and P2 to execute their loads and stores out of
1846 program order, but of course they are allowed 1765 program order, but of course they are allowed to do so. And as you
1847 can see, the Grace Period Guarantee is not vi 1766 can see, the Grace Period Guarantee is not violated: The critical
1848 section in P0 both starts before P1's grace p 1767 section in P0 both starts before P1's grace period does and ends
1849 before it does, and the critical section in P 1768 before it does, and the critical section in P2 both starts after P1's
1850 grace period does and ends after it does. 1769 grace period does and ends after it does.
1851 1770
1852 The LKMM supports SRCU (Sleepable Read-Copy-U !! 1771 Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
1853 normal RCU. The ideas involved are much the !! 1772 addition to normal RCU. The ideas involved are much the same as
1854 relations srcu-gp and srcu-rscsi added to rep !! 1773 above, with new relations srcu-gp and srcu-rscsi added to represent
1855 and read-side critical sections. However, th !! 1774 SRCU grace periods and read-side critical sections. There is a
1856 differences between RCU read-side critical se !! 1775 restriction on the srcu-gp and srcu-rscsi links that can appear in an
1857 counterparts, as described in the next sectio !! 1776 rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
1858 !! 1777 links having the same SRCU domain with proper nesting); the details
1859 !! 1778 are relatively unimportant.
1860 SRCU READ-SIDE CRITICAL SECTIONS <<
1861 -------------------------------- <<
1862 <<
1863 The LKMM uses the srcu-rscsi relation to mode <<
1864 sections. They differ from RCU read-side cri <<
1865 following respects: <<
1866 <<
1867 1. Unlike the analogous RCU primitives, <<
1868 srcu_read_lock(), and srcu_read_unloc <<
1869 struct srcu_struct as an argument. T <<
1870 an SRCU domain, and calls linked by s <<
1871 same domain. Read-side critical sect <<
1872 associated with different domains are <<
1873 another; the SRCU version of the RCU <<
1874 to pairs of critical sections and gra <<
1875 same domain. <<
1876 <<
1877 2. srcu_read_lock() returns a value, cal <<
1878 be passed to the matching srcu_read_u <<
1879 rcu_read_lock() and rcu_read_unlock() <<
1880 call does not always have to match th <<
1881 srcu_read_unlock(). In fact, it is p <<
1882 read-side critical sections to overla <<
1883 following example (where s is an srcu <<
1884 are integer variables): <<
1885 <<
1886 idx1 = srcu_read_lock(&s); <<
1887 idx2 = srcu_read_lock(&s); <<
1888 srcu_read_unlock(&s, idx1); <<
1889 srcu_read_unlock(&s, idx2); <<
1890 <<
1891 The matching is determined entirely b <<
1892 index value. By contrast, if the cal <<
1893 rcu_read_lock() and rcu_read_unlock() <<
1894 created two nested (fully overlapping <<
1895 sections: an inner one and an outer o <<
1896 <<
1897 3. The srcu_down_read() and srcu_up_read <<
1898 exactly like srcu_read_lock() and src <<
1899 that matching calls don't have to exe <<
1900 (The names are meant to be suggestive <<
1901 semaphores.) Since the matching is d <<
1902 pointer and index value, these primit <<
1903 an SRCU read-side critical section to <<
1904 on another, so to speak. <<
1905 <<
1906 In order to account for these properties of S <<
1907 srcu_read_lock() as a special type of load ev <<
1908 appropriate, since it takes a memory location <<
1909 a value, just as a load does) and srcu_read_u <<
1910 of store event (again appropriate, since it t <<
1911 memory location and a value). These loads an <<
1912 belonging to the "srcu-lock" and "srcu-unlock <<
1913 respectively. <<
1914 <<
1915 This approach allows the LKMM to tell whether <<
1916 associated with the same SRCU domain, simply <<
1917 access the same memory location (i.e., they a <<
1918 relation). It also gives a way to tell which <<
1919 particular lock, by checking for the presence <<
1920 from the load (srcu-lock) to the store (srcu- <<
1921 given the situation outlined earlier (with st <<
1922 <<
1923 A: idx1 = srcu_read_lock(&s); <<
1924 B: idx2 = srcu_read_lock(&s); <<
1925 C: srcu_read_unlock(&s, idx1); <<
1926 D: srcu_read_unlock(&s, idx2); <<
1927 <<
1928 the LKMM will treat A and B as loads from s y <<
1929 idx1 and idx2 respectively. Similarly, it wi <<
1930 though they stored the values from idx1 and i <<
1931 is much as if we had written: <<
1932 <<
1933 A: idx1 = READ_ONCE(s); <<
1934 B: idx2 = READ_ONCE(s); <<
1935 C: WRITE_ONCE(s, idx1); <<
1936 D: WRITE_ONCE(s, idx2); <<
1937 <<
1938 except for the presence of the special srcu-l <<
1939 annotations. You can see at once that we hav <<
1940 B ->data D. These dependencies tell the LKMM <<
1941 srcu-unlock event matching srcu-lock event A, <<
1942 srcu-unlock event matching srcu-lock event B. <<
1943 <<
1944 This approach is admittedly a hack, and it ha <<
1945 to problems. For example, in: <<
1946 <<
1947 idx1 = srcu_read_lock(&s); <<
1948 srcu_read_unlock(&s, idx1); <<
1949 idx2 = srcu_read_lock(&s); <<
1950 srcu_read_unlock(&s, idx2); <<
1951 <<
1952 the LKMM will believe that idx2 must have the <<
1953 since it reads from the immediately preceding <<
1954 Fortunately this won't matter, assuming that <<
1955 anything with SRCU index values other than pa <<
1956 srcu_read_unlock() or srcu_up_read() calls. <<
1957 <<
1958 However, sometimes it is necessary to store a <<
1959 shared variable temporarily. In fact, this i <<
1960 srcu_down_read() to pass the index it gets to <<
1961 on a different CPU. In more detail, we might <<
1962 <<
1963 struct srcu_struct s; <<
1964 int x; <<
1965 <<
1966 P0() <<
1967 { <<
1968 int r0; <<
1969 <<
1970 A: r0 = srcu_down_read(&s); <<
1971 B: WRITE_ONCE(x, r0); <<
1972 } <<
1973 <<
1974 P1() <<
1975 { <<
1976 int r1; <<
1977 <<
1978 C: r1 = READ_ONCE(x); <<
1979 D: srcu_up_read(&s, r1); <<
1980 } <<
1981 <<
1982 Assuming that P1 executes after P0 and does r <<
1983 stored in x, we can write this (using bracket <<
1984 annotations) as: <<
1985 <<
1986 A[srcu-lock] ->data B[once] ->rf C[on <<
1987 <<
1988 The LKMM defines a carry-srcu-data relation t <<
1989 it permits an arbitrarily long sequence of <<
1990 <<
1991 data ; rf <<
1992 <<
1993 pairs (that is, a data link followed by an rf <<
1994 an srcu-lock event and the final data depende <<
1995 matching srcu-unlock event. carry-srcu-data <<
1996 need to ensure that none of the intermediate <<
1997 sequence are instances of srcu-unlock. This <<
1998 pattern like the one above: <<
1999 <<
2000 A: idx1 = srcu_read_lock(&s); <<
2001 B: srcu_read_unlock(&s, idx1); <<
2002 C: idx2 = srcu_read_lock(&s); <<
2003 D: srcu_read_unlock(&s, idx2); <<
2004 <<
2005 the LKMM treats B as a store to the variable <<
2006 that variable, creating an undesirable rf lin <<
2007 <<
2008 A ->data B ->rf C ->data D. <<
2009 <<
2010 This would cause carry-srcu-data to mistakenl <<
2011 dependency from A to D, giving the impression <<
2012 srcu-unlock event matching A's srcu-lock. To <<
2013 carry-srcu-data does not accept sequences in <<
2014 the intermediate ->data links (B above) is an <<
2015 1779
2016 1780
2017 LOCKING 1781 LOCKING
2018 ------- 1782 -------
2019 1783
2020 The LKMM includes locking. In fact, there is 1784 The LKMM includes locking. In fact, there is special code for locking
2021 in the formal model, added in order to make t 1785 in the formal model, added in order to make tools run faster.
