1 .. _rcu_dereference_doc: 1 .. _rcu_dereference_doc:
2 2
3 PROPER CARE AND FEEDING OF RETURN VALUES FROM 3 PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
4 ============================================== 4 ===============================================================
5 5
6 Proper care and feeding of address and data de !! 6 Most of the time, you can use values from rcu_dereference() or one of
7 important to correct use of things like RCU. !! 7 the similar primitives without worries. Dereferencing (prefix "*"),
8 returned from the rcu_dereference() family of !! 8 field selection ("->"), assignment ("="), address-of ("&"), addition and
9 data dependencies. These dependencies extend !! 9 subtraction of constants, and casts all work quite naturally and safely.
10 macro's load of the pointer to the later use o !! 10
11 either the address of a later memory access (r !! 11 It is nevertheless possible to get into trouble with other operations.
12 dependency) or the value written by a later me !! 12 Follow these rules to keep your RCU code working properly:
13 a data dependency). <<
14 <<
15 Most of the time, these dependencies are prese <<
16 freely use values from rcu_dereference(). For <<
17 (prefix "*"), field selection ("->"), assignme <<
18 ("&"), casts, and addition or subtraction of c <<
19 naturally and safely. However, because curren <<
20 either address or data dependencies into accou <<
21 to get into trouble. <<
22 <<
23 Follow these rules to preserve the address and <<
24 from your calls to rcu_dereference() and frien <<
25 readers working properly: <<
26 13
27 - You must use one of the rcu_dereferenc 14 - You must use one of the rcu_dereference() family of primitives
28 to load an RCU-protected pointer, othe 15 to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
29 will complain. Worse yet, your code c 16 will complain. Worse yet, your code can see random memory-corruption
30 bugs due to games that compilers and D 17 bugs due to games that compilers and DEC Alpha can play.
31 Without one of the rcu_dereference() p 18 Without one of the rcu_dereference() primitives, compilers
32 can reload the value, and won't your c 19 can reload the value, and won't your code have fun with two
33 different values for a single pointer! 20 different values for a single pointer! Without rcu_dereference(),
34 DEC Alpha can load a pointer, derefere 21 DEC Alpha can load a pointer, dereference that pointer, and
35 return data preceding initialization t !! 22 return data preceding initialization that preceded the store of
36 of the pointer. (As noted later, in r !! 23 the pointer.
37 also prevents DEC Alpha from playing t <<
38 24
39 In addition, the volatile cast in rcu_ 25 In addition, the volatile cast in rcu_dereference() prevents the
40 compiler from deducing the resulting p 26 compiler from deducing the resulting pointer value. Please see
41 the section entitled "EXAMPLE WHERE TH 27 the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
42 for an example where the compiler can 28 for an example where the compiler can in fact deduce the exact
43 value of the pointer, and thus cause m 29 value of the pointer, and thus cause misordering.
44 30
45 - In the special case where data is adde 31 - In the special case where data is added but is never removed
46 while readers are accessing the struct 32 while readers are accessing the structure, READ_ONCE() may be used
47 instead of rcu_dereference(). In this 33 instead of rcu_dereference(). In this case, use of READ_ONCE()
48 takes on the role of the lockless_dere 34 takes on the role of the lockless_dereference() primitive that
49 was removed in v4.15. 35 was removed in v4.15.
50 36
51 - You are only permitted to use rcu_dere !! 37 - You are only permitted to use rcu_dereference on pointer values.
52 The compiler simply knows too much abo 38 The compiler simply knows too much about integral values to
53 trust it to carry dependencies through 39 trust it to carry dependencies through integer operations.
54 There are a very few exceptions, namel 40 There are a very few exceptions, namely that you can temporarily
55 cast the pointer to uintptr_t in order 41 cast the pointer to uintptr_t in order to:
56 42
57 - Set bits and clear bits down i 43 - Set bits and clear bits down in the must-be-zero low-order
58 bits of that pointer. This cl 44 bits of that pointer. This clearly means that the pointer
59 must have alignment constraint 45 must have alignment constraints, for example, this does
60 *not* work in general for char 46 *not* work in general for char* pointers.
