From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932846Ab0DGP7g (ORCPT ); Wed, 7 Apr 2010 11:59:36 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:42947 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932581Ab0DGP7f (ORCPT ); Wed, 7 Apr 2010 11:59:35 -0400 Date: Wed, 7 Apr 2010 08:59:29 -0700 From: "Paul E. McKenney" To: Eric Dumazet Cc: David Howells , Trond.Myklebust@netapp.com, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] rcu: add rcu_access_pointer and rcu_dereference_protect Message-ID: <20100407155929.GB2481@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100407135732.12414.16416.stgit@warthog.procyon.org.uk> <1270652210.8141.9.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1270652210.8141.9.camel@edumazet-laptop> 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 04:56:50PM +0200, Eric Dumazet wrote: > Le mercredi 07 avril 2010 à 14:57 +0100, David Howells a écrit : > > From: Paul E. McKenney > > > > This patch adds variants of rcu_dereference() that handle situations > > where the RCU-protected data structure cannot change, perhaps due to > > our holding the update-side lock, or where the RCU-protected pointer is > > only to be fetched, not dereferenced. > > > > The new rcu_access_pointer() primitive is for the case where the pointer > > is be fetch and not dereferenced. This primitive may be used without > > protection, RCU or otherwise, due to the fact that it uses ACCESS_ONCE(). > > > > The new rcu_dereference_protect() primitive is for the case where updates > > are prevented, for example, due to holding the update-side lock. This > > primitive does neither ACCESS_ONCE() nor smp_read_barrier_depends(), so > > can only be used when updates are somehow prevented. > > > > Suggested-by: David Howells > > Signed-off-by: Paul E. McKenney > > Signed-off-by: David Howells > > --- > > > > include/linux/rcupdate.h | 34 ++++++++++++++++++++++++++++++++++ > > 1 files changed, 34 insertions(+), 0 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 872a98e..a1b14b6 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -209,9 +209,43 @@ static inline int rcu_read_lock_sched_held(void) > > rcu_dereference_raw(p); \ > > }) > > > > +/** > > + * rcu_access_pointer - fetch RCU pointer with no dereferencing > > + * > > + * Return the value of the specified RCU-protected pointer, but omit the > > + * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful > > + * when the value of this pointer is accessed, but the pointer is not > > + * dereferenced, for example, when testing an RCU-protected pointer against > > + * NULL. This may also be used in cases where update-side locks prevent > > + * the value of the pointer from changing, but rcu_dereference_protect() > > + * is a lighter-weight primitive for this use case. > > + */ > > +#define rcu_access_pointer(p) \ > > + ({ \ > > + ACCESS_ONCE(p); \ > > + }) > > + > > +/** > > + * rcu_dereference_protected - fetch RCU pointer when updates prevented > > + * > > + * Return the value of the specified RCU-protected pointer, but omit > > + * both the smp_read_barrier_depends() and the ACCESS_ONCE(). This > > + * is useful in cases where update-side locks prevent the value of the > > + * pointer from changing. Please note that this primitive does -not- > > + * prevent the compiler from repeating this reference or combining it > > + * with other references, so it should not be used without protection > > + * of appropriate locks. > > + */ > > +#define rcu_dereference_protected(p) \ > > + ({ \ > > + (p); \ > > + }) > > + > > #else /* #ifdef CONFIG_PROVE_RCU */ > > > > #define rcu_dereference_check(p, c) rcu_dereference_raw(p) > > +#define rcu_access_pointer(p) ACCESS_ONCE(p) > > +#define rcu_dereference_protect(p) (p) > > > > #endif /* #else #ifdef CONFIG_PROVE_RCU */ > > > > > > -- > > This is not the version Paul posted. I blew the name -- rcu_dereference_protected() is in fact a better name. > Removing checks just to shutup warnings ? > > All the point is to get lockdep assistance, and you throw it away. > > We want to explicit the condition, so that RCU users can explicitly > state what protects their data. What Eric said!!! ;-) Thanx, Paul