2022 However, this special code is intended to be 1786 However, this special code is intended to be more or less equivalent
2023 to concepts we have already covered. A spinl 1787 to concepts we have already covered. A spinlock_t variable is treated
2024 the same as an int, and spin_lock(&s) is trea 1788 the same as an int, and spin_lock(&s) is treated almost the same as:
2025 1789
2026 while (cmpxchg_acquire(&s, 0, 1) != 0 1790 while (cmpxchg_acquire(&s, 0, 1) != 0)
2027 cpu_relax(); 1791 cpu_relax();
2028 1792
2029 This waits until s is equal to 0 and then ato 1793 This waits until s is equal to 0 and then atomically sets it to 1,
2030 and the read part of the cmpxchg operation ac 1794 and the read part of the cmpxchg operation acts as an acquire fence.
2031 An alternate way to express the same thing wo 1795 An alternate way to express the same thing would be:
2032 1796
2033 r = xchg_acquire(&s, 1); 1797 r = xchg_acquire(&s, 1);
2034 1798
2035 along with a requirement that at the end, r = 1799 along with a requirement that at the end, r = 0. Similarly,
2036 spin_trylock(&s) is treated almost the same a 1800 spin_trylock(&s) is treated almost the same as:
2037 1801
2038 return !cmpxchg_acquire(&s, 0, 1); 1802 return !cmpxchg_acquire(&s, 0, 1);
2039 1803
2040 which atomically sets s to 1 if it is current 1804 which atomically sets s to 1 if it is currently equal to 0 and returns
2041 true if it succeeds (the read part of the cmp 1805 true if it succeeds (the read part of the cmpxchg operation acts as an
2042 acquire fence only if the operation is succes 1806 acquire fence only if the operation is successful). spin_unlock(&s)
2043 is treated almost the same as: 1807 is treated almost the same as:
2044 1808
2045 smp_store_release(&s, 0); 1809 smp_store_release(&s, 0);
2046 1810
2047 The "almost" qualifiers above need some expla 1811 The "almost" qualifiers above need some explanation. In the LKMM, the
2048 store-release in a spin_unlock() and the load 1812 store-release in a spin_unlock() and the load-acquire which forms the
2049 first half of the atomic rmw update in a spin 1813 first half of the atomic rmw update in a spin_lock() or a successful
2050 spin_trylock() -- we can call these things lo 1814 spin_trylock() -- we can call these things lock-releases and
2051 lock-acquires -- have two properties beyond t 1815 lock-acquires -- have two properties beyond those of ordinary releases
2052 and acquires. 1816 and acquires.
2053 1817
2054 First, when a lock-acquire reads from or is p !! 1818 First, when a lock-acquire reads from a lock-release, the LKMM
2055 the LKMM requires that every instruction po-b !! 1819 requires that every instruction po-before the lock-release must
2056 must execute before any instruction po-after !! 1820 execute before any instruction po-after the lock-acquire. This would
2057 would naturally hold if the release and acqui !! 1821 naturally hold if the release and acquire operations were on different
2058 different CPUs and accessed the same lock var !! 1822 CPUs, but the LKMM says it holds even when they are on the same CPU.
2059 it also holds when they are on the same CPU, !! 1823 For example:
2060 different lock variables. For example: <<
2061 1824
2062 int x, y; 1825 int x, y;
2063 spinlock_t s, t; !! 1826 spinlock_t s;
2064 1827
2065 P0() 1828 P0()
2066 { 1829 {
2067 int r1, r2; 1830 int r1, r2;
2068 1831
2069 spin_lock(&s); 1832 spin_lock(&s);
2070 r1 = READ_ONCE(x); 1833 r1 = READ_ONCE(x);
2071 spin_unlock(&s); 1834 spin_unlock(&s);
2072 spin_lock(&t); !! 1835 spin_lock(&s);
2073 r2 = READ_ONCE(y); 1836 r2 = READ_ONCE(y);
2074 spin_unlock(&t); !! 1837 spin_unlock(&s);
2075 } 1838 }
2076 1839
2077 P1() 1840 P1()
2078 { 1841 {
2079 WRITE_ONCE(y, 1); 1842 WRITE_ONCE(y, 1);
2080 smp_wmb(); 1843 smp_wmb();
2081 WRITE_ONCE(x, 1); 1844 WRITE_ONCE(x, 1);
2082 } 1845 }
2083 1846
2084 Here the second spin_lock() is po-after the f !! 1847 Here the second spin_lock() reads from the first spin_unlock(), and
2085 therefore the load of x must execute before t !! 1848 therefore the load of x must execute before the load of y. Thus we
2086 the two locking operations use different lock !! 1849 cannot have r1 = 1 and r2 = 0 at the end (this is an instance of the
2087 r1 = 1 and r2 = 0 at the end (this is an inst !! 1850 MP pattern).
2088 1851
2089 This requirement does not apply to ordinary r 1852 This requirement does not apply to ordinary release and acquire
2090 fences, only to lock-related operations. For 1853 fences, only to lock-related operations. For instance, suppose P0()
2091 in the example had been written as: 1854 in the example had been written as:
2092 1855
2093 P0() 1856 P0()
2094 { 1857 {
2095 int r1, r2, r3; 1858 int r1, r2, r3;
2096 1859
2097 r1 = READ_ONCE(x); 1860 r1 = READ_ONCE(x);
2098 smp_store_release(&s, 1); 1861 smp_store_release(&s, 1);
2099 r3 = smp_load_acquire(&s); 1862 r3 = smp_load_acquire(&s);
2100 r2 = READ_ONCE(y); 1863 r2 = READ_ONCE(y);
2101 } 1864 }
2102 1865
2103 Then the CPU would be allowed to forward the 1866 Then the CPU would be allowed to forward the s = 1 value from the
2104 smp_store_release() to the smp_load_acquire() 1867 smp_store_release() to the smp_load_acquire(), executing the
2105 instructions in the following order: 1868 instructions in the following order:
2106 1869
2107 r3 = smp_load_acquire(&s); 1870 r3 = smp_load_acquire(&s); // Obtains r3 = 1
2108 r2 = READ_ONCE(y); 1871 r2 = READ_ONCE(y);
2109 r1 = READ_ONCE(x); 1872 r1 = READ_ONCE(x);
2110 smp_store_release(&s, 1); 1873 smp_store_release(&s, 1); // Value is forwarded
2111 1874
2112 and thus it could load y before x, obtaining 1875 and thus it could load y before x, obtaining r2 = 0 and r1 = 1.
2113 1876
2114 Second, when a lock-acquire reads from or is !! 1877 Second, when a lock-acquire reads from a lock-release, and some other
2115 and some other stores W and W' occur po-befor !! 1878 stores W and W' occur po-before the lock-release and po-after the
2116 po-after the lock-acquire respectively, the L !! 1879 lock-acquire respectively, the LKMM requires that W must propagate to
2117 propagate to each CPU before W' does. For ex !! 1880 each CPU before W' does. For example, consider:
2118 1881
2119 int x, y; 1882 int x, y;
2120 spinlock_t s; !! 1883 spinlock_t x;
2121 1884
2122 P0() 1885 P0()
2123 { 1886 {
2124 spin_lock(&s); 1887 spin_lock(&s);
2125 WRITE_ONCE(x, 1); 1888 WRITE_ONCE(x, 1);
2126 spin_unlock(&s); 1889 spin_unlock(&s);
2127 } 1890 }
2128 1891
2129 P1() 1892 P1()
2130 { 1893 {
2131 int r1; 1894 int r1;
2132 1895
2133 spin_lock(&s); 1896 spin_lock(&s);
2134 r1 = READ_ONCE(x); 1897 r1 = READ_ONCE(x);
2135 WRITE_ONCE(y, 1); 1898 WRITE_ONCE(y, 1);
2136 spin_unlock(&s); 1899 spin_unlock(&s);
2137 } 1900 }
2138 1901
2139 P2() 1902 P2()
2140 { 1903 {
2141 int r2, r3; 1904 int r2, r3;
2142 1905
2143 r2 = READ_ONCE(y); 1906 r2 = READ_ONCE(y);
2144 smp_rmb(); 1907 smp_rmb();
2145 r3 = READ_ONCE(x); 1908 r3 = READ_ONCE(x);
2146 } 1909 }
2147 1910
2148 If r1 = 1 at the end then the spin_lock() in 1911 If r1 = 1 at the end then the spin_lock() in P1 must have read from
2149 the spin_unlock() in P0. Hence the store to 1912 the spin_unlock() in P0. Hence the store to x must propagate to P2
2150 before the store to y does, so we cannot have !! 1913 before the store to y does, so we cannot have r2 = 1 and r3 = 0.