61 47
62 - XOR bits to translate pointers 48 - XOR bits to translate pointers, as is done in some
63 classic buddy-allocator algori 49 classic buddy-allocator algorithms.
64 50
65 It is important to cast the value back 51 It is important to cast the value back to pointer before
66 doing much of anything else with it. 52 doing much of anything else with it.
67 53
68 - Avoid cancellation when using the "+" 54 - Avoid cancellation when using the "+" and "-" infix arithmetic
69 operators. For example, for a given v 55 operators. For example, for a given variable "x", avoid
70 "(x-(uintptr_t)x)" for char* pointers. 56 "(x-(uintptr_t)x)" for char* pointers. The compiler is within its
71 rights to substitute zero for this sor 57 rights to substitute zero for this sort of expression, so that
72 subsequent accesses no longer depend o 58 subsequent accesses no longer depend on the rcu_dereference(),
73 again possibly resulting in bugs due t 59 again possibly resulting in bugs due to misordering.
74 60
75 Of course, if "p" is a pointer from rc 61 Of course, if "p" is a pointer from rcu_dereference(), and "a"
76 and "b" are integers that happen to be 62 and "b" are integers that happen to be equal, the expression
77 "p+a-b" is safe because its value stil 63 "p+a-b" is safe because its value still necessarily depends on
78 the rcu_dereference(), thus maintainin 64 the rcu_dereference(), thus maintaining proper ordering.
79 65
80 - If you are using RCU to protect JITed 66 - If you are using RCU to protect JITed functions, so that the
81 "()" function-invocation operator is a 67 "()" function-invocation operator is applied to a value obtained
82 (directly or indirectly) from rcu_dere 68 (directly or indirectly) from rcu_dereference(), you may need to
83 interact directly with the hardware to 69 interact directly with the hardware to flush instruction caches.
84 This issue arises on some systems when 70 This issue arises on some systems when a newly JITed function is
85 using the same memory that was used by 71 using the same memory that was used by an earlier JITed function.
86 72
87 - Do not use the results from relational 73 - Do not use the results from relational operators ("==", "!=",
88 ">", ">=", "<", or "<=") when derefere 74 ">", ">=", "<", or "<=") when dereferencing. For example,
89 the following (quite strange) code is 75 the following (quite strange) code is buggy::
90 76
91 int *p; 77 int *p;
92 int *q; 78 int *q;
93 79
94 ... 80 ...
95 81
96 p = rcu_dereference(gp) 82 p = rcu_dereference(gp)
97 q = &global_q; 83 q = &global_q;
98 q += p > &oom_p; 84 q += p > &oom_p;
99 r1 = *q; /* BUGGY!!! */ 85 r1 = *q; /* BUGGY!!! */
100 86
101 As before, the reason this is buggy is 87 As before, the reason this is buggy is that relational operators
102 are often compiled using branches. An 88 are often compiled using branches. And as before, although
103 weak-memory machines such as ARM or Po 89 weak-memory machines such as ARM or PowerPC do order stores
104 after such branches, but can speculate 90 after such branches, but can speculate loads, which can again
105 result in misordering bugs. 91 result in misordering bugs.
106 92
107 - Be very careful about comparing pointe 93 - Be very careful about comparing pointers obtained from
108 rcu_dereference() against non-NULL val 94 rcu_dereference() against non-NULL values. As Linus Torvalds
109 explained, if the two pointers are equ 95 explained, if the two pointers are equal, the compiler could
110 substitute the pointer you are compari 96 substitute the pointer you are comparing against for the pointer
111 obtained from rcu_dereference(). For 97 obtained from rcu_dereference(). For example::
112 98
113 p = rcu_dereference(gp); 99 p = rcu_dereference(gp);
114 if (p == &default_struct) 100 if (p == &default_struct)
115 do_default(p->a); 101 do_default(p->a);
116 102
117 Because the compiler now knows that th 103 Because the compiler now knows that the value of "p" is exactly
118 the address of the variable "default_s 104 the address of the variable "default_struct", it is free to
119 transform this code into the following 105 transform this code into the following::
120 106
121 p = rcu_dereference(gp); 107 p = rcu_dereference(gp);
122 if (p == &default_struct) 108 if (p == &default_struct)
123 do_default(default_str 109 do_default(default_struct.a);
124 110
125 On ARM and Power hardware, the load fr 111 On ARM and Power hardware, the load from "default_struct.a"
126 can now be speculated, such that it mi 112 can now be speculated, such that it might happen before the
127 rcu_dereference(). This could result 113 rcu_dereference(). This could result in bugs due to misordering.
