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058cc23b MCC |
1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ======================== | |
c598a070 | 4 | RCU and lockdep checking |
058cc23b | 5 | ======================== |
c598a070 PM |
6 | |
7 | All flavors of RCU have lockdep checking available, so that lockdep is | |
8 | aware of when each task enters and leaves any flavor of RCU read-side | |
9 | critical section. Each flavor of RCU is tracked separately (but note | |
10 | that this is not the case in 2.6.32 and earlier). This allows lockdep's | |
11 | tracking to include RCU state, which can sometimes help when debugging | |
12 | deadlocks and the like. | |
13 | ||
14 | In addition, RCU provides the following primitives that check lockdep's | |
058cc23b | 15 | state:: |
c598a070 PM |
16 | |
17 | rcu_read_lock_held() for normal RCU. | |
18 | rcu_read_lock_bh_held() for RCU-bh. | |
19 | rcu_read_lock_sched_held() for RCU-sched. | |
20 | srcu_read_lock_held() for SRCU. | |
21 | ||
22 | These functions are conservative, and will therefore return 1 if they | |
23 | aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). | |
24 | This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false | |
25 | positives when lockdep is disabled. | |
26 | ||
27 | In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables | |
28 | checking of rcu_dereference() primitives: | |
29 | ||
30 | rcu_dereference(p): | |
31 | Check for RCU read-side critical section. | |
32 | rcu_dereference_bh(p): | |
33 | Check for RCU-bh read-side critical section. | |
34 | rcu_dereference_sched(p): | |
35 | Check for RCU-sched read-side critical section. | |
36 | srcu_dereference(p, sp): | |
37 | Check for SRCU read-side critical section. | |
38 | rcu_dereference_check(p, c): | |
8cd889cb PM |
39 | Use explicit check expression "c" along with |
40 | rcu_read_lock_held(). This is useful in code that is | |
41 | invoked by both RCU readers and updaters. | |
42 | rcu_dereference_bh_check(p, c): | |
43 | Use explicit check expression "c" along with | |
44 | rcu_read_lock_bh_held(). This is useful in code that | |
45 | is invoked by both RCU-bh readers and updaters. | |
46 | rcu_dereference_sched_check(p, c): | |
47 | Use explicit check expression "c" along with | |
48 | rcu_read_lock_sched_held(). This is useful in code that | |
49 | is invoked by both RCU-sched readers and updaters. | |
50 | srcu_dereference_check(p, c): | |
51 | Use explicit check expression "c" along with | |
053f8fc7 | 52 | srcu_read_lock_held(). This is useful in code that |
8cd889cb | 53 | is invoked by both SRCU readers and updaters. |
8cd889cb | 54 | rcu_dereference_raw(p): |
c598a070 | 55 | Don't check. (Use sparingly, if at all.) |
50aec002 PM |
56 | rcu_dereference_protected(p, c): |
57 | Use explicit check expression "c", and omit all barriers | |
58 | and compiler constraints. This is useful when the data | |
59 | structure cannot change, for example, in code that is | |
60 | invoked only by updaters. | |
61 | rcu_access_pointer(p): | |
62 | Return the value of the pointer and omit all barriers, | |
63 | but retain the compiler constraints that prevent duplicating | |
64 | or coalescsing. This is useful when when testing the | |
65 | value of the pointer itself, for example, against NULL. | |
c598a070 PM |
66 | |
67 | The rcu_dereference_check() check expression can be any boolean | |
e5177ec7 MH |
68 | expression, but would normally include a lockdep expression. However, |
69 | any boolean expression can be used. For a moderately ornate example, | |
058cc23b | 70 | consider the following:: |
c598a070 PM |
71 | |
72 | file = rcu_dereference_check(fdt->fd[fd], | |
c598a070 PM |
73 | lockdep_is_held(&files->file_lock) || |
74 | atomic_read(&files->count) == 1); | |
75 | ||
76 | This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner, | |
77 | and, if CONFIG_PROVE_RCU is configured, verifies that this expression | |
78 | is used in: | |
79 | ||
e5177ec7 | 80 | 1. An RCU read-side critical section (implicit), or |
c598a070 PM |
81 | 2. with files->file_lock held, or |
82 | 3. on an unshared files_struct. | |
83 | ||
84 | In case (1), the pointer is picked up in an RCU-safe manner for vanilla | |
85 | RCU read-side critical sections, in case (2) the ->file_lock prevents | |
86 | any change from taking place, and finally, in case (3) the current task | |
87 | is the only task accessing the file_struct, again preventing any change | |
50aec002 | 88 | from taking place. If the above statement was invoked only from updater |
058cc23b | 89 | code, it could instead be written as follows:: |
50aec002 PM |
90 | |
91 | file = rcu_dereference_protected(fdt->fd[fd], | |
92 | lockdep_is_held(&files->file_lock) || | |
93 | atomic_read(&files->count) == 1); | |
94 | ||
95 | This would verify cases #2 and #3 above, and furthermore lockdep would | |
96 | complain if this was used in an RCU read-side critical section unless one | |
97 | of these two cases held. Because rcu_dereference_protected() omits all | |
98 | barriers and compiler constraints, it generates better code than do the | |
99 | other flavors of rcu_dereference(). On the other hand, it is illegal | |
100 | to use rcu_dereference_protected() if either the RCU-protected pointer | |
101 | or the RCU-protected data that it points to can change concurrently. | |
c598a070 | 102 | |
45271064 JFG |
103 | Like rcu_dereference(), when lockdep is enabled, RCU list and hlist |
104 | traversal primitives check for being called from within an RCU read-side | |
105 | critical section. However, a lockdep expression can be passed to them | |
106 | as a additional optional argument. With this lockdep expression, these | |
107 | traversal primitives will complain only if the lockdep expression is | |
108 | false and they are called from outside any RCU read-side critical section. | |
109 | ||
110 | For example, the workqueue for_each_pwq() macro is intended to be used | |
111 | either within an RCU read-side critical section or with wq->mutex held. | |
058cc23b | 112 | It is thus implemented as follows:: |
45271064 JFG |
113 | |
114 | #define for_each_pwq(pwq, wq) | |
115 | list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node, | |
116 | lock_is_held(&(wq->mutex).dep_map)) |