Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754153Ab3JaM4x (ORCPT ); Thu, 31 Oct 2013 08:56:53 -0400 Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:60859 "EHLO e06smtp16.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753970Ab3JaM4v (ORCPT ); Thu, 31 Oct 2013 08:56:51 -0400 In-Reply-To: References: <20131023141948.GB3566@localhost.localdomain> <20131025173749.GG19466@laptop.lan> <20131028132634.GO19466@laptop.lan> <20131028163418.GD4126@linux.vnet.ibm.com> <20131028201735.GA15629@redhat.com> <20131030092725.GL4126@linux.vnet.ibm.com> <20131031043258.GQ4126@linux.vnet.ibm.com> Subject: RE: perf events ring buffer memory barrier on powerpc X-KeepSent: F094B66E:899BE5FF-42257C15:0045F3A1; type=4; name=$KeepSent To: "David Laight" Cc: "Anton Blanchard" , "Frederic Weisbecker" , "LKML" , "Linux PPC dev" , "Mathieu Desnoyers" , "Michael Neuling" , "Oleg Nesterov" , paulmck@linux.vnet.ibm.com, "Peter Zijlstra" X-Mailer: Lotus Notes Release 8.5.3 September 15, 2011 Message-ID: From: Victor Kaplansky Date: Thu, 31 Oct 2013 14:55:25 +0200 X-MIMETrack: Serialize by Router on D06ML319/06/M/IBM(Release 8.5.3FP5|July 31, 2013) at 31/10/2013 14:55:18 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13103112-3548-0000-0000-000007018ECA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 760 Lines: 18 "David Laight" wrote on 10/31/2013 02:28:56 PM: > So even though the wmb() in the writer ensures the writes are correctly > ordered, the reader can read the old value from the second location from > its local cache. In case of circular buffer, the only thing that producer reads is @tail, and nothing wrong will happen if producer reads old value of @tail. Moreover, adherents of smp_mb() insert it *after* the read of @tail, so it cannot prevent reading of old value anyway. -- Victor -- 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/