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Documentation / RCU / lockdep.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:50 EST.

1	RCU and lockdep checking
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.
10	In addition, RCU provides the following primitives that check lockdep's
11	state:
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.
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.
23	In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
24	checking of rcu_dereference() primitives:
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_raw(p):
51			Don't check.  (Use sparingly, if at all.)
52		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.
63	The rcu_dereference_check() check expression can be any boolean
64	expression, but would normally include a lockdep expression.  However,
65	any boolean expression can be used.  For a moderately ornate example,
66	consider the following:
68		file = rcu_dereference_check(fdt->fd[fd],
69					     lockdep_is_held(&files->file_lock) ||
70					     atomic_read(&files->count) == 1);
72	This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
73	and, if CONFIG_PROVE_RCU is configured, verifies that this expression
74	is used in:
76	1.	An RCU read-side critical section (implicit), or
77	2.	with files->file_lock held, or
78	3.	on an unshared files_struct.
80	In case (1), the pointer is picked up in an RCU-safe manner for vanilla
81	RCU read-side critical sections, in case (2) the ->file_lock prevents
82	any change from taking place, and finally, in case (3) the current task
83	is the only task accessing the file_struct, again preventing any change
84	from taking place.  If the above statement was invoked only from updater
85	code, it could instead be written as follows:
87		file = rcu_dereference_protected(fdt->fd[fd],
88						 lockdep_is_held(&files->file_lock) ||
89						 atomic_read(&files->count) == 1);
91	This would verify cases #2 and #3 above, and furthermore lockdep would
92	complain if this was used in an RCU read-side critical section unless one
93	of these two cases held.  Because rcu_dereference_protected() omits all
94	barriers and compiler constraints, it generates better code than do the
95	other flavors of rcu_dereference().  On the other hand, it is illegal
96	to use rcu_dereference_protected() if either the RCU-protected pointer
97	or the RCU-protected data that it points to can change concurrently.
99	There are currently only "universal" versions of the rcu_assign_pointer()
100	and RCU list-/tree-traversal primitives, which do not (yet) check for
101	being in an RCU read-side critical section.  In the future, separate
102	versions of these primitives might be created.
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