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