Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965897AbbDQOGs (ORCPT ); Fri, 17 Apr 2015 10:06:48 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:41900 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753506AbbDQOGo (ORCPT ); Fri, 17 Apr 2015 10:06:44 -0400 Date: Fri, 17 Apr 2015 07:06:38 -0700 From: "Paul E. McKenney" To: Milos Vyletel Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Jonathan Corbet , "open list:READ-COPY UPDATE..." , "open list:DOCUMENTATION" , jeff.haran@citrix.com Subject: Re: [PATCH] rcu: small rcu_dereference doc update Message-ID: <20150417140637.GL23685@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1429266816-27004-1-git-send-email-milos@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1429266816-27004-1-git-send-email-milos@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15041714-8236-0000-0000-00000AD70315 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1641 Lines: 43 On Fri, Apr 17, 2015 at 12:33:36PM +0200, Milos Vyletel wrote: > Make a note stating that repeated calls of rcu_dereference() may not > return the same pointer if update happens while in critical section. > > Reported-by: Jeff Haran > Signed-off-by: Milos Vyletel Hmmm... Seems like that should be obvious, but on the other hand, I have been using RCU for more than twenty years, so my obviousness sensors might need recalibration. Queued for 4.2. Thanx, Paul > --- > Documentation/RCU/whatisRCU.txt | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt > index 88dfce1..82b1b2c 100644 > --- a/Documentation/RCU/whatisRCU.txt > +++ b/Documentation/RCU/whatisRCU.txt > @@ -256,7 +256,9 @@ rcu_dereference() > If you are going to be fetching multiple fields from the > RCU-protected structure, using the local variable is of > course preferred. Repeated rcu_dereference() calls look > - ugly and incur unnecessary overhead on Alpha CPUs. > + ugly, do not guarantee that same pointer will be returned > + if update happened while in critical section and incur > + unnecessary overhead on Alpha CPUs. > > Note that the value returned by rcu_dereference() is valid > only within the enclosing RCU read-side critical section. > -- > 2.1.0 > -- 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/