From: David Howells Date: Fri, 9 Apr 2010 22:39:11 +0000 (-0700) Subject: rcu: Better explain the condition parameter of rcu_dereference_check() X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=c08c68dd76bd6b776bc0eb45a5e8f354ed772cdf;p=openwrt%2Fstaging%2Fblogic.git 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 Signed-off-by: Paul E. McKenney 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 --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 8fe86609441f..9f1ddfef84b5 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -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) \ ({ \