128 114
129 However, comparisons are OK in the fol 115 However, comparisons are OK in the following cases:
130 116
131 - The comparison was against the 117 - The comparison was against the NULL pointer. If the
132 compiler knows that the pointe 118 compiler knows that the pointer is NULL, you had better
133 not be dereferencing it anyway 119 not be dereferencing it anyway. If the comparison is
134 non-equal, the compiler is non 120 non-equal, the compiler is none the wiser. Therefore,
135 it is safe to compare pointers 121 it is safe to compare pointers from rcu_dereference()
136 against NULL pointers. 122 against NULL pointers.
137 123
138 - The pointer is never dereferen 124 - The pointer is never dereferenced after being compared.
139 Since there are no subsequent 125 Since there are no subsequent dereferences, the compiler
140 cannot use anything it learned 126 cannot use anything it learned from the comparison
141 to reorder the non-existent su 127 to reorder the non-existent subsequent dereferences.
142 This sort of comparison occurs 128 This sort of comparison occurs frequently when scanning
143 RCU-protected circular linked 129 RCU-protected circular linked lists.
144 130
145 Note that if the pointer compa 131 Note that if the pointer comparison is done outside
146 of an RCU read-side critical s 132 of an RCU read-side critical section, and the pointer
147 is never dereferenced, rcu_acc 133 is never dereferenced, rcu_access_pointer() should be
148 used in place of rcu_dereferen 134 used in place of rcu_dereference(). In most cases,
149 it is best to avoid accidental 135 it is best to avoid accidental dereferences by testing
150 the rcu_access_pointer() retur 136 the rcu_access_pointer() return value directly, without
151 assigning it to a variable. 137 assigning it to a variable.
152 138
153 Within an RCU read-side critic 139 Within an RCU read-side critical section, there is little
154 reason to use rcu_access_point 140 reason to use rcu_access_pointer().
155 141
156 - The comparison is against a po 142 - The comparison is against a pointer that references memory
157 that was initialized "a long t 143 that was initialized "a long time ago." The reason
158 this is safe is that even if m 144 this is safe is that even if misordering occurs, the
159 misordering will not affect th 145 misordering will not affect the accesses that follow
160 the comparison. So exactly ho 146 the comparison. So exactly how long ago is "a long
161 time ago"? Here are some poss 147 time ago"? Here are some possibilities:
162 148
163 - Compile time. 149 - Compile time.
164 150
165 - Boot time. 151 - Boot time.
166 152
167 - Module-init time for m 153 - Module-init time for module code.
168 154
169 - Prior to kthread creat 155 - Prior to kthread creation for kthread code.
170 156
171 - During some prior acqu 157 - During some prior acquisition of the lock that
172 we now hold. 158 we now hold.
173 159
174 - Before mod_timer() tim 160 - Before mod_timer() time for a timer handler.
175 161
176 There are many other possibili 162 There are many other possibilities involving the Linux
177 kernel's wide array of primiti 163 kernel's wide array of primitives that cause code to
178 be invoked at a later time. 164 be invoked at a later time.
179 165
180 - The pointer being compared aga 166 - The pointer being compared against also came from
181 rcu_dereference(). In this ca 167 rcu_dereference(). In this case, both pointers depend
182 on one rcu_dereference() or an 168 on one rcu_dereference() or another, so you get proper
183 ordering either way. 169 ordering either way.