2151 if P1 had used a lock variable different from <<
2152 propagated in either order. (On the other ha <<
2153 P1 had all executed on a single CPU, as in th <<
2154 one, then the writes would have propagated in <<
2155 critical sections used different lock variabl <<
2156 1914
2157 These two special requirements for lock-relea 1915 These two special requirements for lock-release and lock-acquire do
2158 not arise from the operational model. Nevert 1916 not arise from the operational model. Nevertheless, kernel developers
2159 have come to expect and rely on them because 1917 have come to expect and rely on them because they do hold on all
2160 architectures supported by the Linux kernel, 1918 architectures supported by the Linux kernel, albeit for various
2161 differing reasons. 1919 differing reasons.
2162 1920
2163 1921
2164 PLAIN ACCESSES AND DATA RACES 1922 PLAIN ACCESSES AND DATA RACES
2165 ----------------------------- 1923 -----------------------------
2166 1924
2167 In the LKMM, memory accesses such as READ_ONC 1925 In the LKMM, memory accesses such as READ_ONCE(x), atomic_inc(&y),
2168 smp_load_acquire(&z), and so on are collectiv 1926 smp_load_acquire(&z), and so on are collectively referred to as
2169 "marked" accesses, because they are all annot 1927 "marked" accesses, because they are all annotated with special
2170 operations of one kind or another. Ordinary 1928 operations of one kind or another. Ordinary C-language memory
2171 accesses such as x or y = 0 are simply called 1929 accesses such as x or y = 0 are simply called "plain" accesses.
2172 1930
2173 Early versions of the LKMM had nothing to say 1931 Early versions of the LKMM had nothing to say about plain accesses.
2174 The C standard allows compilers to assume tha 1932 The C standard allows compilers to assume that the variables affected
2175 by plain accesses are not concurrently read o 1933 by plain accesses are not concurrently read or written by any other
2176 threads or CPUs. This leaves compilers free 1934 threads or CPUs. This leaves compilers free to implement all manner
2177 of transformations or optimizations of code c 1935 of transformations or optimizations of code containing plain accesses,
2178 making such code very difficult for a memory 1936 making such code very difficult for a memory model to handle.
2179 1937
2180 Here is just one example of a possible pitfal 1938 Here is just one example of a possible pitfall:
2181 1939
2182 int a = 6; 1940 int a = 6;
2183 int *x = &a; 1941 int *x = &a;
2184 1942
2185 P0() 1943 P0()
2186 { 1944 {
2187 int *r1; 1945 int *r1;
2188 int r2 = 0; 1946 int r2 = 0;
2189 1947
2190 r1 = x; 1948 r1 = x;
2191 if (r1 != NULL) 1949 if (r1 != NULL)
2192 r2 = READ_ONCE(*r1); 1950 r2 = READ_ONCE(*r1);
2193 } 1951 }
2194 1952
2195 P1() 1953 P1()
2196 { 1954 {
2197 WRITE_ONCE(x, NULL); 1955 WRITE_ONCE(x, NULL);
2198 } 1956 }
2199 1957
2200 On the face of it, one would expect that when 1958 On the face of it, one would expect that when this code runs, the only
2201 possible final values for r2 are 6 and 0, dep 1959 possible final values for r2 are 6 and 0, depending on whether or not
2202 P1's store to x propagates to P0 before P0's 1960 P1's store to x propagates to P0 before P0's load from x executes.
2203 But since P0's load from x is a plain access, 1961 But since P0's load from x is a plain access, the compiler may decide
2204 to carry out the load twice (for the comparis 1962 to carry out the load twice (for the comparison against NULL, then again
2205 for the READ_ONCE()) and eliminate the tempor 1963 for the READ_ONCE()) and eliminate the temporary variable r1. The
2206 object code generated for P0 could therefore 1964 object code generated for P0 could therefore end up looking rather
2207 like this: 1965 like this:
2208 1966
2209 P0() 1967 P0()
2210 { 1968 {
2211 int r2 = 0; 1969 int r2 = 0;
2212 1970
2213 if (x != NULL) 1971 if (x != NULL)
2214 r2 = READ_ONCE(*x); 1972 r2 = READ_ONCE(*x);
2215 } 1973 }
2216 1974
2217 And now it is obvious that this code runs the 1975 And now it is obvious that this code runs the risk of dereferencing a
2218 NULL pointer, because P1's store to x might p 1976 NULL pointer, because P1's store to x might propagate to P0 after the
2219 test against NULL has been made but before th 1977 test against NULL has been made but before the READ_ONCE() executes.
2220 If the original code had said "r1 = READ_ONCE 1978 If the original code had said "r1 = READ_ONCE(x)" instead of "r1 = x",
2221 the compiler would not have performed this op 1979 the compiler would not have performed this optimization and there
2222 would be no possibility of a NULL-pointer der 1980 would be no possibility of a NULL-pointer dereference.
2223 1981
2224 Given the possibility of transformations like 1982 Given the possibility of transformations like this one, the LKMM
2225 doesn't try to predict all possible outcomes 1983 doesn't try to predict all possible outcomes of code containing plain
2226 accesses. It is instead content to determine 1984 accesses. It is instead content to determine whether the code
2227 violates the compiler's assumptions, which wo 1985 violates the compiler's assumptions, which would render the ultimate
2228 outcome undefined. 1986 outcome undefined.
2229 1987
2230 In technical terms, the compiler is allowed t 1988 In technical terms, the compiler is allowed to assume that when the
2231 program executes, there will not be any data 1989 program executes, there will not be any data races. A "data race"
2232 occurs when there are two memory accesses suc !! 1990 occurs when two conflicting memory accesses execute concurrently;
2233 !! 1991 two memory accesses "conflict" if:
2234 1. they access the same location, <<
2235 1992
2236 2. at least one of them is a store, !! 1993 they access the same location,
2237 1994
2238 3. at least one of them is plain, !! 1995 they occur on different CPUs (or in different threads on the
>> 1996 same CPU),
2239 1997
2240 4. they occur on different CPUs (or in d !! 1998 at least one of them is a plain access,
2241 same CPU), and <<
2242 1999
2243 5. they execute concurrently. !! 2000 and at least one of them is a store.
2244 2001
2245 In the literature, two accesses are said to " !! 2002 The LKMM tries to determine whether a program contains two conflicting
2246 1 and 2 above. We'll go a little farther and !! 2003 accesses which may execute concurrently; if it does then the LKMM says
2247 are "race candidates" if they satisfy 1 - 4. !! 2004 there is a potential data race and makes no predictions about the
2248 race candidates actually do race in a given e !! 2005 program's outcome.
2249 whether they are concurrent. <<
2250 2006
2251 The LKMM tries to determine whether a program !! 2007 Determining whether two accesses conflict is easy; you can see that
2252 which may execute concurrently; if it does th !! 2008 all the concepts involved in the definition above are already part of
2253 a potential data race and makes no prediction !! 2009 the memory model. The hard part is telling whether they may execute
2254 outcome. !! 2010 concurrently. The LKMM takes a conservative attitude, assuming that
2255 !! 2011 accesses may be concurrent unless it can prove they cannot.
2256 Determining whether two accesses are race can <<
2257 see that all the concepts involved in the def <<
2258 part of the memory model. The hard part is t <<
2259 execute concurrently. The LKMM takes a conse <<
2260 assuming that accesses may be concurrent unle <<
2261 are not. <<
2262 2012
2263 If two memory accesses aren't concurrent then 2013 If two memory accesses aren't concurrent then one must execute before
2264 the other. Therefore the LKMM decides two ac 2014 the other. Therefore the LKMM decides two accesses aren't concurrent
2265 if they can be connected by a sequence of hb, 2015 if they can be connected by a sequence of hb, pb, and rb links
2266 (together referred to as xb, for "executes be 2016 (together referred to as xb, for "executes before"). However, there
2267 are two complicating factors. 2017 are two complicating factors.
