1 PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
3 Most of the time, you can use values from rcu_dereference() or one of
4 the similar primitives without worries. Dereferencing (prefix "*"),
5 field selection ("->"), assignment ("="), address-of ("&"), addition and
6 subtraction of constants, and casts all work quite naturally and safely.
8 It is nevertheless possible to get into trouble with other operations.
9 Follow these rules to keep your RCU code working properly:
11 o You must use one of the rcu_dereference() family of primitives
12 to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
13 will complain. Worse yet, your code can see random memory-corruption
14 bugs due to games that compilers and DEC Alpha can play.
15 Without one of the rcu_dereference() primitives, compilers
16 can reload the value, and won't your code have fun with two
17 different values for a single pointer! Without rcu_dereference(),
18 DEC Alpha can load a pointer, dereference that pointer, and
19 return data preceding initialization that preceded the store of
22 In addition, the volatile cast in rcu_dereference() prevents the
23 compiler from deducing the resulting pointer value. Please see
24 the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
25 for an example where the compiler can in fact deduce the exact
26 value of the pointer, and thus cause misordering.
28 o You are only permitted to use rcu_dereference on pointer values.
29 The compiler simply knows too much about integral values to
30 trust it to carry dependencies through integer operations.
31 There are a very few exceptions, namely that you can temporarily
32 cast the pointer to uintptr_t in order to:
34 o Set bits and clear bits down in the must-be-zero low-order
35 bits of that pointer. This clearly means that the pointer
36 must have alignment constraints, for example, this does
37 -not- work in general for char* pointers.
39 o XOR bits to translate pointers, as is done in some
40 classic buddy-allocator algorithms.
42 It is important to cast the value back to pointer before
43 doing much of anything else with it.
45 o Avoid cancellation when using the "+" and "-" infix arithmetic
46 operators. For example, for a given variable "x", avoid
47 "(x-(uintptr_t)x)" for char* pointers. The compiler is within its
48 rights to substitute zero for this sort of expression, so that
49 subsequent accesses no longer depend on the rcu_dereference(),
50 again possibly resulting in bugs due to misordering.
52 Of course, if "p" is a pointer from rcu_dereference(), and "a"
53 and "b" are integers that happen to be equal, the expression
54 "p+a-b" is safe because its value still necessarily depends on
55 the rcu_dereference(), thus maintaining proper ordering.
57 o If you are using RCU to protect JITed functions, so that the
58 "()" function-invocation operator is applied to a value obtained
59 (directly or indirectly) from rcu_dereference(), you may need to
60 interact directly with the hardware to flush instruction caches.
61 This issue arises on some systems when a newly JITed function is
62 using the same memory that was used by an earlier JITed function.
64 o Do not use the results from relational operators ("==", "!=",
65 ">", ">=", "<", or "<=") when dereferencing. For example,
66 the following (quite strange) code is buggy:
73 p = rcu_dereference(gp)
76 r1 = *q; /* BUGGY!!! */
78 As before, the reason this is buggy is that relational operators
79 are often compiled using branches. And as before, although
80 weak-memory machines such as ARM or PowerPC do order stores
81 after such branches, but can speculate loads, which can again
82 result in misordering bugs.
84 o Be very careful about comparing pointers obtained from
85 rcu_dereference() against non-NULL values. As Linus Torvalds
86 explained, if the two pointers are equal, the compiler could
87 substitute the pointer you are comparing against for the pointer
88 obtained from rcu_dereference(). For example:
90 p = rcu_dereference(gp);
91 if (p == &default_struct)
94 Because the compiler now knows that the value of "p" is exactly
95 the address of the variable "default_struct", it is free to
96 transform this code into the following:
98 p = rcu_dereference(gp);
99 if (p == &default_struct)
100 do_default(default_struct.a);
102 On ARM and Power hardware, the load from "default_struct.a"
103 can now be speculated, such that it might happen before the
104 rcu_dereference(). This could result in bugs due to misordering.
106 However, comparisons are OK in the following cases:
108 o The comparison was against the NULL pointer. If the
109 compiler knows that the pointer is NULL, you had better
110 not be dereferencing it anyway. If the comparison is
111 non-equal, the compiler is none the wiser. Therefore,
112 it is safe to compare pointers from rcu_dereference()
113 against NULL pointers.
115 o The pointer is never dereferenced after being compared.
116 Since there are no subsequent dereferences, the compiler
117 cannot use anything it learned from the comparison
118 to reorder the non-existent subsequent dereferences.
119 This sort of comparison occurs frequently when scanning
120 RCU-protected circular linked lists.
122 Note that if checks for being within an RCU read-side
123 critical section are not required and the pointer is never
124 dereferenced, rcu_access_pointer() should be used in place
125 of rcu_dereference().