184 170
185 That said, this situation can 171 That said, this situation can make certain RCU usage
186 bugs more likely to happen. W 172 bugs more likely to happen. Which can be a good thing,
187 at least if they happen during 173 at least if they happen during testing. An example
188 of such an RCU usage bug is sh 174 of such an RCU usage bug is shown in the section titled
189 "EXAMPLE OF AMPLIFIED RCU-USAG 175 "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
190 176
191 - All of the accesses following 177 - All of the accesses following the comparison are stores,
192 so that a control dependency p 178 so that a control dependency preserves the needed ordering.
193 That said, it is easy to get c 179 That said, it is easy to get control dependencies wrong.
194 Please see the "CONTROL DEPEND 180 Please see the "CONTROL DEPENDENCIES" section of
195 Documentation/memory-barriers. 181 Documentation/memory-barriers.txt for more details.
196 182
197 - The pointers are not equal *an 183 - The pointers are not equal *and* the compiler does
198 not have enough information to 184 not have enough information to deduce the value of the
199 pointer. Note that the volati 185 pointer. Note that the volatile cast in rcu_dereference()
200 will normally prevent the comp 186 will normally prevent the compiler from knowing too much.
201 187
202 However, please note that if t 188 However, please note that if the compiler knows that the
203 pointer takes on only one of t 189 pointer takes on only one of two values, a not-equal
204 comparison will provide exactl 190 comparison will provide exactly the information that the
205 compiler needs to deduce the v 191 compiler needs to deduce the value of the pointer.
206 192
207 - Disable any value-speculation optimiza 193 - Disable any value-speculation optimizations that your compiler
208 might provide, especially if you are m 194 might provide, especially if you are making use of feedback-based
209 optimizations that take data collected 195 optimizations that take data collected from prior runs. Such
210 value-speculation optimizations reorde 196 value-speculation optimizations reorder operations by design.
211 197
212 There is one exception to this rule: 198 There is one exception to this rule: Value-speculation
213 optimizations that leverage the branch 199 optimizations that leverage the branch-prediction hardware are
214 safe on strongly ordered systems (such 200 safe on strongly ordered systems (such as x86), but not on weakly
215 ordered systems (such as ARM or Power) 201 ordered systems (such as ARM or Power). Choose your compiler
216 command-line options wisely! 202 command-line options wisely!
217 203
218 204
219 EXAMPLE OF AMPLIFIED RCU-USAGE BUG 205 EXAMPLE OF AMPLIFIED RCU-USAGE BUG
220 ---------------------------------- 206 ----------------------------------
221 207
222 Because updaters can run concurrently with RCU 208 Because updaters can run concurrently with RCU readers, RCU readers can
223 see stale and/or inconsistent values. If RCU 209 see stale and/or inconsistent values. If RCU readers need fresh or
224 consistent values, which they sometimes do, th 210 consistent values, which they sometimes do, they need to take proper
225 precautions. To see this, consider the follow 211 precautions. To see this, consider the following code fragment::
226 212
227 struct foo { 213 struct foo {
228 int a; 214 int a;
229 int b; 215 int b;
230 int c; 216 int c;
231 }; 217 };
232 struct foo *gp1; 218 struct foo *gp1;
233 struct foo *gp2; 219 struct foo *gp2;
234 220
235 void updater(void) 221 void updater(void)
236 { 222 {
237 struct foo *p; 223 struct foo *p;
238 224
239 p = kmalloc(...); 225 p = kmalloc(...);
240 if (p == NULL) 226 if (p == NULL)
241 deal_with_it(); 227 deal_with_it();
242 p->a = 42; /* Each field in i 228 p->a = 42; /* Each field in its own cache line. */
243 p->b = 43; 229 p->b = 43;
244 p->c = 44; 230 p->c = 44;