2268 2018
2269 If X is a load and X executes before a store 2019 If X is a load and X executes before a store Y, then indeed there is
2270 no danger of X and Y being concurrent. After 2020 no danger of X and Y being concurrent. After all, Y can't have any
2271 effect on the value obtained by X until the m 2021 effect on the value obtained by X until the memory subsystem has
2272 propagated Y from its own CPU to X's CPU, whi 2022 propagated Y from its own CPU to X's CPU, which won't happen until
2273 some time after Y executes and thus after X e 2023 some time after Y executes and thus after X executes. But if X is a
2274 store, then even if X executes before Y it is 2024 store, then even if X executes before Y it is still possible that X
2275 will propagate to Y's CPU just as Y is execut 2025 will propagate to Y's CPU just as Y is executing. In such a case X
2276 could very well interfere somehow with Y, and 2026 could very well interfere somehow with Y, and we would have to
2277 consider X and Y to be concurrent. 2027 consider X and Y to be concurrent.
2278 2028
2279 Therefore when X is a store, for X and Y to b 2029 Therefore when X is a store, for X and Y to be non-concurrent the LKMM
2280 requires not only that X must execute before 2030 requires not only that X must execute before Y but also that X must
2281 propagate to Y's CPU before Y executes. (Or 2031 propagate to Y's CPU before Y executes. (Or vice versa, of course, if
2282 Y executes before X -- then Y must propagate 2032 Y executes before X -- then Y must propagate to X's CPU before X
2283 executes if Y is a store.) This is expressed 2033 executes if Y is a store.) This is expressed by the visibility
2284 relation (vis), where X ->vis Y is defined to 2034 relation (vis), where X ->vis Y is defined to hold if there is an
2285 intermediate event Z such that: 2035 intermediate event Z such that:
2286 2036
2287 X is connected to Z by a possibly emp 2037 X is connected to Z by a possibly empty sequence of
2288 cumul-fence links followed by an opti 2038 cumul-fence links followed by an optional rfe link (if none of
2289 these links are present, X and Z are 2039 these links are present, X and Z are the same event),
2290 2040
2291 and either: 2041 and either:
2292 2042
2293 Z is connected to Y by a strong-fence 2043 Z is connected to Y by a strong-fence link followed by a
2294 possibly empty sequence of xb links, 2044 possibly empty sequence of xb links,
2295 2045
2296 or: 2046 or:
2297 2047
2298 Z is on the same CPU as Y and is conn 2048 Z is on the same CPU as Y and is connected to Y by a possibly
2299 empty sequence of xb links (again, if 2049 empty sequence of xb links (again, if the sequence is empty it
2300 means Z and Y are the same event). 2050 means Z and Y are the same event).
2301 2051
2302 The motivations behind this definition are st 2052 The motivations behind this definition are straightforward:
2303 2053
2304 cumul-fence memory barriers force sto 2054 cumul-fence memory barriers force stores that are po-before
2305 the barrier to propagate to other CPU 2055 the barrier to propagate to other CPUs before stores that are
2306 po-after the barrier. 2056 po-after the barrier.
2307 2057
2308 An rfe link from an event W to an eve 2058 An rfe link from an event W to an event R says that R reads
2309 from W, which certainly means that W 2059 from W, which certainly means that W must have propagated to
2310 R's CPU before R executed. 2060 R's CPU before R executed.
2311 2061
2312 strong-fence memory barriers force st 2062 strong-fence memory barriers force stores that are po-before
2313 the barrier, or that propagate to the 2063 the barrier, or that propagate to the barrier's CPU before the
2314 barrier executes, to propagate to all 2064 barrier executes, to propagate to all CPUs before any events
2315 po-after the barrier can execute. 2065 po-after the barrier can execute.
2316 2066
2317 To see how this works out in practice, consid 2067 To see how this works out in practice, consider our old friend, the MP
2318 pattern (with fences and statement labels, bu 2068 pattern (with fences and statement labels, but without the conditional
2319 test): 2069 test):
2320 2070
2321 int buf = 0, flag = 0; 2071 int buf = 0, flag = 0;
2322 2072
2323 P0() 2073 P0()
2324 { 2074 {
2325 X: WRITE_ONCE(buf, 1); 2075 X: WRITE_ONCE(buf, 1);
2326 smp_wmb(); 2076 smp_wmb();
2327 W: WRITE_ONCE(flag, 1); 2077 W: WRITE_ONCE(flag, 1);
2328 } 2078 }
2329 2079
2330 P1() 2080 P1()
2331 { 2081 {
2332 int r1; 2082 int r1;
2333 int r2 = 0; 2083 int r2 = 0;
2334 2084
2335 Z: r1 = READ_ONCE(flag); 2085 Z: r1 = READ_ONCE(flag);
2336 smp_rmb(); 2086 smp_rmb();
2337 Y: r2 = READ_ONCE(buf); 2087 Y: r2 = READ_ONCE(buf);
2338 } 2088 }
2339 2089
2340 The smp_wmb() memory barrier gives a cumul-fe 2090 The smp_wmb() memory barrier gives a cumul-fence link from X to W, and
2341 assuming r1 = 1 at the end, there is an rfe l 2091 assuming r1 = 1 at the end, there is an rfe link from W to Z. This
2342 means that the store to buf must propagate fr 2092 means that the store to buf must propagate from P0 to P1 before Z
2343 executes. Next, Z and Y are on the same CPU 2093 executes. Next, Z and Y are on the same CPU and the smp_rmb() fence
2344 provides an xb link from Z to Y (i.e., it for 2094 provides an xb link from Z to Y (i.e., it forces Z to execute before
2345 Y). Therefore we have X ->vis Y: X must prop 2095 Y). Therefore we have X ->vis Y: X must propagate to Y's CPU before Y
2346 executes. 2096 executes.
2347 2097
2348 The second complicating factor mentioned abov 2098 The second complicating factor mentioned above arises from the fact
2349 that when we are considering data races, some 2099 that when we are considering data races, some of the memory accesses
2350 are plain. Now, although we have not said so 2100 are plain. Now, although we have not said so explicitly, up to this
2351 point most of the relations defined by the LK 2101 point most of the relations defined by the LKMM (ppo, hb, prop,
2352 cumul-fence, pb, and so on -- including vis) 2102 cumul-fence, pb, and so on -- including vis) apply only to marked
2353 accesses. 2103 accesses.
2354 2104
2355 There are good reasons for this restriction. 2105 There are good reasons for this restriction. The compiler is not
2356 allowed to apply fancy transformations to mar 2106 allowed to apply fancy transformations to marked accesses, and
2357 consequently each such access in the source c 2107 consequently each such access in the source code corresponds more or
2358 less directly to a single machine instruction 2108 less directly to a single machine instruction in the object code. But
2359 plain accesses are a different story; the com 2109 plain accesses are a different story; the compiler may combine them,
2360 split them up, duplicate them, eliminate them 2110 split them up, duplicate them, eliminate them, invent new ones, and
2361 who knows what else. Seeing a plain access i 2111 who knows what else. Seeing a plain access in the source code tells
2362 you almost nothing about what machine instruc 2112 you almost nothing about what machine instructions will end up in the
2363 object code. 2113 object code.
2364 2114
2365 Fortunately, the compiler isn't completely fr 2115 Fortunately, the compiler isn't completely free; it is subject to some
2366 limitations. For one, it is not allowed to i 2116 limitations. For one, it is not allowed to introduce a data race into
2367 the object code if the source code does not a 2117 the object code if the source code does not already contain a data
2368 race (if it could, memory models would be use 2118 race (if it could, memory models would be useless and no multithreaded
2369 code would be safe!). For another, it cannot 2119 code would be safe!). For another, it cannot move a plain access past
2370 a compiler barrier. 2120 a compiler barrier.
