1 .. _rcu_dereference_doc:
3 PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
4 ===============================================================
6 Most of the time, you can use values from rcu_dereference() or one of
7 the similar primitives without worries. Dereferencing (prefix "*"),
8 field selection ("->"), assignment ("="), address-of ("&"), addition and
9 subtraction of constants, and casts all work quite naturally and safely.
11 It is nevertheless possible to get into trouble with other operations.
12 Follow these rules to keep your RCU code working properly:
14 - You must use one of the rcu_dereference() family of primitives
15 to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
16 will complain. Worse yet, your code can see random memory-corruption
17 bugs due to games that compilers and DEC Alpha can play.
18 Without one of the rcu_dereference() primitives, compilers
19 can reload the value, and won't your code have fun with two
20 different values for a single pointer! Without rcu_dereference(),
21 DEC Alpha can load a pointer, dereference that pointer, and
22 return data preceding initialization that preceded the store
23 of the pointer. (As noted later, in recent kernels READ_ONCE()
24 also prevents DEC Alpha from playing these tricks.)
26 In addition, the volatile cast in rcu_dereference() prevents the
27 compiler from deducing the resulting pointer value. Please see
28 the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
29 for an example where the compiler can in fact deduce the exact
30 value of the pointer, and thus cause misordering.
32 - In the special case where data is added but is never removed
33 while readers are accessing the structure, READ_ONCE() may be used
34 instead of rcu_dereference(). In this case, use of READ_ONCE()
35 takes on the role of the lockless_dereference() primitive that
38 - You are only permitted to use rcu_dereference() on pointer values.
39 The compiler simply knows too much about integral values to
40 trust it to carry dependencies through integer operations.
41 There are a very few exceptions, namely that you can temporarily
42 cast the pointer to uintptr_t in order to:
44 - Set bits and clear bits down in the must-be-zero low-order
45 bits of that pointer. This clearly means that the pointer
46 must have alignment constraints, for example, this does
47 *not* work in general for char* pointers.
49 - XOR bits to translate pointers, as is done in some
50 classic buddy-allocator algorithms.
52 It is important to cast the value back to pointer before
53 doing much of anything else with it.
55 - Avoid cancellation when using the "+" and "-" infix arithmetic
56 operators. For example, for a given variable "x", avoid
57 "(x-(uintptr_t)x)" for char* pointers. The compiler is within its
58 rights to substitute zero for this sort of expression, so that
59 subsequent accesses no longer depend on the rcu_dereference(),
60 again possibly resulting in bugs due to misordering.
62 Of course, if "p" is a pointer from rcu_dereference(), and "a"
63 and "b" are integers that happen to be equal, the expression
64 "p+a-b" is safe because its value still necessarily depends on
65 the rcu_dereference(), thus maintaining proper ordering.
67 - If you are using RCU to protect JITed functions, so that the
68 "()" function-invocation operator is applied to a value obtained
69 (directly or indirectly) from rcu_dereference(), you may need to
70 interact directly with the hardware to flush instruction caches.
71 This issue arises on some systems when a newly JITed function is
72 using the same memory that was used by an earlier JITed function.
74 - Do not use the results from relational operators ("==", "!=",
75 ">", ">=", "<", or "<=") when dereferencing. For example,
76 the following (quite strange) code is buggy::
83 p = rcu_dereference(gp)
86 r1 = *q; /* BUGGY!!! */
88 As before, the reason this is buggy is that relational operators
89 are often compiled using branches. And as before, although
90 weak-memory machines such as ARM or PowerPC do order stores
91 after such branches, but can speculate loads, which can again
92 result in misordering bugs.
94 - Be very careful about comparing pointers obtained from
95 rcu_dereference() against non-NULL values. As Linus Torvalds
96 explained, if the two pointers are equal, the compiler could
97 substitute the pointer you are comparing against for the pointer
98 obtained from rcu_dereference(). For example::
100 p = rcu_dereference(gp);
101 if (p == &default_struct)
104 Because the compiler now knows that the value of "p" is exactly
105 the address of the variable "default_struct", it is free to
106 transform this code into the following::
108 p = rcu_dereference(gp);
109 if (p == &default_struct)
110 do_default(default_struct.a);
112 On ARM and Power hardware, the load from "default_struct.a"
113 can now be speculated, such that it might happen before the
114 rcu_dereference(). This could result in bugs due to misordering.
116 However, comparisons are OK in the following cases:
118 - The comparison was against the NULL pointer. If the
119 compiler knows that the pointer is NULL, you had better
120 not be dereferencing it anyway. If the comparison is
121 non-equal, the compiler is none the wiser. Therefore,
122 it is safe to compare pointers from rcu_dereference()
123 against NULL pointers.
125 - The pointer is never dereferenced after being compared.
126 Since there are no subsequent dereferences, the compiler
127 cannot use anything it learned from the comparison
128 to reorder the non-existent subsequent dereferences.