245 rcu_assign_pointer(gp1, p); 231 rcu_assign_pointer(gp1, p);
246 p->b = 143; 232 p->b = 143;
247 p->c = 144; 233 p->c = 144;
248 rcu_assign_pointer(gp2, p); 234 rcu_assign_pointer(gp2, p);
249 } 235 }
250 236
251 void reader(void) 237 void reader(void)
252 { 238 {
253 struct foo *p; 239 struct foo *p;
254 struct foo *q; 240 struct foo *q;
255 int r1, r2; 241 int r1, r2;
256 242
257 rcu_read_lock(); <<
258 p = rcu_dereference(gp2); 243 p = rcu_dereference(gp2);
259 if (p == NULL) 244 if (p == NULL)
260 return; 245 return;
261 r1 = p->b; /* Guaranteed to g 246 r1 = p->b; /* Guaranteed to get 143. */
262 q = rcu_dereference(gp1); /* 247 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
263 if (p == q) { 248 if (p == q) {
264 /* The compiler decide 249 /* The compiler decides that q->c is same as p->c. */
265 r2 = p->c; /* Could ge 250 r2 = p->c; /* Could get 44 on weakly order system. */
266 } else { <<
267 r2 = p->c - r1; /* Unc <<
268 } 251 }
269 rcu_read_unlock(); <<
270 do_something_with(r1, r2); 252 do_something_with(r1, r2);
271 } 253 }
272 254
273 You might be surprised that the outcome (r1 == 255 You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
274 but you should not be. After all, the updater 256 but you should not be. After all, the updater might have been invoked
275 a second time between the time reader() loaded 257 a second time between the time reader() loaded into "r1" and the time
276 that it loaded into "r2". The fact that this 258 that it loaded into "r2". The fact that this same result can occur due
277 to some reordering from the compiler and CPUs 259 to some reordering from the compiler and CPUs is beside the point.
278 260
279 But suppose that the reader needs a consistent 261 But suppose that the reader needs a consistent view?
280 262
281 Then one approach is to use locking, for examp 263 Then one approach is to use locking, for example, as follows::
282 264
283 struct foo { 265 struct foo {
284 int a; 266 int a;
285 int b; 267 int b;
286 int c; 268 int c;
287 spinlock_t lock; 269 spinlock_t lock;
288 }; 270 };
289 struct foo *gp1; 271 struct foo *gp1;
290 struct foo *gp2; 272 struct foo *gp2;
291 273
292 void updater(void) 274 void updater(void)
293 { 275 {
294 struct foo *p; 276 struct foo *p;
295 277
296 p = kmalloc(...); 278 p = kmalloc(...);
297 if (p == NULL) 279 if (p == NULL)
298 deal_with_it(); 280 deal_with_it();
299 spin_lock(&p->lock); 281 spin_lock(&p->lock);
300 p->a = 42; /* Each field in i 282 p->a = 42; /* Each field in its own cache line. */
301 p->b = 43; 283 p->b = 43;
302 p->c = 44; 284 p->c = 44;
303 spin_unlock(&p->lock); 285 spin_unlock(&p->lock);
304 rcu_assign_pointer(gp1, p); 286 rcu_assign_pointer(gp1, p);
305 spin_lock(&p->lock); 287 spin_lock(&p->lock);
306 p->b = 143; 288 p->b = 143;
307 p->c = 144; 289 p->c = 144;
308 spin_unlock(&p->lock); 290 spin_unlock(&p->lock);
309 rcu_assign_pointer(gp2, p); 291 rcu_assign_pointer(gp2, p);
310 } 292 }
311 293
312 void reader(void) 294 void reader(void)
313 { 295 {
314 struct foo *p; 296 struct foo *p;
315 struct foo *q; 297 struct foo *q;
316 int r1, r2; 298 int r1, r2;
317 299
318 rcu_read_lock(); <<
319 p = rcu_dereference(gp2); 300 p = rcu_dereference(gp2);
320 if (p == NULL) 301 if (p == NULL)
321 return; 302 return;
322 spin_lock(&p->lock); 303 spin_lock(&p->lock);
323 r1 = p->b; /* Guaranteed to g 304 r1 = p->b; /* Guaranteed to get 143. */
324 q = rcu_dereference(gp1); /* 305 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
325 if (p == q) { 306 if (p == q) {
326 /* The compiler decide 307 /* The compiler decides that q->c is same as p->c. */
327 r2 = p->c; /* Locking 308 r2 = p->c; /* Locking guarantees r2 == 144. */
328 } else { <<
329 spin_lock(&q->lock); <<
330 r2 = q->c - r1; <<
331 spin_unlock(&q->lock); <<
332 } 309 }
333 rcu_read_unlock(); <<
334 spin_unlock(&p->lock); 310 spin_unlock(&p->lock);
335 do_something_with(r1, r2); 311 do_something_with(r1, r2);
