From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754456Ab2HOOts (ORCPT ); Wed, 15 Aug 2012 10:49:48 -0400 Received: from mail-qa0-f46.google.com ([209.85.216.46]:35287 "EHLO mail-qa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751164Ab2HOOtq (ORCPT ); Wed, 15 Aug 2012 10:49:46 -0400 Date: Wed, 15 Aug 2012 10:49:43 -0400 (EDT) From: Nicolas Pitre To: Ben Hutchings cc: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Linus Torvalds , Andrew Morton , alan@lxorguk.ukuu.org.uk, Arnd Bergmann , Shan Kang , Will Deacon , Russell King Subject: Re: [ 20/82] ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+ In-Reply-To: <1345039901.824.221.camel@deadeye.wl.decadent.org.uk> Message-ID: References: <20120813201746.448504360@linuxfoundation.org> <20120813201748.202917513@linuxfoundation.org> <1345038982.824.216.camel@deadeye.wl.decadent.org.uk> <20120815140808.GD22871@kroah.com> <1345039901.824.221.camel@deadeye.wl.decadent.org.uk> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 15 Aug 2012, Ben Hutchings wrote: > On Wed, 2012-08-15 at 07:08 -0700, Greg Kroah-Hartman wrote: > > On Wed, Aug 15, 2012 at 02:56:22PM +0100, Ben Hutchings wrote: > > > On Mon, 2012-08-13 at 13:18 -0700, Greg Kroah-Hartman wrote: > > > > From: Greg KH > > > > > > > > 3.5-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. > > > [...] > > > > > > Here also, I think this should be deferred. > > > > "also"? Am I missing some context here? Why should we deferr this one? > > What do we need to wait for? > > This is the same as 3.4.9-rc1 patch 10/65, which I queried as it > apparently caused a regression. > > Will Deacon wrote: > > 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. If you wish to really be stable, both this patch _and_ http://git.kernel.org/?p=linux/kernel/git/tip/tip.git;a=patch;h=0bce9c46bf must be applied. The later is going to hit mainline soon. Nicolas