1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ======================== 3 ========================
4 RCU and lockdep checking 4 RCU and lockdep checking
5 ======================== 5 ========================
6 6
7 All flavors of RCU have lockdep checking avail 7 All flavors of RCU have lockdep checking available, so that lockdep is
8 aware of when each task enters and leaves any 8 aware of when each task enters and leaves any flavor of RCU read-side
9 critical section. Each flavor of RCU is track 9 critical section. Each flavor of RCU is tracked separately (but note
10 that this is not the case in 2.6.32 and earlie 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 somet 11 tracking to include RCU state, which can sometimes help when debugging
12 deadlocks and the like. 12 deadlocks and the like.
13 13
14 In addition, RCU provides the following primit 14 In addition, RCU provides the following primitives that check lockdep's
15 state:: 15 state::
16 16
17 rcu_read_lock_held() for normal RCU. 17 rcu_read_lock_held() for normal RCU.
18 rcu_read_lock_bh_held() for RCU-bh. 18 rcu_read_lock_bh_held() for RCU-bh.
19 rcu_read_lock_sched_held() for RCU-sch 19 rcu_read_lock_sched_held() for RCU-sched.
20 rcu_read_lock_any_held() for any of no 20 rcu_read_lock_any_held() for any of normal RCU, RCU-bh, and RCU-sched.
21 srcu_read_lock_held() for SRCU. 21 srcu_read_lock_held() for SRCU.
22 rcu_read_lock_trace_held() for RCU Tas 22 rcu_read_lock_trace_held() for RCU Tasks Trace.
23 23
24 These functions are conservative, and will the 24 These functions are conservative, and will therefore return 1 if they
25 aren't certain (for example, if CONFIG_DEBUG_L 25 aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
26 This prevents things like WARN_ON(!rcu_read_lo 26 This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
27 positives when lockdep is disabled. 27 positives when lockdep is disabled.
28 28
29 In addition, a separate kernel config paramete 29 In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
30 checking of rcu_dereference() primitives: 30 checking of rcu_dereference() primitives:
31 31
32 rcu_dereference(p): 32 rcu_dereference(p):
33 Check for RCU read-side critic 33 Check for RCU read-side critical section.
34 rcu_dereference_bh(p): 34 rcu_dereference_bh(p):
35 Check for RCU-bh read-side cri 35 Check for RCU-bh read-side critical section.
36 rcu_dereference_sched(p): 36 rcu_dereference_sched(p):
37 Check for RCU-sched read-side 37 Check for RCU-sched read-side critical section.
38 srcu_dereference(p, sp): 38 srcu_dereference(p, sp):
39 Check for SRCU read-side criti 39 Check for SRCU read-side critical section.
40 rcu_dereference_check(p, c): 40 rcu_dereference_check(p, c):
41 Use explicit check expression 41 Use explicit check expression "c" along with
42 rcu_read_lock_held(). This is 42 rcu_read_lock_held(). This is useful in code that is
43 invoked by both RCU readers an 43 invoked by both RCU readers and updaters.
44 rcu_dereference_bh_check(p, c): 44 rcu_dereference_bh_check(p, c):
45 Use explicit check expression 45 Use explicit check expression "c" along with
46 rcu_read_lock_bh_held(). This 46 rcu_read_lock_bh_held(). This is useful in code that
47 is invoked by both RCU-bh read 47 is invoked by both RCU-bh readers and updaters.
48 rcu_dereference_sched_check(p, c): 48 rcu_dereference_sched_check(p, c):
49 Use explicit check expression 49 Use explicit check expression "c" along with
50 rcu_read_lock_sched_held(). T 50 rcu_read_lock_sched_held(). This is useful in code that
51 is invoked by both RCU-sched r 51 is invoked by both RCU-sched readers and updaters.
52 srcu_dereference_check(p, c): 52 srcu_dereference_check(p, c):
53 Use explicit check expression 53 Use explicit check expression "c" along with
54 srcu_read_lock_held(). This i 54 srcu_read_lock_held(). This is useful in code that
55 is invoked by both SRCU reader 55 is invoked by both SRCU readers and updaters.
56 rcu_dereference_raw(p): 56 rcu_dereference_raw(p):
57 Don't check. (Use sparingly, 57 Don't check. (Use sparingly, if at all.)
58 rcu_dereference_raw_check(p): 58 rcu_dereference_raw_check(p):
59 Don't do lockdep at all. (Use 59 Don't do lockdep at all. (Use sparingly, if at all.)
60 rcu_dereference_protected(p, c): 60 rcu_dereference_protected(p, c):
61 Use explicit check expression 61 Use explicit check expression "c", and omit all barriers
62 and compiler constraints. Thi 62 and compiler constraints. This is useful when the data
63 structure cannot change, for e 63 structure cannot change, for example, in code that is
64 invoked only by updaters. 64 invoked only by updaters.