127 o The comparison is against a pointer that references memory
128 that was initialized "a long time ago." The reason
129 this is safe is that even if misordering occurs, the
130 misordering will not affect the accesses that follow
131 the comparison. So exactly how long ago is "a long
132 time ago"? Here are some possibilities:
138 o Module-init time for module code.
140 o Prior to kthread creation for kthread code.
142 o During some prior acquisition of the lock that
145 o Before mod_timer() time for a timer handler.
147 There are many other possibilities involving the Linux
148 kernel's wide array of primitives that cause code to
149 be invoked at a later time.
151 o The pointer being compared against also came from
152 rcu_dereference(). In this case, both pointers depend
153 on one rcu_dereference() or another, so you get proper
156 That said, this situation can make certain RCU usage
157 bugs more likely to happen. Which can be a good thing,
158 at least if they happen during testing. An example
159 of such an RCU usage bug is shown in the section titled
160 "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
162 o All of the accesses following the comparison are stores,
163 so that a control dependency preserves the needed ordering.
164 That said, it is easy to get control dependencies wrong.
165 Please see the "CONTROL DEPENDENCIES" section of
166 Documentation/memory-barriers.txt for more details.
168 o The pointers are not equal -and- the compiler does
169 not have enough information to deduce the value of the
170 pointer. Note that the volatile cast in rcu_dereference()
171 will normally prevent the compiler from knowing too much.
173 However, please note that if the compiler knows that the
174 pointer takes on only one of two values, a not-equal
175 comparison will provide exactly the information that the
176 compiler needs to deduce the value of the pointer.
178 o Disable any value-speculation optimizations that your compiler
179 might provide, especially if you are making use of feedback-based
180 optimizations that take data collected from prior runs. Such
181 value-speculation optimizations reorder operations by design.
183 There is one exception to this rule: Value-speculation
184 optimizations that leverage the branch-prediction hardware are
185 safe on strongly ordered systems (such as x86), but not on weakly
186 ordered systems (such as ARM or Power). Choose your compiler
187 command-line options wisely!
190 EXAMPLE OF AMPLIFIED RCU-USAGE BUG
192 Because updaters can run concurrently with RCU readers, RCU readers can
193 see stale and/or inconsistent values. If RCU readers need fresh or
194 consistent values, which they sometimes do, they need to take proper
195 precautions. To see this, consider the following code fragment:
212 p->a = 42; /* Each field in its own cache line. */
215 rcu_assign_pointer(gp1, p);
218 rcu_assign_pointer(gp2, p);
227 p = rcu_dereference(gp2);
230 r1 = p->b; /* Guaranteed to get 143. */
231 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
233 /* The compiler decides that q->c is same as p->c. */
234 r2 = p->c; /* Could get 44 on weakly order system. */
236 do_something_with(r1, r2);
239 You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
240 but you should not be. After all, the updater might have been invoked
241 a second time between the time reader() loaded into "r1" and the time
242 that it loaded into "r2". The fact that this same result can occur due
243 to some reordering from the compiler and CPUs is beside the point.
245 But suppose that the reader needs a consistent view?
247 Then one approach is to use locking, for example, as follows:
266 p->a = 42; /* Each field in its own cache line. */
269 spin_unlock(&p->lock);
270 rcu_assign_pointer(gp1, p);
274 spin_unlock(&p->lock);
275 rcu_assign_pointer(gp2, p);
284 p = rcu_dereference(gp2);
288 r1 = p->b; /* Guaranteed to get 143. */
289 q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
291 /* The compiler decides that q->c is same as p->c. */
292 r2 = p->c; /* Locking guarantees r2 == 144. */
294 spin_unlock(&p->lock);
295 do_something_with(r1, r2);
298 As always, use the right tool for the job!
301 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
303 If a pointer obtained from rcu_dereference() compares not-equal to some
304 other pointer, the compiler normally has no clue what the value of the
305 first pointer might be. This lack of knowledge prevents the compiler
306 from carrying out optimizations that otherwise might destroy the ordering
307 guarantees that RCU depends on. And the volatile cast in rcu_dereference()
308 should prevent the compiler from guessing the value.
310 But without rcu_dereference(), the compiler knows more than you might
311 expect. Consider the following code fragment:
317 static struct foo variable1;
318 static struct foo variable2;
319 static struct foo *gp = &variable1;
323 initialize_foo(&variable2);
324 rcu_assign_pointer(gp, &variable2);
326 * The above is the only store to gp in this translation unit,
327 * and the address of gp is not exported in any way.