336 } 312 }
337 313
338 As always, use the right tool for the job! 314 As always, use the right tool for the job!
339 315
340 316
341 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH 317 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
342 ----------------------------------------- 318 -----------------------------------------
343 319
344 If a pointer obtained from rcu_dereference() c 320 If a pointer obtained from rcu_dereference() compares not-equal to some
345 other pointer, the compiler normally has no cl 321 other pointer, the compiler normally has no clue what the value of the
346 first pointer might be. This lack of knowledg 322 first pointer might be. This lack of knowledge prevents the compiler
347 from carrying out optimizations that otherwise 323 from carrying out optimizations that otherwise might destroy the ordering
348 guarantees that RCU depends on. And the volat 324 guarantees that RCU depends on. And the volatile cast in rcu_dereference()
349 should prevent the compiler from guessing the 325 should prevent the compiler from guessing the value.
350 326
351 But without rcu_dereference(), the compiler kn 327 But without rcu_dereference(), the compiler knows more than you might
352 expect. Consider the following code fragment: 328 expect. Consider the following code fragment::
353 329
354 struct foo { 330 struct foo {
355 int a; 331 int a;
356 int b; 332 int b;
357 }; 333 };
358 static struct foo variable1; 334 static struct foo variable1;
359 static struct foo variable2; 335 static struct foo variable2;
360 static struct foo *gp = &variable1; 336 static struct foo *gp = &variable1;
361 337
362 void updater(void) 338 void updater(void)
363 { 339 {
364 initialize_foo(&variable2); 340 initialize_foo(&variable2);
365 rcu_assign_pointer(gp, &variab 341 rcu_assign_pointer(gp, &variable2);
366 /* 342 /*
367 * The above is the only store 343 * The above is the only store to gp in this translation unit,
368 * and the address of gp is no 344 * and the address of gp is not exported in any way.
369 */ 345 */
370 } 346 }
371 347
372 int reader(void) 348 int reader(void)
373 { 349 {
374 struct foo *p; 350 struct foo *p;
375 351
376 p = gp; 352 p = gp;
377 barrier(); 353 barrier();
378 if (p == &variable1) 354 if (p == &variable1)
379 return p->a; /* Must b 355 return p->a; /* Must be variable1.a. */
380 else 356 else
381 return p->b; /* Must b 357 return p->b; /* Must be variable2.b. */
382 } 358 }
383 359
384 Because the compiler can see all stores to "gp 360 Because the compiler can see all stores to "gp", it knows that the only
385 possible values of "gp" are "variable1" on the 361 possible values of "gp" are "variable1" on the one hand and "variable2"
386 on the other. The comparison in reader() ther 362 on the other. The comparison in reader() therefore tells the compiler
387 the exact value of "p" even in the not-equals 363 the exact value of "p" even in the not-equals case. This allows the
388 compiler to make the return values independent 364 compiler to make the return values independent of the load from "gp",
389 in turn destroying the ordering between this l 365 in turn destroying the ordering between this load and the loads of the
390 return values. This can result in "p->b" retu 366 return values. This can result in "p->b" returning pre-initialization
391 garbage values on weakly ordered systems. !! 367 garbage values.
392 368
393 In short, rcu_dereference() is *not* optional 369 In short, rcu_dereference() is *not* optional when you are going to
394 dereference the resulting pointer. 370 dereference the resulting pointer.