2371 2121
2372 A compiler barrier is a kind of fence, but as 2122 A compiler barrier is a kind of fence, but as the name implies, it
2373 only affects the compiler; it does not necess 2123 only affects the compiler; it does not necessarily have any effect on
2374 how instructions are executed by the CPU. In 2124 how instructions are executed by the CPU. In Linux kernel source
2375 code, the barrier() function is a compiler ba 2125 code, the barrier() function is a compiler barrier. It doesn't give
2376 rise directly to any machine instructions in 2126 rise directly to any machine instructions in the object code; rather,
2377 it affects how the compiler generates the res 2127 it affects how the compiler generates the rest of the object code.
2378 Given source code like this: 2128 Given source code like this:
2379 2129
2380 ... some memory accesses ... 2130 ... some memory accesses ...
2381 barrier(); 2131 barrier();
2382 ... some other memory accesses ... 2132 ... some other memory accesses ...
2383 2133
2384 the barrier() function ensures that the machi 2134 the barrier() function ensures that the machine instructions
2385 corresponding to the first group of accesses 2135 corresponding to the first group of accesses will all end po-before
2386 any machine instructions corresponding to the 2136 any machine instructions corresponding to the second group of accesses
2387 -- even if some of the accesses are plain. ( 2137 -- even if some of the accesses are plain. (Of course, the CPU may
2388 then execute some of those accesses out of pr 2138 then execute some of those accesses out of program order, but we
2389 already know how to deal with such issues.) 2139 already know how to deal with such issues.) Without the barrier()
2390 there would be no such guarantee; the two gro 2140 there would be no such guarantee; the two groups of accesses could be
2391 intermingled or even reversed in the object c 2141 intermingled or even reversed in the object code.
2392 2142
2393 The LKMM doesn't say much about the barrier() 2143 The LKMM doesn't say much about the barrier() function, but it does
2394 require that all fences are also compiler bar 2144 require that all fences are also compiler barriers. In addition, it
2395 requires that the ordering properties of memo 2145 requires that the ordering properties of memory barriers such as
2396 smp_rmb() or smp_store_release() apply to pla 2146 smp_rmb() or smp_store_release() apply to plain accesses as well as to
2397 marked accesses. 2147 marked accesses.
2398 2148
2399 This is the key to analyzing data races. Con 2149 This is the key to analyzing data races. Consider the MP pattern
2400 again, now using plain accesses for buf: 2150 again, now using plain accesses for buf:
2401 2151
2402 int buf = 0, flag = 0; 2152 int buf = 0, flag = 0;
2403 2153
2404 P0() 2154 P0()
2405 { 2155 {
2406 U: buf = 1; 2156 U: buf = 1;
2407 smp_wmb(); 2157 smp_wmb();
2408 X: WRITE_ONCE(flag, 1); 2158 X: WRITE_ONCE(flag, 1);
2409 } 2159 }
2410 2160
2411 P1() 2161 P1()
2412 { 2162 {
2413 int r1; 2163 int r1;
2414 int r2 = 0; 2164 int r2 = 0;
2415 2165
2416 Y: r1 = READ_ONCE(flag); 2166 Y: r1 = READ_ONCE(flag);
2417 if (r1) { 2167 if (r1) {
2418 smp_rmb(); 2168 smp_rmb();
2419 V: r2 = buf; 2169 V: r2 = buf;
2420 } 2170 }
2421 } 2171 }
2422 2172
2423 This program does not contain a data race. A 2173 This program does not contain a data race. Although the U and V
2424 accesses are race candidates, the LKMM can pr !! 2174 accesses conflict, the LKMM can prove they are not concurrent as
2425 concurrent as follows: !! 2175 follows:
2426 2176
2427 The smp_wmb() fence in P0 is both a c 2177 The smp_wmb() fence in P0 is both a compiler barrier and a
2428 cumul-fence. It guarantees that no m 2178 cumul-fence. It guarantees that no matter what hash of
2429 machine instructions the compiler gen 2179 machine instructions the compiler generates for the plain
2430 access U, all those instructions will 2180 access U, all those instructions will be po-before the fence.
2431 Consequently U's store to buf, no mat 2181 Consequently U's store to buf, no matter how it is carried out
2432 at the machine level, must propagate 2182 at the machine level, must propagate to P1 before X's store to
2433 flag does. 2183 flag does.
2434 2184
2435 X and Y are both marked accesses. He 2185 X and Y are both marked accesses. Hence an rfe link from X to
2436 Y is a valid indicator that X propaga 2186 Y is a valid indicator that X propagated to P1 before Y
2437 executed, i.e., X ->vis Y. (And if t 2187 executed, i.e., X ->vis Y. (And if there is no rfe link then
2438 r1 will be 0, so V will not be execut 2188 r1 will be 0, so V will not be executed and ipso facto won't
2439 race with U.) 2189 race with U.)
2440 2190
2441 The smp_rmb() fence in P1 is a compil 2191 The smp_rmb() fence in P1 is a compiler barrier as well as a
2442 fence. It guarantees that all the ma 2192 fence. It guarantees that all the machine-level instructions
2443 corresponding to the access V will be 2193 corresponding to the access V will be po-after the fence, and
2444 therefore any loads among those instr 2194 therefore any loads among those instructions will execute
2445 after the fence does and hence after 2195 after the fence does and hence after Y does.
2446 2196
2447 Thus U's store to buf is forced to propagate 2197 Thus U's store to buf is forced to propagate to P1 before V's load
2448 executes (assuming V does execute), ruling ou 2198 executes (assuming V does execute), ruling out the possibility of a
2449 data race between them. 2199 data race between them.
2450 2200
2451 This analysis illustrates how the LKMM deals 2201 This analysis illustrates how the LKMM deals with plain accesses in
2452 general. Suppose R is a plain load and we wa 2202 general. Suppose R is a plain load and we want to show that R
2453 executes before some marked access E. We can 2203 executes before some marked access E. We can do this by finding a
2454 marked access X such that R and X are ordered 2204 marked access X such that R and X are ordered by a suitable fence and
2455 X ->xb* E. If E was also a plain access, we 2205 X ->xb* E. If E was also a plain access, we would also look for a
2456 marked access Y such that X ->xb* Y, and Y an 2206 marked access Y such that X ->xb* Y, and Y and E are ordered by a
2457 fence. We describe this arrangement by sayin 2207 fence. We describe this arrangement by saying that R is
2458 "post-bounded" by X and E is "pre-bounded" by 2208 "post-bounded" by X and E is "pre-bounded" by Y.
2459 2209
2460 In fact, we go one step further: Since R is a 2210 In fact, we go one step further: Since R is a read, we say that R is
2461 "r-post-bounded" by X. Similarly, E would be 2211 "r-post-bounded" by X. Similarly, E would be "r-pre-bounded" or
2462 "w-pre-bounded" by Y, depending on whether E 2212 "w-pre-bounded" by Y, depending on whether E was a store or a load.
2463 This distinction is needed because some fence 2213 This distinction is needed because some fences affect only loads
2464 (i.e., smp_rmb()) and some affect only stores 2214 (i.e., smp_rmb()) and some affect only stores (smp_wmb()); otherwise
2465 the two types of bounds are the same. And as 2215 the two types of bounds are the same. And as a degenerate case, we
2466 say that a marked access pre-bounds and post- 2216 say that a marked access pre-bounds and post-bounds itself (e.g., if R
2467 above were a marked load then X could simply 2217 above were a marked load then X could simply be taken to be R itself.)
2468 2218
2469 The need to distinguish between r- and w-boun 2219 The need to distinguish between r- and w-bounding raises yet another
2470 issue. When the source code contains a plain 2220 issue. When the source code contains a plain store, the compiler is
2471 allowed to put plain loads of the same locati 2221 allowed to put plain loads of the same location into the object code.
2472 For example, given the source code: 2222 For example, given the source code:
2473 2223
2474 x = 1; 2224 x = 1;
2475 2225
2476 the compiler is theoretically allowed to gene 2226 the compiler is theoretically allowed to generate object code that
2477 looks like: 2227 looks like:
2478 2228
2479 if (x != 1) 2229 if (x != 1)
2480 x = 1; 2230 x = 1;
2481 2231
2482 thereby adding a load (and possibly replacing 2232 thereby adding a load (and possibly replacing the store entirely).
