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Paul E. McKenneyc598a072010-02-22 17:04:57 -08001RCU and lockdep checking
2
3All flavors of RCU have lockdep checking available, so that lockdep is
4aware of when each task enters and leaves any flavor of RCU read-side
5critical section. Each flavor of RCU is tracked separately (but note
6that this is not the case in 2.6.32 and earlier). This allows lockdep's
7tracking to include RCU state, which can sometimes help when debugging
8deadlocks and the like.
9
10In addition, RCU provides the following primitives that check lockdep's
11state:
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
18These functions are conservative, and will therefore return 1 if they
19aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
20This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
21positives when lockdep is disabled.
22
23In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
24checking 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):
Paul E. McKenney8cd889c2011-07-08 09:07:35 -070035 Use explicit check expression "c" along with
36 rcu_read_lock_held(). This is useful in code that is
37 invoked by both RCU readers and updaters.
38 rcu_dereference_bh_check(p, c):
39 Use explicit check expression "c" along with
40 rcu_read_lock_bh_held(). This is useful in code that
41 is invoked by both RCU-bh readers and updaters.
42 rcu_dereference_sched_check(p, c):
43 Use explicit check expression "c" along with
44 rcu_read_lock_sched_held(). This is useful in code that
45 is invoked by both RCU-sched readers and updaters.
46 srcu_dereference_check(p, c):
47 Use explicit check expression "c" along with
48 srcu_read_lock_held()(). This is useful in code that
49 is invoked by both SRCU readers and updaters.
Paul E. McKenney8cd889c2011-07-08 09:07:35 -070050 rcu_dereference_raw(p):
Paul E. McKenneyc598a072010-02-22 17:04:57 -080051 Don't check. (Use sparingly, if at all.)
Paul E. McKenney50aec002010-04-09 15:39:12 -070052 rcu_dereference_protected(p, c):
53 Use explicit check expression "c", and omit all barriers
54 and compiler constraints. This is useful when the data
55 structure cannot change, for example, in code that is
56 invoked only by updaters.
57 rcu_access_pointer(p):
58 Return the value of the pointer and omit all barriers,
59 but retain the compiler constraints that prevent duplicating
60 or coalescsing. This is useful when when testing the
61 value of the pointer itself, for example, against NULL.
Paul E. McKenneyc598a072010-02-22 17:04:57 -080062
63The rcu_dereference_check() check expression can be any boolean
Michal Hockoe5177ec2011-07-08 08:48:24 -070064expression, but would normally include a lockdep expression. However,
65any boolean expression can be used. For a moderately ornate example,
66consider the following:
Paul E. McKenneyc598a072010-02-22 17:04:57 -080067
68 file = rcu_dereference_check(fdt->fd[fd],
Paul E. McKenneyc598a072010-02-22 17:04:57 -080069 lockdep_is_held(&files->file_lock) ||
70 atomic_read(&files->count) == 1);
71
72This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
73and, if CONFIG_PROVE_RCU is configured, verifies that this expression
74is used in:
75
Michal Hockoe5177ec2011-07-08 08:48:24 -0700761. An RCU read-side critical section (implicit), or
Paul E. McKenneyc598a072010-02-22 17:04:57 -0800772. with files->file_lock held, or
783. on an unshared files_struct.
79
80In case (1), the pointer is picked up in an RCU-safe manner for vanilla
81RCU read-side critical sections, in case (2) the ->file_lock prevents
82any change from taking place, and finally, in case (3) the current task
83is the only task accessing the file_struct, again preventing any change
Paul E. McKenney50aec002010-04-09 15:39:12 -070084from taking place. If the above statement was invoked only from updater
85code, it could instead be written as follows:
86
87 file = rcu_dereference_protected(fdt->fd[fd],
88 lockdep_is_held(&files->file_lock) ||
89 atomic_read(&files->count) == 1);
90
91This would verify cases #2 and #3 above, and furthermore lockdep would
92complain if this was used in an RCU read-side critical section unless one
93of these two cases held. Because rcu_dereference_protected() omits all
94barriers and compiler constraints, it generates better code than do the
95other flavors of rcu_dereference(). On the other hand, it is illegal
96to use rcu_dereference_protected() if either the RCU-protected pointer
97or the RCU-protected data that it points to can change concurrently.
Paul E. McKenneyc598a072010-02-22 17:04:57 -080098
99There are currently only "universal" versions of the rcu_assign_pointer()
100and RCU list-/tree-traversal primitives, which do not (yet) check for
101being in an RCU read-side critical section. In the future, separate
102versions of these primitives might be created.