338 return p->a; /* Must be variable1.a. */
340 return p->b; /* Must be variable2.b. */
343 Because the compiler can see all stores to "gp", it knows that the only
344 possible values of "gp" are "variable1" on the one hand and "variable2"
345 on the other. The comparison in reader() therefore tells the compiler
346 the exact value of "p" even in the not-equals case. This allows the
347 compiler to make the return values independent of the load from "gp",
348 in turn destroying the ordering between this load and the loads of the
349 return values. This can result in "p->b" returning pre-initialization
352 In short, rcu_dereference() is -not- optional when you are going to
353 dereference the resulting pointer.
356 WHICH MEMBER OF THE rcu_dereference() FAMILY SHOULD YOU USE?
358 First, please avoid using rcu_dereference_raw() and also please avoid
359 using rcu_dereference_check() and rcu_dereference_protected() with a
360 second argument with a constant value of 1 (or true, for that matter).
361 With that caution out of the way, here is some guidance for which
362 member of the rcu_dereference() to use in various situations:
364 1. If the access needs to be within an RCU read-side critical
365 section, use rcu_dereference(). With the new consolidated
366 RCU flavors, an RCU read-side critical section is entered
367 using rcu_read_lock(), anything that disables bottom halves,
368 anything that disables interrupts, or anything that disables
371 2. If the access might be within an RCU read-side critical section
372 on the one hand, or protected by (say) my_lock on the other,
373 use rcu_dereference_check(), for example:
375 p1 = rcu_dereference_check(p->rcu_protected_pointer,
376 lockdep_is_held(&my_lock));
379 3. If the access might be within an RCU read-side critical section
380 on the one hand, or protected by either my_lock or your_lock on
381 the other, again use rcu_dereference_check(), for example:
383 p1 = rcu_dereference_check(p->rcu_protected_pointer,
384 lockdep_is_held(&my_lock) ||
385 lockdep_is_held(&your_lock));
387 4. If the access is on the update side, so that it is always protected
388 by my_lock, use rcu_dereference_protected():
390 p1 = rcu_dereference_protected(p->rcu_protected_pointer,
391 lockdep_is_held(&my_lock));
393 This can be extended to handle multiple locks as in #3 above,
394 and both can be extended to check other conditions as well.
396 5. If the protection is supplied by the caller, and is thus unknown
397 to this code, that is the rare case when rcu_dereference_raw()
398 is appropriate. In addition, rcu_dereference_raw() might be
399 appropriate when the lockdep expression would be excessively
400 complex, except that a better approach in that case might be to
401 take a long hard look at your synchronization design. Still,
402 there are data-locking cases where any one of a very large number
403 of locks or reference counters suffices to protect the pointer,
404 so rcu_dereference_raw() does have its place.
406 However, its place is probably quite a bit smaller than one
407 might expect given the number of uses in the current kernel.
408 Ditto for its synonym, rcu_dereference_check( ... , 1), and
409 its close relative, rcu_dereference_protected(... , 1).
412 SPARSE CHECKING OF RCU-PROTECTED POINTERS
414 The sparse static-analysis tool checks for direct access to RCU-protected
415 pointers, which can result in "interesting" bugs due to compiler
416 optimizations involving invented loads and perhaps also load tearing.
417 For example, suppose someone mistakenly does something like this:
419 p = q->rcu_protected_pointer;
420 do_something_with(p->a);
421 do_something_else_with(p->b);
423 If register pressure is high, the compiler might optimize "p" out
424 of existence, transforming the code to something like this:
426 do_something_with(q->rcu_protected_pointer->a);
427 do_something_else_with(q->rcu_protected_pointer->b);
429 This could fatally disappoint your code if q->rcu_protected_pointer
430 changed in the meantime. Nor is this a theoretical problem: Exactly
431 this sort of bug cost Paul E. McKenney (and several of his innocent
432 colleagues) a three-day weekend back in the early 1990s.
434 Load tearing could of course result in dereferencing a mashup of a pair
435 of pointers, which also might fatally disappoint your code.
437 These problems could have been avoided simply by making the code instead
440 p = rcu_dereference(q->rcu_protected_pointer);
441 do_something_with(p->a);
442 do_something_else_with(p->b);
444 Unfortunately, these sorts of bugs can be extremely hard to spot during
445 review. This is where the sparse tool comes into play, along with the
446 "__rcu" marker. If you mark a pointer declaration, whether in a structure
447 or as a formal parameter, with "__rcu", which tells sparse to complain if
448 this pointer is accessed directly. It will also cause sparse to complain
449 if a pointer not marked with "__rcu" is accessed using rcu_dereference()
450 and friends. For example, ->rcu_protected_pointer might be declared as
453 struct foo __rcu *rcu_protected_pointer;
455 Use of "__rcu" is opt-in. If you choose not to use it, then you should
456 ignore the sparse warnings.