2483 For this reason, whenever the LKMM requires a 2233 For this reason, whenever the LKMM requires a plain store to be
2484 w-pre-bounded or w-post-bounded by a marked a 2234 w-pre-bounded or w-post-bounded by a marked access, it also requires
2485 the store to be r-pre-bounded or r-post-bound 2235 the store to be r-pre-bounded or r-post-bounded, so as to handle cases
2486 where the compiler adds a load. 2236 where the compiler adds a load.
2487 2237
2488 (This may be overly cautious. We don't know 2238 (This may be overly cautious. We don't know of any examples where a
2489 compiler has augmented a store with a load in 2239 compiler has augmented a store with a load in this fashion, and the
2490 Linux kernel developers would probably fight 2240 Linux kernel developers would probably fight pretty hard to change a
2491 compiler if it ever did this. Still, better 2241 compiler if it ever did this. Still, better safe than sorry.)
2492 2242
2493 Incidentally, the other tranformation -- augm 2243 Incidentally, the other tranformation -- augmenting a plain load by
2494 adding in a store to the same location -- is 2244 adding in a store to the same location -- is not allowed. This is
2495 because the compiler cannot know whether any 2245 because the compiler cannot know whether any other CPUs might perform
2496 a concurrent load from that location. Two co 2246 a concurrent load from that location. Two concurrent loads don't
2497 constitute a race (they can't interfere with 2247 constitute a race (they can't interfere with each other), but a store
2498 does race with a concurrent load. Thus addin 2248 does race with a concurrent load. Thus adding a store might create a
2499 data race where one was not already present i 2249 data race where one was not already present in the source code,
2500 something the compiler is forbidden to do. A 2250 something the compiler is forbidden to do. Augmenting a store with a
2501 load, on the other hand, is acceptable becaus 2251 load, on the other hand, is acceptable because doing so won't create a
2502 data race unless one already existed. 2252 data race unless one already existed.
2503 2253
2504 The LKMM includes a second way to pre-bound p 2254 The LKMM includes a second way to pre-bound plain accesses, in
2505 addition to fences: an address dependency fro 2255 addition to fences: an address dependency from a marked load. That
2506 is, in the sequence: 2256 is, in the sequence:
2507 2257
2508 p = READ_ONCE(ptr); 2258 p = READ_ONCE(ptr);
2509 r = *p; 2259 r = *p;
2510 2260
2511 the LKMM says that the marked load of ptr pre 2261 the LKMM says that the marked load of ptr pre-bounds the plain load of
2512 *p; the marked load must execute before any o 2262 *p; the marked load must execute before any of the machine
2513 instructions corresponding to the plain load. 2263 instructions corresponding to the plain load. This is a reasonable
2514 stipulation, since after all, the CPU can't p 2264 stipulation, since after all, the CPU can't perform the load of *p
2515 until it knows what value p will hold. Furth 2265 until it knows what value p will hold. Furthermore, without some
2516 assumption like this one, some usages typical 2266 assumption like this one, some usages typical of RCU would count as
2517 data races. For example: 2267 data races. For example:
2518 2268
2519 int a = 1, b; 2269 int a = 1, b;
2520 int *ptr = &a; 2270 int *ptr = &a;
2521 2271
2522 P0() 2272 P0()
2523 { 2273 {
2524 b = 2; 2274 b = 2;
2525 rcu_assign_pointer(ptr, &b); 2275 rcu_assign_pointer(ptr, &b);
2526 } 2276 }
2527 2277
2528 P1() 2278 P1()
2529 { 2279 {
2530 int *p; 2280 int *p;
2531 int r; 2281 int r;
2532 2282
2533 rcu_read_lock(); 2283 rcu_read_lock();
2534 p = rcu_dereference(ptr); 2284 p = rcu_dereference(ptr);
2535 r = *p; 2285 r = *p;
2536 rcu_read_unlock(); 2286 rcu_read_unlock();
2537 } 2287 }
2538 2288
2539 (In this example the rcu_read_lock() and rcu_ 2289 (In this example the rcu_read_lock() and rcu_read_unlock() calls don't
2540 really do anything, because there aren't any 2290 really do anything, because there aren't any grace periods. They are
2541 included merely for the sake of good form; ty 2291 included merely for the sake of good form; typically P0 would call
2542 synchronize_rcu() somewhere after the rcu_ass 2292 synchronize_rcu() somewhere after the rcu_assign_pointer().)
2543 2293
2544 rcu_assign_pointer() performs a store-release 2294 rcu_assign_pointer() performs a store-release, so the plain store to b
2545 is definitely w-post-bounded before the store 2295 is definitely w-post-bounded before the store to ptr, and the two
2546 stores will propagate to P1 in that order. H 2296 stores will propagate to P1 in that order. However, rcu_dereference()
2547 is only equivalent to READ_ONCE(). While it 2297 is only equivalent to READ_ONCE(). While it is a marked access, it is
2548 not a fence or compiler barrier. Hence the o 2298 not a fence or compiler barrier. Hence the only guarantee we have
2549 that the load of ptr in P1 is r-pre-bounded b 2299 that the load of ptr in P1 is r-pre-bounded before the load of *p
2550 (thus avoiding a race) is the assumption abou 2300 (thus avoiding a race) is the assumption about address dependencies.
2551 2301
2552 This is a situation where the compiler can un 2302 This is a situation where the compiler can undermine the memory model,
2553 and a certain amount of care is required when 2303 and a certain amount of care is required when programming constructs
2554 like this one. In particular, comparisons be 2304 like this one. In particular, comparisons between the pointer and
2555 other known addresses can cause trouble. If 2305 other known addresses can cause trouble. If you have something like:
2556 2306
2557 p = rcu_dereference(ptr); 2307 p = rcu_dereference(ptr);
2558 if (p == &x) 2308 if (p == &x)
2559 r = *p; 2309 r = *p;
2560 2310
2561 then the compiler just might generate object 2311 then the compiler just might generate object code resembling:
2562 2312
2563 p = rcu_dereference(ptr); 2313 p = rcu_dereference(ptr);
2564 if (p == &x) 2314 if (p == &x)
2565 r = x; 2315 r = x;
2566 2316
2567 or even: 2317 or even:
2568 2318
2569 rtemp = x; 2319 rtemp = x;
2570 p = rcu_dereference(ptr); 2320 p = rcu_dereference(ptr);
2571 if (p == &x) 2321 if (p == &x)
2572 r = rtemp; 2322 r = rtemp;
2573 2323
2574 which would invalidate the memory model's ass 2324 which would invalidate the memory model's assumption, since the CPU
2575 could now perform the load of x before the lo 2325 could now perform the load of x before the load of ptr (there might be
2576 a control dependency but no address dependenc 2326 a control dependency but no address dependency at the machine level).
2577 2327
2578 Finally, it turns out there is a situation in 2328 Finally, it turns out there is a situation in which a plain write does
2579 not need to be w-post-bounded: when it is sep !! 2329 not need to be w-post-bounded: when it is separated from the
2580 race-candidate access by a fence. At first g !! 2330 conflicting access by a fence. At first glance this may seem
2581 impossible. After all, to be race candidates !! 2331 impossible. After all, to be conflicting the second access has to be
2582 be on different CPUs, and fences don't link e !! 2332 on a different CPU from the first, and fences don't link events on
2583 Well, normal fences don't -- but rcu-fence ca !! 2333 different CPUs. Well, normal fences don't -- but rcu-fence can!
