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".  This is useful in
36		code that is invoked by both readers and updaters.
37	rcu_dereference_raw(p)
38		Don't check.  (Use sparingly, if at all.)
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.
49
50The rcu_dereference_check() check expression can be any boolean
51expression, but would normally include one of the rcu_read_lock_held()
52family of functions and a lockdep expression.  However, any boolean
53expression can be used.  For a moderately ornate example, consider
54the 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
61This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
62and, if CONFIG_PROVE_RCU is configured, verifies that this expression
63is used in:
64
651.	An RCU read-side critical section, or
662.	with files->file_lock held, or
673.	on an unshared files_struct.
68
69In case (1), the pointer is picked up in an RCU-safe manner for vanilla
70RCU read-side critical sections, in case (2) the ->file_lock prevents
71any change from taking place, and finally, in case (3) the current task
72is the only task accessing the file_struct, again preventing any change
73from taking place.  If the above statement was invoked only from updater
74code, 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
80This would verify cases #2 and #3 above, and furthermore lockdep would
81complain if this was used in an RCU read-side critical section unless one
82of these two cases held.  Because rcu_dereference_protected() omits all
83barriers and compiler constraints, it generates better code than do the
84other flavors of rcu_dereference().  On the other hand, it is illegal
85to use rcu_dereference_protected() if either the RCU-protected pointer
86or the RCU-protected data that it points to can change concurrently.
87
88There are currently only "universal" versions of the rcu_assign_pointer()
89and RCU list-/tree-traversal primitives, which do not (yet) check for
90being in an RCU read-side critical section.  In the future, separate
91versions of these primitives might be created.
92