From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932714Ab0DGO47 (ORCPT ); Wed, 7 Apr 2010 10:56:59 -0400 Received: from mail-bw0-f209.google.com ([209.85.218.209]:45040 "EHLO mail-bw0-f209.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932581Ab0DGO45 (ORCPT ); Wed, 7 Apr 2010 10:56:57 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=NtGNMPJW3oZ/4e7QeY1Lr8aRuXa4JBBWnFHkioCIhYjg9sl1FshzTZSIvPhXaB6dyD rZOWYRU8poc/VqaHEe+a9khen8Gr92O9udFlOrSQtzafBb2nwRuksILrJSvropZ4/rGs XyjFCILmRQB99AQUCfjUsR+UN7Tg4r9S0YpNo= Subject: Re: [PATCH 1/2] rcu: add rcu_access_pointer and rcu_dereference_protect From: Eric Dumazet To: David Howells Cc: paulmck@linux.vnet.ibm.com, Trond.Myklebust@netapp.com, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20100407135732.12414.16416.stgit@warthog.procyon.org.uk> References: <20100407135732.12414.16416.stgit@warthog.procyon.org.uk> Content-Type: text/plain; charset="UTF-8" Date: Wed, 07 Apr 2010 16:56:50 +0200 Message-ID: <1270652210.8141.9.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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.