From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751410Ab0DHEg3 (ORCPT ); Thu, 8 Apr 2010 00:36:29 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:60191 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750746Ab0DHEg0 (ORCPT ); Thu, 8 Apr 2010 00:36:26 -0400 Date: Wed, 7 Apr 2010 21:36:23 -0700 From: "Paul E. McKenney" To: Yong Zhang Cc: David Howells , eric.dumazet@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] RCU: Better explain the condition parameter of rcu_dereference_check() Message-ID: <20100408043623.GB2887@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100407171500.27710.88994.stgit@warthog.procyon.org.uk> <20100408025050.GB12905@windriver.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100408025050.GB12905@windriver.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 08, 2010 at 10:50:50AM +0800, Yong Zhang wrote: > 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. > > > > Signed-off-by: David Howells > > --- > > > > 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), > ^^ > || ? Good catch, fixed! Thanx, Paul > Thanks, > Yong > > > + * 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@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/