129 This sort of comparison occurs frequently when scanning
130 RCU-protected circular linked lists.
132 Note that if the pointer comparison is done outside
133 of an RCU read-side critical section, and the pointer
134 is never dereferenced, rcu_access_pointer() should be
135 used in place of rcu_dereference(). In most cases,
136 it is best to avoid accidental dereferences by testing
137 the rcu_access_pointer() return value directly, without
138 assigning it to a variable.
140 Within an RCU read-side critical section, there is little
141 reason to use rcu_access_pointer().
143 - The comparison is against a pointer that references memory
144 that was initialized "a long time ago." The reason
145 this is safe is that even if misordering occurs, the
146 misordering will not affect the accesses that follow
147 the comparison. So exactly how long ago is "a long
148 time ago"? Here are some possibilities:
154 - Module-init time for module code.
156 - Prior to kthread creation for kthread code.
158 - During some prior acquisition of the lock that
161 - Before mod_timer() time for a timer handler.
163 There are many other possibilities involving the Linux
164 kernel's wide array of primitives that cause code to
165 be invoked at a later time.
167 - The pointer being compared against also came from
168 rcu_dereference(). In this case, both pointers depend
169 on one rcu_dereference() or another, so you get proper
172 That said, this situation can make certain RCU usage
173 bugs more likely to happen. Which can be a good thing,
174 at least if they happen during testing. An example
175 of such an RCU usage bug is shown in the section titled
176 "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
178 - All of the accesses following the comparison are stores,
179 so that a control dependency preserves the needed ordering.
180 That said, it is easy to get control dependencies wrong.
181 Please see the "CONTROL DEPENDENCIES" section of
182 Documentation/memory-barriers.txt for more details.
184 - The pointers are not equal *and* the compiler does
185 not have enough information to deduce the value of the
186 pointer. Note that the volatile cast in rcu_dereference()
187 will normally prevent the compiler from knowing too much.
189 However, please note that if the compiler knows that the
190 pointer takes on only one of two values, a not-equal
191 comparison will provide exactly the information that the
192 compiler needs to deduce the value of the pointer.
194 - Disable any value-speculation optimizations that your compiler
195 might provide, especially if you are making use of feedback-based
196 optimizations that take data collected from prior runs. Such
197 value-speculation optimizations reorder operations by design.
199 There is one exception to this rule: Value-speculation
200 optimizations that leverage the branch-prediction hardware are
201 safe on strongly ordered systems (such as x86), but not on weakly
202 ordered systems (such as ARM or Power). Choose your compiler
203 command-line options wisely!
206 EXAMPLE OF AMPLIFIED RCU-USAGE BUG
207 ----------------------------------
209 Because updaters can run concurrently with RCU readers, RCU readers can
210 see stale and/or inconsistent values. If RCU readers need fresh or
211 consistent values, which they sometimes do, they need to take proper
212 precautions. To see this, consider the following code fragment::
229 p->a = 42; /* Each field in its own cache line. */
232 rcu_assign_pointer(gp1, p);
235 rcu_assign_pointer(gp2, p);
245 p = rcu_dereference(gp2);
248 r1 = p->b; /* Guaranteed to get 143. */
249 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
251 /* The compiler decides that q->c is same as p->c. */
252 r2 = p->c; /* Could get 44 on weakly order system. */
254 r2 = p->c - r1; /* Unconditional access to p->c. */
257 do_something_with(r1, r2);
260 You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
261 but you should not be. After all, the updater might have been invoked
262 a second time between the time reader() loaded into "r1" and the time
263 that it loaded into "r2". The fact that this same result can occur due
264 to some reordering from the compiler and CPUs is beside the point.
266 But suppose that the reader needs a consistent view?
268 Then one approach is to use locking, for example, as follows::
287 p->a = 42; /* Each field in its own cache line. */
290 spin_unlock(&p->lock);
291 rcu_assign_pointer(gp1, p);
295 spin_unlock(&p->lock);
296 rcu_assign_pointer(gp2, p);
306 p = rcu_dereference(gp2);
310 r1 = p->b; /* Guaranteed to get 143. */
311 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
313 /* The compiler decides that q->c is same as p->c. */
314 r2 = p->c; /* Locking guarantees r2 == 144. */
318 spin_unlock(&q->lock);
321 spin_unlock(&p->lock);
322 do_something_with(r1, r2);
325 As always, use the right tool for the job!
328 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
329 -----------------------------------------
331 If a pointer obtained from rcu_dereference() compares not-equal to some
332 other pointer, the compiler normally has no clue what the value of the
333 first pointer might be. This lack of knowledge prevents the compiler
334 from carrying out optimizations that otherwise might destroy the ordering
335 guarantees that RCU depends on. And the volatile cast in rcu_dereference()
336 should prevent the compiler from guessing the value.
338 But without rcu_dereference(), the compiler knows more than you might
339 expect. Consider the following code fragment::
345 static struct foo variable1;
346 static struct foo variable2;
347 static struct foo *gp = &variable1;
351 initialize_foo(&variable2);
352 rcu_assign_pointer(gp, &variable2);
354 * The above is the only store to gp in this translation unit,
355 * and the address of gp is not exported in any way.
