From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758187Ab0DGR2a (ORCPT ); Wed, 7 Apr 2010 13:28:30 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:47880 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751125Ab0DGR22 (ORCPT ); Wed, 7 Apr 2010 13:28:28 -0400 Date: Wed, 7 Apr 2010 10:28:24 -0700 From: "Paul E. McKenney" To: David Howells Cc: eric.dumazet@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] RCU: Better explain the condition parameter of rcu_dereference_check() Message-ID: <20100407172824.GG2481@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100407171500.27710.88994.stgit@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100407171500.27710.88994.stgit@warthog.procyon.org.uk> 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 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 > --- > > 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) \ > ({ \ >