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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

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):
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		rcu_access_index(idx):
68			Return the value of the index and omit all barriers, but
69			retain the compiler constraints that prevent duplicating
70			or coalescsing.  This is useful when when testing the
71			value of the index itself, for example, against -1.
72	
73	The rcu_dereference_check() check expression can be any boolean
74	expression, but would normally include a lockdep expression.  However,
75	any boolean expression can be used.  For a moderately ornate example,
76	consider the following:
77	
78		file = rcu_dereference_check(fdt->fd[fd],
79					     lockdep_is_held(&files->file_lock) ||
80					     atomic_read(&files->count) == 1);
81	
82	This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
83	and, if CONFIG_PROVE_RCU is configured, verifies that this expression
84	is used in:
85	
86	1.	An RCU read-side critical section (implicit), or
87	2.	with files->file_lock held, or
88	3.	on an unshared files_struct.
89	
90	In case (1), the pointer is picked up in an RCU-safe manner for vanilla
91	RCU read-side critical sections, in case (2) the ->file_lock prevents
92	any change from taking place, and finally, in case (3) the current task
93	is the only task accessing the file_struct, again preventing any change
94	from taking place.  If the above statement was invoked only from updater
95	code, it could instead be written as follows:
96	
97		file = rcu_dereference_protected(fdt->fd[fd],
98						 lockdep_is_held(&files->file_lock) ||
99						 atomic_read(&files->count) == 1);
100	
101	This would verify cases #2 and #3 above, and furthermore lockdep would
102	complain if this was used in an RCU read-side critical section unless one
103	of these two cases held.  Because rcu_dereference_protected() omits all
104	barriers and compiler constraints, it generates better code than do the
105	other flavors of rcu_dereference().  On the other hand, it is illegal
106	to use rcu_dereference_protected() if either the RCU-protected pointer
107	or the RCU-protected data that it points to can change concurrently.
108	
109	There are currently only "universal" versions of the rcu_assign_pointer()
110	and RCU list-/tree-traversal primitives, which do not (yet) check for
111	being in an RCU read-side critical section.  In the future, separate
112	versions of these primitives might be created.
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