366 return p->a; /* Must be variable1.a. */
368 return p->b; /* Must be variable2.b. */
371 Because the compiler can see all stores to "gp", it knows that the only
372 possible values of "gp" are "variable1" on the one hand and "variable2"
373 on the other. The comparison in reader() therefore tells the compiler
374 the exact value of "p" even in the not-equals case. This allows the
375 compiler to make the return values independent of the load from "gp",
376 in turn destroying the ordering between this load and the loads of the
377 return values. This can result in "p->b" returning pre-initialization
378 garbage values on weakly ordered systems.
380 In short, rcu_dereference() is *not* optional when you are going to
381 dereference the resulting pointer.
384 WHICH MEMBER OF THE rcu_dereference() FAMILY SHOULD YOU USE?
385 ------------------------------------------------------------
387 First, please avoid using rcu_dereference_raw() and also please avoid
388 using rcu_dereference_check() and rcu_dereference_protected() with a
389 second argument with a constant value of 1 (or true, for that matter).
390 With that caution out of the way, here is some guidance for which
391 member of the rcu_dereference() to use in various situations:
393 1. If the access needs to be within an RCU read-side critical
394 section, use rcu_dereference(). With the new consolidated
395 RCU flavors, an RCU read-side critical section is entered
396 using rcu_read_lock(), anything that disables bottom halves,
397 anything that disables interrupts, or anything that disables
400 2. If the access might be within an RCU read-side critical section
401 on the one hand, or protected by (say) my_lock on the other,
402 use rcu_dereference_check(), for example::
404 p1 = rcu_dereference_check(p->rcu_protected_pointer,
405 lockdep_is_held(&my_lock));
408 3. If the access might be within an RCU read-side critical section
409 on the one hand, or protected by either my_lock or your_lock on
410 the other, again use rcu_dereference_check(), for example::
412 p1 = rcu_dereference_check(p->rcu_protected_pointer,
413 lockdep_is_held(&my_lock) ||
414 lockdep_is_held(&your_lock));
416 4. If the access is on the update side, so that it is always protected
417 by my_lock, use rcu_dereference_protected()::
419 p1 = rcu_dereference_protected(p->rcu_protected_pointer,
420 lockdep_is_held(&my_lock));
422 This can be extended to handle multiple locks as in #3 above,
423 and both can be extended to check other conditions as well.
425 5. If the protection is supplied by the caller, and is thus unknown
426 to this code, that is the rare case when rcu_dereference_raw()
427 is appropriate. In addition, rcu_dereference_raw() might be
428 appropriate when the lockdep expression would be excessively
429 complex, except that a better approach in that case might be to
430 take a long hard look at your synchronization design. Still,
431 there are data-locking cases where any one of a very large number
432 of locks or reference counters suffices to protect the pointer,
433 so rcu_dereference_raw() does have its place.
435 However, its place is probably quite a bit smaller than one
436 might expect given the number of uses in the current kernel.
437 Ditto for its synonym, rcu_dereference_check( ... , 1), and
438 its close relative, rcu_dereference_protected(... , 1).
441 SPARSE CHECKING OF RCU-PROTECTED POINTERS
442 -----------------------------------------
444 The sparse static-analysis tool checks for non-RCU access to RCU-protected
445 pointers, which can result in "interesting" bugs due to compiler
446 optimizations involving invented loads and perhaps also load tearing.
447 For example, suppose someone mistakenly does something like this::
449 p = q->rcu_protected_pointer;
450 do_something_with(p->a);
451 do_something_else_with(p->b);
453 If register pressure is high, the compiler might optimize "p" out
454 of existence, transforming the code to something like this::
456 do_something_with(q->rcu_protected_pointer->a);
457 do_something_else_with(q->rcu_protected_pointer->b);
459 This could fatally disappoint your code if q->rcu_protected_pointer
460 changed in the meantime. Nor is this a theoretical problem: Exactly
461 this sort of bug cost Paul E. McKenney (and several of his innocent
462 colleagues) a three-day weekend back in the early 1990s.
464 Load tearing could of course result in dereferencing a mashup of a pair
465 of pointers, which also might fatally disappoint your code.
467 These problems could have been avoided simply by making the code instead
470 p = rcu_dereference(q->rcu_protected_pointer);
471 do_something_with(p->a);
472 do_something_else_with(p->b);
474 Unfortunately, these sorts of bugs can be extremely hard to spot during
475 review. This is where the sparse tool comes into play, along with the
476 "__rcu" marker. If you mark a pointer declaration, whether in a structure
477 or as a formal parameter, with "__rcu", which tells sparse to complain if
478 this pointer is accessed directly. It will also cause sparse to complain
479 if a pointer not marked with "__rcu" is accessed using rcu_dereference()
480 and friends. For example, ->rcu_protected_pointer might be declared as
483 struct foo __rcu *rcu_protected_pointer;
485 Use of "__rcu" is opt-in. If you choose not to use it, then you should
486 ignore the sparse warnings.