395 371
396 372
397 WHICH MEMBER OF THE rcu_dereference() FAMILY S 373 WHICH MEMBER OF THE rcu_dereference() FAMILY SHOULD YOU USE?
398 ---------------------------------------------- 374 ------------------------------------------------------------
399 375
400 First, please avoid using rcu_dereference_raw( 376 First, please avoid using rcu_dereference_raw() and also please avoid
401 using rcu_dereference_check() and rcu_derefere 377 using rcu_dereference_check() and rcu_dereference_protected() with a
402 second argument with a constant value of 1 (or 378 second argument with a constant value of 1 (or true, for that matter).
403 With that caution out of the way, here is some 379 With that caution out of the way, here is some guidance for which
404 member of the rcu_dereference() to use in vari 380 member of the rcu_dereference() to use in various situations:
405 381
406 1. If the access needs to be within an RC 382 1. If the access needs to be within an RCU read-side critical
407 section, use rcu_dereference(). With 383 section, use rcu_dereference(). With the new consolidated
408 RCU flavors, an RCU read-side critical 384 RCU flavors, an RCU read-side critical section is entered
409 using rcu_read_lock(), anything that d 385 using rcu_read_lock(), anything that disables bottom halves,
410 anything that disables interrupts, or 386 anything that disables interrupts, or anything that disables
411 preemption. Please note that spinlock !! 387 preemption.
412 are also implied RCU read-side critica <<
413 they are preemptible, as they are in k <<
414 CONFIG_PREEMPT_RT=y. <<
415 388
416 2. If the access might be within an RCU r 389 2. If the access might be within an RCU read-side critical section
417 on the one hand, or protected by (say) 390 on the one hand, or protected by (say) my_lock on the other,
418 use rcu_dereference_check(), for examp 391 use rcu_dereference_check(), for example::
419 392
420 p1 = rcu_dereference_check(p-> 393 p1 = rcu_dereference_check(p->rcu_protected_pointer,
421 loc 394 lockdep_is_held(&my_lock));
422 395
423 396
424 3. If the access might be within an RCU r 397 3. If the access might be within an RCU read-side critical section
425 on the one hand, or protected by eithe 398 on the one hand, or protected by either my_lock or your_lock on
426 the other, again use rcu_dereference_c 399 the other, again use rcu_dereference_check(), for example::
427 400
428 p1 = rcu_dereference_check(p-> 401 p1 = rcu_dereference_check(p->rcu_protected_pointer,
429 loc 402 lockdep_is_held(&my_lock) ||
430 loc 403 lockdep_is_held(&your_lock));
431 404
432 4. If the access is on the update side, s 405 4. If the access is on the update side, so that it is always protected
433 by my_lock, use rcu_dereference_protec 406 by my_lock, use rcu_dereference_protected()::
434 407
435 p1 = rcu_dereference_protected 408 p1 = rcu_dereference_protected(p->rcu_protected_pointer,
436 409 lockdep_is_held(&my_lock));
437 410
438 This can be extended to handle multipl 411 This can be extended to handle multiple locks as in #3 above,
439 and both can be extended to check othe 412 and both can be extended to check other conditions as well.
440 413
441 5. If the protection is supplied by the c 414 5. If the protection is supplied by the caller, and is thus unknown
442 to this code, that is the rare case wh 415 to this code, that is the rare case when rcu_dereference_raw()
443 is appropriate. In addition, rcu_dere 416 is appropriate. In addition, rcu_dereference_raw() might be
444 appropriate when the lockdep expressio 417 appropriate when the lockdep expression would be excessively
445 complex, except that a better approach 418 complex, except that a better approach in that case might be to
446 take a long hard look at your synchron 419 take a long hard look at your synchronization design. Still,
447 there are data-locking cases where any 420 there are data-locking cases where any one of a very large number
448 of locks or reference counters suffice 421 of locks or reference counters suffices to protect the pointer,
449 so rcu_dereference_raw() does have its 422 so rcu_dereference_raw() does have its place.
