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Date: Wed, 7 Apr 2010 10:28:24 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: David Howells <dhowells@...hat.com> Cc: eric.dumazet@...il.com, linux-kernel@...r.kernel.org Subject: Re: [PATCH] RCU: Better explain the condition parameter of rcu_dereference_check() On Wed, Apr 07, 2010 at 06:15:00PM +0100, David Howells wrote: > Better explain the condition parameter of rcu_dereference_check() that > describes the conditions under which the dereference is permitted to take > place. This condition is only checked under lockdep proving. Very good, I have queued this. Thank you! Thanx, Paul > Signed-off-by: David Howells <dhowells@...hat.com> > --- > > include/linux/rcupdate.h | 28 +++++++++++++++++++++++----- > 1 files changed, 23 insertions(+), 5 deletions(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 872a98e..acf9958 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) \ > ({ \ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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