From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752446Ab2HOJK6 (ORCPT ); Wed, 15 Aug 2012 05:10:58 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:59078 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751081Ab2HOJK5 (ORCPT ); Wed, 15 Aug 2012 05:10:57 -0400 Date: Wed, 15 Aug 2012 10:10:29 +0100 From: Will Deacon To: Ben Hutchings Cc: Russell King , "linux-kernel@vger.kernel.org" , "stable@vger.kernel.org" , "torvalds@linux-foundation.org" , "akpm@linux-foundation.org" , "alan@lxorguk.ukuu.org.uk" , Arnd Bergmann , Nicolas Pitre , Shan Kang , Greg Kroah-Hartman Subject: Re: [ 10/65] ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+ Message-ID: <20120815091028.GA29154@mudshark.cambridge.arm.com> References: <20120813221414.965154048@linuxfoundation.org> <20120813221415.872342783@linuxfoundation.org> <1345004966.824.214.camel@deadeye.wl.decadent.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1345004966.824.214.camel@deadeye.wl.decadent.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ben, On Wed, Aug 15, 2012 at 05:29:26AM +0100, Ben Hutchings wrote: > On Mon, 2012-08-13 at 15:13 -0700, Greg Kroah-Hartman wrote: > > From: Greg KH > > > > 3.4-stable review patch. If anyone has any objections, please let me know. > > > > ------------------ > > > > From: Will Deacon > > > > commit a76d7bd96d65fa5119adba97e1b58d95f2e78829 upstream. > > > > The open-coded mutex implementation for ARMv6+ cores suffers from a > > severe lack of barriers, so in the uncontended case we don't actually > > protect any accesses performed during the critical section. > > > > Furthermore, the code is largely a duplication of the ARMv6+ atomic_dec > > code but optimised to remove a branch instruction, as the mutex fastpath > > was previously inlined. Now that this is executed out-of-line, we can > > reuse the atomic access code for the locking (in fact, we use the xchg > > code as this produces shorter critical sections). > > > > This patch uses the generic xchg based implementation for mutexes on > > ARMv6+, which introduces barriers to the lock/unlock operations and also > > has the benefit of removing a fair amount of inline assembly code. > [...] > > I understand that a further fix is needed on top of this > but it's > not in Linus's tree yet. Is it better to apply this on its own or to > wait for the complete fix? The additional patch should also be CC'd to stable and is sitting in -tip somewhere I believe, so it shouldn't be long before it does hit mainline. Without this patch there's a memory-ordering bug (which we seem to have hit once in > 5 years). With the patch there's a mutex lockup issue on SMP systems that I can provoke with enough hackbenching, so you may want to hold off for now. Will