1RCU 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):
35		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.
50	rcu_dereference_index_check(p, c):
51		Use explicit check expression "c", but the caller
52		must supply one of the rcu_read_lock_held() functions.
53		This is useful in code that uses RCU-protected arrays
54		that is invoked by both RCU readers and updaters.
55	rcu_dereference_raw(p):
56		Don't check.  (Use sparingly, if at all.)
57	rcu_dereference_protected(p, c):
58		Use explicit check expression "c", and omit all barriers
59		and compiler constraints.  This is useful when the data
60		structure cannot change, for example, in code that is
61		invoked only by updaters.
62	rcu_access_pointer(p):
63		Return the value of the pointer and omit all barriers,
64		but retain the compiler constraints that prevent duplicating
65		or coalescsing.  This is useful when when testing the
66		value of the pointer itself, for example, against NULL.
67
68The rcu_dereference_check() check expression can be any boolean
69expression, but would normally include a lockdep expression.  However,
70any boolean expression can be used.  For a moderately ornate example,
71consider the following:
72
73	file = rcu_dereference_check(fdt->fd[fd],
74				     lockdep_is_held(&files->file_lock) ||
75				     atomic_read(&files->count) == 1);
76
77This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
78and, if CONFIG_PROVE_RCU is configured, verifies that this expression
79is used in:
80
811.	An RCU read-side critical section (implicit), or
822.	with files->file_lock held, or
833.	on an unshared files_struct.
84
85In case (1), the pointer is picked up in an RCU-safe manner for vanilla
86RCU read-side critical sections, in case (2) the ->file_lock prevents
87any change from taking place, and finally, in case (3) the current task
88is the only task accessing the file_struct, again preventing any change
89from taking place.  If the above statement was invoked only from updater
90code, it could instead be written as follows:
91
92	file = rcu_dereference_protected(fdt->fd[fd],
93					 lockdep_is_held(&files->file_lock) ||
94					 atomic_read(&files->count) == 1);
95
96This would verify cases #2 and #3 above, and furthermore lockdep would
97complain if this was used in an RCU read-side critical section unless one
98of these two cases held.  Because rcu_dereference_protected() omits all
99barriers and compiler constraints, it generates better code than do the
100other flavors of rcu_dereference().  On the other hand, it is illegal
101to use rcu_dereference_protected() if either the RCU-protected pointer
102or the RCU-protected data that it points to can change concurrently.
103
104There are currently only "universal" versions of the rcu_assign_pointer()
105and RCU list-/tree-traversal primitives, which do not (yet) check for
106being in an RCU read-side critical section.  In the future, separate
107versions of these primitives might be created.
108