65 rcu_access_pointer(p): 65 rcu_access_pointer(p):
66 Return the value of the pointe 66 Return the value of the pointer and omit all barriers,
67 but retain the compiler constr 67 but retain the compiler constraints that prevent duplicating
68 or coalescing. This is useful !! 68 or coalescsing. This is useful when testing the
69 value of the pointer itself, f 69 value of the pointer itself, for example, against NULL.
70 70
71 The rcu_dereference_check() check expression c 71 The rcu_dereference_check() check expression can be any boolean
72 expression, but would normally include a lockd !! 72 expression, but would normally include a lockdep expression. However,
73 moderately ornate example, consider the follow !! 73 any boolean expression can be used. For a moderately ornate example,
>> 74 consider the following::
74 75
75 file = rcu_dereference_check(fdt->fd[f 76 file = rcu_dereference_check(fdt->fd[fd],
76 lockdep_i 77 lockdep_is_held(&files->file_lock) ||
77 atomic_re 78 atomic_read(&files->count) == 1);
78 79
79 This expression picks up the pointer "fdt->fd[ 80 This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
80 and, if CONFIG_PROVE_RCU is configured, verifi 81 and, if CONFIG_PROVE_RCU is configured, verifies that this expression
81 is used in: 82 is used in:
82 83
83 1. An RCU read-side critical section (imp 84 1. An RCU read-side critical section (implicit), or
84 2. with files->file_lock held, or 85 2. with files->file_lock held, or
85 3. on an unshared files_struct. 86 3. on an unshared files_struct.
86 87
87 In case (1), the pointer is picked up in an RC 88 In case (1), the pointer is picked up in an RCU-safe manner for vanilla
88 RCU read-side critical sections, in case (2) t 89 RCU read-side critical sections, in case (2) the ->file_lock prevents
89 any change from taking place, and finally, in 90 any change from taking place, and finally, in case (3) the current task
90 is the only task accessing the file_struct, ag 91 is the only task accessing the file_struct, again preventing any change
91 from taking place. If the above statement was 92 from taking place. If the above statement was invoked only from updater
92 code, it could instead be written as follows:: 93 code, it could instead be written as follows::
93 94
94 file = rcu_dereference_protected(fdt-> 95 file = rcu_dereference_protected(fdt->fd[fd],
95 lockd 96 lockdep_is_held(&files->file_lock) ||
96 atomi 97 atomic_read(&files->count) == 1);
97 98
98 This would verify cases #2 and #3 above, and f 99 This would verify cases #2 and #3 above, and furthermore lockdep would
99 complain even if this was used in an RCU read- !! 100 complain if this was used in an RCU read-side critical section unless one
100 one of these two cases held. Because rcu_dere !! 101 of these two cases held. Because rcu_dereference_protected() omits all
101 all barriers and compiler constraints, it gene !! 102 barriers and compiler constraints, it generates better code than do the
102 the other flavors of rcu_dereference(). On th !! 103 other flavors of rcu_dereference(). On the other hand, it is illegal
103 to use rcu_dereference_protected() if either t 104 to use rcu_dereference_protected() if either the RCU-protected pointer
104 or the RCU-protected data that it points to ca 105 or the RCU-protected data that it points to can change concurrently.
105 106
106 Like rcu_dereference(), when lockdep is enable 107 Like rcu_dereference(), when lockdep is enabled, RCU list and hlist
107 traversal primitives check for being called fr 108 traversal primitives check for being called from within an RCU read-side
108 critical section. However, a lockdep expressi 109 critical section. However, a lockdep expression can be passed to them
109 as a additional optional argument. With this 110 as a additional optional argument. With this lockdep expression, these
110 traversal primitives will complain only if the 111 traversal primitives will complain only if the lockdep expression is
111 false and they are called from outside any RCU 112 false and they are called from outside any RCU read-side critical section.
112 113
113 For example, the workqueue for_each_pwq() macr 114 For example, the workqueue for_each_pwq() macro is intended to be used
114 either within an RCU read-side critical sectio 115 either within an RCU read-side critical section or with wq->mutex held.
115 It is thus implemented as follows:: 116 It is thus implemented as follows::
116 117
117 #define for_each_pwq(pwq, wq) 118 #define for_each_pwq(pwq, wq)
118 list_for_each_entry_rcu((pwq), 119 list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node,
119 lock_i 120 lock_is_held(&(wq->mutex).dep_map))
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