>> 2334 Here's an example:
2584 2335
2585 int x, y; 2336 int x, y;
2586 2337
2587 P0() 2338 P0()
2588 { 2339 {
2589 WRITE_ONCE(x, 1); 2340 WRITE_ONCE(x, 1);
2590 synchronize_rcu(); 2341 synchronize_rcu();
2591 y = 3; 2342 y = 3;
2592 } 2343 }
2593 2344
2594 P1() 2345 P1()
2595 { 2346 {
2596 rcu_read_lock(); 2347 rcu_read_lock();
2597 if (READ_ONCE(x) == 0) 2348 if (READ_ONCE(x) == 0)
2598 y = 2; 2349 y = 2;
2599 rcu_read_unlock(); 2350 rcu_read_unlock();
2600 } 2351 }
2601 2352
2602 Do the plain stores to y race? Clearly not i 2353 Do the plain stores to y race? Clearly not if P1 reads a non-zero
2603 value for x, so let's assume the READ_ONCE(x) 2354 value for x, so let's assume the READ_ONCE(x) does obtain 0. This
2604 means that the read-side critical section in 2355 means that the read-side critical section in P1 must finish executing
2605 before the grace period in P0 does, because R 2356 before the grace period in P0 does, because RCU's Grace-Period
2606 Guarantee says that otherwise P0's store to x 2357 Guarantee says that otherwise P0's store to x would have propagated to
2607 P1 before the critical section started and so 2358 P1 before the critical section started and so would have been visible
2608 to the READ_ONCE(). (Another way of putting 2359 to the READ_ONCE(). (Another way of putting it is that the fre link
2609 from the READ_ONCE() to the WRITE_ONCE() give 2360 from the READ_ONCE() to the WRITE_ONCE() gives rise to an rcu-link
2610 between those two events.) 2361 between those two events.)
2611 2362
2612 This means there is an rcu-fence link from P1 2363 This means there is an rcu-fence link from P1's "y = 2" store to P0's
2613 "y = 3" store, and consequently the first mus 2364 "y = 3" store, and consequently the first must propagate from P1 to P0
2614 before the second can execute. Therefore the 2365 before the second can execute. Therefore the two stores cannot be
2615 concurrent and there is no race, even though 2366 concurrent and there is no race, even though P1's plain store to y
2616 isn't w-post-bounded by any marked accesses. 2367 isn't w-post-bounded by any marked accesses.
2617 2368
2618 Putting all this material together yields the 2369 Putting all this material together yields the following picture. For
2619 race-candidate stores W and W', where W ->co !! 2370 two conflicting stores W and W', where W ->co W', the LKMM says the
2620 stores don't race if W can be linked to W' by 2371 stores don't race if W can be linked to W' by a
2621 2372
2622 w-post-bounded ; vis ; w-pre-bounded 2373 w-post-bounded ; vis ; w-pre-bounded
2623 2374
2624 sequence. If W is plain then they also have 2375 sequence. If W is plain then they also have to be linked by an
2625 2376
2626 r-post-bounded ; xb* ; w-pre-bounded 2377 r-post-bounded ; xb* ; w-pre-bounded
2627 2378
2628 sequence, and if W' is plain then they also h 2379 sequence, and if W' is plain then they also have to be linked by a
2629 2380
2630 w-post-bounded ; vis ; r-pre-bounded 2381 w-post-bounded ; vis ; r-pre-bounded
2631 2382
2632 sequence. For race-candidate load R and stor !! 2383 sequence. For a conflicting load R and store W, the LKMM says the two
2633 two accesses don't race if R can be linked to !! 2384 accesses don't race if R can be linked to W by an
2634 2385
2635 r-post-bounded ; xb* ; w-pre-bounded 2386 r-post-bounded ; xb* ; w-pre-bounded
2636 2387
2637 sequence or if W can be linked to R by a 2388 sequence or if W can be linked to R by a
2638 2389
2639 w-post-bounded ; vis ; r-pre-bounded 2390 w-post-bounded ; vis ; r-pre-bounded
2640 2391
2641 sequence. For the cases involving a vis link 2392 sequence. For the cases involving a vis link, the LKMM also accepts
2642 sequences in which W is linked to W' or R by 2393 sequences in which W is linked to W' or R by a
2643 2394
2644 strong-fence ; xb* ; {w and/or r}-pre 2395 strong-fence ; xb* ; {w and/or r}-pre-bounded
2645 2396
2646 sequence with no post-bounding, and in every 2397 sequence with no post-bounding, and in every case the LKMM also allows
2647 the link simply to be a fence with no boundin 2398 the link simply to be a fence with no bounding at all. If no sequence
2648 of the appropriate sort exists, the LKMM says 2399 of the appropriate sort exists, the LKMM says that the accesses race.
2649 2400
2650 There is one more part of the LKMM related to 2401 There is one more part of the LKMM related to plain accesses (although
2651 not to data races) we should discuss. Recall 2402 not to data races) we should discuss. Recall that many relations such
2652 as hb are limited to marked accesses only. A 2403 as hb are limited to marked accesses only. As a result, the
2653 happens-before, propagates-before, and rcu ax 2404 happens-before, propagates-before, and rcu axioms (which state that
2654 various relation must not contain a cycle) do 2405 various relation must not contain a cycle) doesn't apply to plain
2655 accesses. Nevertheless, we do want to rule o 2406 accesses. Nevertheless, we do want to rule out such cycles, because
2656 they don't make sense even for plain accesses 2407 they don't make sense even for plain accesses.
2657 2408
2658 To this end, the LKMM imposes three extra res 2409 To this end, the LKMM imposes three extra restrictions, together
2659 called the "plain-coherence" axiom because of 2410 called the "plain-coherence" axiom because of their resemblance to the
2660 rules used by the operational model to ensure 2411 rules used by the operational model to ensure cache coherence (that
2661 is, the rules governing the memory subsystem' 2412 is, the rules governing the memory subsystem's choice of a store to
2662 satisfy a load request and its determination 2413 satisfy a load request and its determination of where a store will
2663 fall in the coherence order): 2414 fall in the coherence order):
2664 2415
2665 If R and W are race candidates and it !! 2416 If R and W conflict and it is possible to link R to W by one
2666 W by one of the xb* sequences listed !! 2417 of the xb* sequences listed above, then W ->rfe R is not
2667 not allowed (i.e., a load cannot read !! 2418 allowed (i.e., a load cannot read from a store that it
2668 executes before, even if one or both 2419 executes before, even if one or both is plain).
2669 2420
2670 If W and R are race candidates and it !! 2421 If W and R conflict and it is possible to link W to R by one
2671 R by one of the vis sequences listed !! 2422 of the vis sequences listed above, then R ->fre W is not
2672 not allowed (i.e., if a store is visi !! 2423 allowed (i.e., if a store is visible to a load then the load
2673 load must read from that store or one !! 2424 must read from that store or one coherence-after it).
2674 !! 2425
2675 If W and W' are race candidates and i !! 2426 If W and W' conflict and it is possible to link W to W' by one
2676 to W' by one of the vis sequences lis !! 2427 of the vis sequences listed above, then W' ->co W is not
2677 is not allowed (i.e., if one store is !! 2428 allowed (i.e., if one store is visible to a second then the
2678 the second must come after the first !! 2429 second must come after the first in the coherence order).
2679 2430
2680 This is the extent to which the LKMM deals wi 2431 This is the extent to which the LKMM deals with plain accesses.
2681 Perhaps it could say more (for example, plain 2432 Perhaps it could say more (for example, plain accesses might
2682 contribute to the ppo relation), but at the m 2433 contribute to the ppo relation), but at the moment it seems that this
2683 minimal, conservative approach is good enough 2434 minimal, conservative approach is good enough.
2684 2435
2685 2436
2686 ODDS AND ENDS 2437 ODDS AND ENDS
2687 ------------- 2438 -------------
2688 2439
2689 This section covers material that didn't quit 2440 This section covers material that didn't quite fit anywhere in the
2690 earlier sections. 2441 earlier sections.
