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c598a070 PM |
1 | RCU and lockdep checking |
2 | ||
3 | All flavors of RCU have lockdep checking available, so that lockdep is | |
4 | aware of when each task enters and leaves any flavor of RCU read-side | |
5 | critical section. Each flavor of RCU is tracked separately (but note | |
6 | that this is not the case in 2.6.32 and earlier). This allows lockdep's | |
7 | tracking to include RCU state, which can sometimes help when debugging | |
8 | deadlocks and the like. | |
9 | ||
10 | In addition, RCU provides the following primitives that check lockdep's | |
11 | state: | |
12 | ||
13 | rcu_read_lock_held() for normal RCU. | |
14 | rcu_read_lock_bh_held() for RCU-bh. | |
15 | rcu_read_lock_sched_held() for RCU-sched. | |
16 | srcu_read_lock_held() for SRCU. | |
17 | ||
18 | These functions are conservative, and will therefore return 1 if they | |
19 | aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). | |
20 | This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false | |
21 | positives when lockdep is disabled. | |
22 | ||
23 | In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables | |
24 | checking of rcu_dereference() primitives: | |
25 | ||
26 | rcu_dereference(p): | |
27 | Check for RCU read-side critical section. | |
28 | rcu_dereference_bh(p): | |
29 | Check for RCU-bh read-side critical section. | |
30 | rcu_dereference_sched(p): | |
31 | Check for RCU-sched read-side critical section. | |
32 | srcu_dereference(p, sp): | |
33 | Check for SRCU read-side critical section. | |
34 | rcu_dereference_check(p, c): | |
50aec002 PM |
35 | Use explicit check expression "c". This is useful in |
36 | code that is invoked by both readers and updaters. | |
c598a070 PM |
37 | rcu_dereference_raw(p) |
38 | Don't check. (Use sparingly, if at all.) | |
50aec002 PM |
39 | rcu_dereference_protected(p, c): |
40 | Use explicit check expression "c", and omit all barriers | |
41 | and compiler constraints. This is useful when the data | |
42 | structure cannot change, for example, in code that is | |
43 | invoked only by updaters. | |
44 | rcu_access_pointer(p): | |
45 | Return the value of the pointer and omit all barriers, | |
46 | but retain the compiler constraints that prevent duplicating | |
47 | or coalescsing. This is useful when when testing the | |
48 | value of the pointer itself, for example, against NULL. | |
c598a070 PM |
49 | |
50 | The rcu_dereference_check() check expression can be any boolean | |
51 | expression, but would normally include one of the rcu_read_lock_held() | |
52 | family of functions and a lockdep expression. However, any boolean | |
53 | expression can be used. For a moderately ornate example, consider | |
54 | the following: | |
55 | ||
56 | file = rcu_dereference_check(fdt->fd[fd], | |
57 | rcu_read_lock_held() || | |
58 | lockdep_is_held(&files->file_lock) || | |
59 | atomic_read(&files->count) == 1); | |
60 | ||
61 | This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner, | |
62 | and, if CONFIG_PROVE_RCU is configured, verifies that this expression | |
63 | is used in: | |
64 | ||
65 | 1. An RCU read-side critical section, or | |
66 | 2. with files->file_lock held, or | |
67 | 3. on an unshared files_struct. | |
68 | ||
69 | In case (1), the pointer is picked up in an RCU-safe manner for vanilla | |
70 | RCU read-side critical sections, in case (2) the ->file_lock prevents | |
71 | any change from taking place, and finally, in case (3) the current task | |
72 | is the only task accessing the file_struct, again preventing any change | |
50aec002 PM |
73 | from taking place. If the above statement was invoked only from updater |
74 | code, it could instead be written as follows: | |
75 | ||
76 | file = rcu_dereference_protected(fdt->fd[fd], | |
77 | lockdep_is_held(&files->file_lock) || | |
78 | atomic_read(&files->count) == 1); | |
79 | ||
80 | This would verify cases #2 and #3 above, and furthermore lockdep would | |
81 | complain if this was used in an RCU read-side critical section unless one | |
82 | of these two cases held. Because rcu_dereference_protected() omits all | |
83 | barriers and compiler constraints, it generates better code than do the | |
84 | other flavors of rcu_dereference(). On the other hand, it is illegal | |
85 | to use rcu_dereference_protected() if either the RCU-protected pointer | |
86 | or the RCU-protected data that it points to can change concurrently. | |
c598a070 PM |
87 | |
88 | There are currently only "universal" versions of the rcu_assign_pointer() | |
89 | and RCU list-/tree-traversal primitives, which do not (yet) check for | |
90 | being in an RCU read-side critical section. In the future, separate | |
91 | versions of these primitives might be created. |