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