2691 2442
2692 The descriptions in this document don't alway 2443 The descriptions in this document don't always match the formal
2693 version of the LKMM exactly. For example, th 2444 version of the LKMM exactly. For example, the actual formal
2694 definition of the prop relation makes the ini 2445 definition of the prop relation makes the initial coe or fre part
2695 optional, and it doesn't require the events l 2446 optional, and it doesn't require the events linked by the relation to
2696 be on the same CPU. These differences are ve 2447 be on the same CPU. These differences are very unimportant; indeed,
2697 instances where the coe/fre part of prop is m 2448 instances where the coe/fre part of prop is missing are of no interest
2698 because all the other parts (fences and rfe) 2449 because all the other parts (fences and rfe) are already included in
2699 hb anyway, and where the formal model adds pr 2450 hb anyway, and where the formal model adds prop into hb, it includes
2700 an explicit requirement that the events being 2451 an explicit requirement that the events being linked are on the same
2701 CPU. 2452 CPU.
2702 2453
2703 Another minor difference has to do with event 2454 Another minor difference has to do with events that are both memory
2704 accesses and fences, such as those correspond 2455 accesses and fences, such as those corresponding to smp_load_acquire()
2705 calls. In the formal model, these events are 2456 calls. In the formal model, these events aren't actually both reads
2706 and fences; rather, they are read events with 2457 and fences; rather, they are read events with an annotation marking
2707 them as acquires. (Or write events annotated 2458 them as acquires. (Or write events annotated as releases, in the case
2708 smp_store_release().) The final effect is th 2459 smp_store_release().) The final effect is the same.
2709 2460
2710 Although we didn't mention it above, the inst 2461 Although we didn't mention it above, the instruction execution
2711 ordering provided by the smp_rmb() fence does 2462 ordering provided by the smp_rmb() fence doesn't apply to read events
2712 that are part of a non-value-returning atomic 2463 that are part of a non-value-returning atomic update. For instance,
2713 given: 2464 given:
2714 2465
2715 atomic_inc(&x); 2466 atomic_inc(&x);
2716 smp_rmb(); 2467 smp_rmb();
2717 r1 = READ_ONCE(y); 2468 r1 = READ_ONCE(y);
2718 2469
2719 it is not guaranteed that the load from y wil 2470 it is not guaranteed that the load from y will execute after the
2720 update to x. This is because the ARMv8 archi 2471 update to x. This is because the ARMv8 architecture allows
2721 non-value-returning atomic operations effecti 2472 non-value-returning atomic operations effectively to be executed off
2722 the CPU. Basically, the CPU tells the memory 2473 the CPU. Basically, the CPU tells the memory subsystem to increment
2723 x, and then the increment is carried out by t 2474 x, and then the increment is carried out by the memory hardware with
2724 no further involvement from the CPU. Since t 2475 no further involvement from the CPU. Since the CPU doesn't ever read
2725 the value of x, there is nothing for the smp_ 2476 the value of x, there is nothing for the smp_rmb() fence to act on.
2726 2477
2727 The LKMM defines a few extra synchronization 2478 The LKMM defines a few extra synchronization operations in terms of
2728 things we have already covered. In particula 2479 things we have already covered. In particular, rcu_dereference() is
2729 treated as READ_ONCE() and rcu_assign_pointer 2480 treated as READ_ONCE() and rcu_assign_pointer() is treated as
2730 smp_store_release() -- which is basically how 2481 smp_store_release() -- which is basically how the Linux kernel treats
2731 them. 2482 them.
2732 2483
2733 Although we said that plain accesses are not 2484 Although we said that plain accesses are not linked by the ppo
2734 relation, they do contribute to it indirectly !! 2485 relation, they do contribute to it indirectly. Namely, when there is
2735 an address dependency from a marked load R to 2486 an address dependency from a marked load R to a plain store W,
2736 followed by smp_wmb() and then a marked store 2487 followed by smp_wmb() and then a marked store W', the LKMM creates a
2737 ppo link from R to W'. The reasoning behind 2488 ppo link from R to W'. The reasoning behind this is perhaps a little
2738 shaky, but essentially it says there is no wa 2489 shaky, but essentially it says there is no way to generate object code
2739 for this source code in which W' could execut 2490 for this source code in which W' could execute before R. Just as with
2740 pre-bounding by address dependencies, it is p 2491 pre-bounding by address dependencies, it is possible for the compiler
2741 to undermine this relation if sufficient care 2492 to undermine this relation if sufficient care is not taken.
2742 2493
2743 Secondly, plain accesses can carry dependenci <<
2744 links a marked load R to a store W, and the s <<
2745 from the same thread, then the data loaded by <<
2746 loaded originally by R. Thus, if R' is linked <<
2747 dependency, R is also linked to access X by t <<
2748 if W' or R' (or both!) are plain. <<
2749 <<
2750 There are a few oddball fences which need spe 2494 There are a few oddball fences which need special treatment:
2751 smp_mb__before_atomic(), smp_mb__after_atomic 2495 smp_mb__before_atomic(), smp_mb__after_atomic(), and
2752 smp_mb__after_spinlock(). The LKMM uses fenc 2496 smp_mb__after_spinlock(). The LKMM uses fence events with special
2753 annotations for them; they act as strong fenc 2497 annotations for them; they act as strong fences just like smp_mb()
2754 except for the sets of events that they order 2498 except for the sets of events that they order. Instead of ordering
2755 all po-earlier events against all po-later ev 2499 all po-earlier events against all po-later events, as smp_mb() does,
2756 they behave as follows: 2500 they behave as follows:
2757 2501
2758 smp_mb__before_atomic() orders all po 2502 smp_mb__before_atomic() orders all po-earlier events against
2759 po-later atomic updates and the event 2503 po-later atomic updates and the events following them;
2760 2504
2761 smp_mb__after_atomic() orders po-earl 2505 smp_mb__after_atomic() orders po-earlier atomic updates and
2762 the events preceding them against all 2506 the events preceding them against all po-later events;
2763 2507
2764 smp_mb__after_spinlock() orders po-ea !! 2508 smp_mb_after_spinlock() orders po-earlier lock acquisition
2765 events and the events preceding them 2509 events and the events preceding them against all po-later
2766 events. 2510 events.
2767 2511
2768 Interestingly, RCU and locking each introduce 2512 Interestingly, RCU and locking each introduce the possibility of
2769 deadlock. When faced with code sequences suc 2513 deadlock. When faced with code sequences such as:
2770 2514
2771 spin_lock(&s); 2515 spin_lock(&s);
2772 spin_lock(&s); 2516 spin_lock(&s);
2773 spin_unlock(&s); 2517 spin_unlock(&s);
2774 spin_unlock(&s); 2518 spin_unlock(&s);
2775 2519
2776 or: 2520 or:
2777 2521
2778 rcu_read_lock(); 2522 rcu_read_lock();
2779 synchronize_rcu(); 2523 synchronize_rcu();
2780 rcu_read_unlock(); 2524 rcu_read_unlock();
2781 2525
2782 what does the LKMM have to say? Answer: It s 2526 what does the LKMM have to say? Answer: It says there are no allowed
2783 executions at all, which makes sense. But th 2527 executions at all, which makes sense. But this can also lead to
2784 misleading results, because if a piece of cod 2528 misleading results, because if a piece of code has multiple possible
2785 executions, some of which deadlock, the model 2529 executions, some of which deadlock, the model will report only on the
2786 non-deadlocking executions. For example: 2530 non-deadlocking executions. For example:
2787 2531
2788 int x, y; 2532 int x, y;
2789 2533
2790 P0() 2534 P0()
2791 { 2535 {
2792 int r0; 2536 int r0;
2793 2537
2794 WRITE_ONCE(x, 1); 2538 WRITE_ONCE(x, 1);
2795 r0 = READ_ONCE(y); 2539 r0 = READ_ONCE(y);
2796 } 2540 }
2797 2541
2798 P1() 2542 P1()
2799 { 2543 {
2800 rcu_read_lock(); 2544 rcu_read_lock();
2801 if (READ_ONCE(x) > 0) { 2545 if (READ_ONCE(x) > 0) {
2802 WRITE_ONCE(y, 36); 2546 WRITE_ONCE(y, 36);
2803 synchronize_rcu(); 2547 synchronize_rcu();
2804 } 2548 }
2805 rcu_read_unlock(); 2549 rcu_read_unlock();
2806 } 2550 }
2807 2551
2808 Is it possible to end up with r0 = 36 at the 2552 Is it possible to end up with r0 = 36 at the end? The LKMM will tell
2809 you it is not, but the model won't mention th 2553 you it is not, but the model won't mention that this is because P1
2810 will self-deadlock in the executions where it 2554 will self-deadlock in the executions where it stores 36 in y.
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