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rcu: Better explain the condition parameter of rcu_dereference_check() 

Better explain the condition parameter of
rcu_dereference_check() that describes the conditions under
which the dereference is permitted to take place (and
incorporate Yong Zhang's suggestion).  This condition is only
checked under lockdep proving.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: eric.dumazet@gmail.com
LKML-Reference: <1270852752-25278-2-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
David Howells 2010-04-09 15:39:11 -07:00 committed by Ingo Molnar
parent b62730baea
commit c08c68dd76
1 changed files with 23 additions and 5 deletions
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28
include/linux/rcupdate.h
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@ -195,12 +195,30 @@ static inline int rcu_read_lock_sched_held(void)
/**
* rcu_dereference_check - rcu_dereference with debug checking
* @p: The pointer to read, prior to dereferencing
* @c: The conditions under which the dereference will take place
*
* Do an rcu_dereference(), but check that the context is correct.
* For example, rcu_dereference_check(gp, rcu_read_lock_held()) to
* ensure that the rcu_dereference_check() executes within an RCU
* read-side critical section. It is also possible to check for
* locks being held, for example, by using lockdep_is_held().
* Do an rcu_dereference(), but check that the conditions under which the
* dereference will take place are correct. Typically the conditions indicate
* the various locking conditions that should be held at that point. The check
* should return true if the conditions are satisfied.
*
* For example:
*
* bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
* lockdep_is_held(&foo->lock));
*
* could be used to indicate to lockdep that foo->bar may only be dereferenced
* if either the RCU read lock is held, or that the lock required to replace
* the bar struct at foo->bar is held.
*
* Note that the list of conditions may also include indications of when a lock
* need not be held, for example during initialisation or destruction of the
* target struct:
*
* bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
* lockdep_is_held(&foo->lock) ||
* atomic_read(&foo->usage) == 0);
*/
#define rcu_dereference_check(p, c) \
({ \