450 423
451 However, its place is probably quite a 424 However, its place is probably quite a bit smaller than one
452 might expect given the number of uses 425 might expect given the number of uses in the current kernel.
453 Ditto for its synonym, rcu_dereference 426 Ditto for its synonym, rcu_dereference_check( ... , 1), and
454 its close relative, rcu_dereference_pr 427 its close relative, rcu_dereference_protected(... , 1).
455 428
456 429
457 SPARSE CHECKING OF RCU-PROTECTED POINTERS 430 SPARSE CHECKING OF RCU-PROTECTED POINTERS
458 ----------------------------------------- 431 -----------------------------------------
459 432
460 The sparse static-analysis tool checks for non !! 433 The sparse static-analysis tool checks for direct access to RCU-protected
461 pointers, which can result in "interesting" bu 434 pointers, which can result in "interesting" bugs due to compiler
462 optimizations involving invented loads and per 435 optimizations involving invented loads and perhaps also load tearing.
463 For example, suppose someone mistakenly does s 436 For example, suppose someone mistakenly does something like this::
464 437
465 p = q->rcu_protected_pointer; 438 p = q->rcu_protected_pointer;
466 do_something_with(p->a); 439 do_something_with(p->a);
467 do_something_else_with(p->b); 440 do_something_else_with(p->b);
468 441
469 If register pressure is high, the compiler mig 442 If register pressure is high, the compiler might optimize "p" out
470 of existence, transforming the code to somethi 443 of existence, transforming the code to something like this::
471 444
472 do_something_with(q->rcu_protected_poi 445 do_something_with(q->rcu_protected_pointer->a);
473 do_something_else_with(q->rcu_protecte 446 do_something_else_with(q->rcu_protected_pointer->b);
474 447
475 This could fatally disappoint your code if q-> 448 This could fatally disappoint your code if q->rcu_protected_pointer
476 changed in the meantime. Nor is this a theore 449 changed in the meantime. Nor is this a theoretical problem: Exactly
477 this sort of bug cost Paul E. McKenney (and se 450 this sort of bug cost Paul E. McKenney (and several of his innocent
478 colleagues) a three-day weekend back in the ea 451 colleagues) a three-day weekend back in the early 1990s.
479 452
480 Load tearing could of course result in derefer 453 Load tearing could of course result in dereferencing a mashup of a pair
481 of pointers, which also might fatally disappoi 454 of pointers, which also might fatally disappoint your code.
482 455
483 These problems could have been avoided simply 456 These problems could have been avoided simply by making the code instead
484 read as follows:: 457 read as follows::
485 458
486 p = rcu_dereference(q->rcu_protected_p 459 p = rcu_dereference(q->rcu_protected_pointer);
487 do_something_with(p->a); 460 do_something_with(p->a);
488 do_something_else_with(p->b); 461 do_something_else_with(p->b);
489 462
490 Unfortunately, these sorts of bugs can be extr 463 Unfortunately, these sorts of bugs can be extremely hard to spot during
491 review. This is where the sparse tool comes i 464 review. This is where the sparse tool comes into play, along with the
492 "__rcu" marker. If you mark a pointer declara 465 "__rcu" marker. If you mark a pointer declaration, whether in a structure
493 or as a formal parameter, with "__rcu", which 466 or as a formal parameter, with "__rcu", which tells sparse to complain if
494 this pointer is accessed directly. It will al 467 this pointer is accessed directly. It will also cause sparse to complain
495 if a pointer not marked with "__rcu" is access 468 if a pointer not marked with "__rcu" is accessed using rcu_dereference()
496 and friends. For example, ->rcu_protected_poi 469 and friends. For example, ->rcu_protected_pointer might be declared as
497 follows:: 470 follows::
498 471
499 struct foo __rcu *rcu_protected_pointe 472 struct foo __rcu *rcu_protected_pointer;
500 473
501 Use of "__rcu" is opt-in. If you choose not t 474 Use of "__rcu" is opt-in. If you choose not to use it, then you should
502 ignore the sparse warnings. 475 ignore the sparse warnings.
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