Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754997AbZCLLAZ (ORCPT ); Thu, 12 Mar 2009 07:00:25 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752572AbZCLLAL (ORCPT ); Thu, 12 Mar 2009 07:00:11 -0400 Received: from www.tglx.de ([62.245.132.106]:41124 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752169AbZCLLAJ (ORCPT ); Thu, 12 Mar 2009 07:00:09 -0400 Date: Thu, 12 Mar 2009 11:58:51 +0100 (CET) From: Thomas Gleixner To: Ingo Molnar cc: Peter Zijlstra , Darren Hart , linux-kernel@vger.kernel.org, Rusty Russell Subject: Re: [PATCH 3/6] futex: add double_unlock_hb() In-Reply-To: <20090312101028.GA7157@elte.hu> Message-ID: References: <20090312075349.9856.83687.stgit@Aeon> <20090312075552.9856.48021.stgit@Aeon> <1236852455.5090.105.camel@laptop> <20090312101028.GA7157@elte.hu> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) 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 Content-Length: 1501 Lines: 43 On Thu, 12 Mar 2009, Ingo Molnar wrote: > * Peter Zijlstra wrote: > > > On Thu, 2009-03-12 at 00:55 -0700, Darren Hart wrote: > > > The futex code uses double_lock_hb() which locks the hb->lock's in pointer > > > value order. There is no parallel unlock routine, and the code unlocks them > > > in name order, ignoring pointer value. This opens up a window for an ABBA > > > deadlock. This patch adds double_unlock_hb() to remove the window as well > > > as refactor the duplicated code segments. > > > > While I don't mind the patch per-se, I'm hard pressed to see > > any deadlock potential in the unordered unlock. > > > > All sites (at least those in the patch) always release both > > locks without taking another in between, therefore one would > > think there's no deadlock possible. > > yeah. I can't see a deadlock either. > The patch is still nice (as you mention), it factors out the > unlock sequence. I'll change the commit message accordingy. We do not need the comparison magic. Can we just put the code into double_unlock_hb() which gets replaced ? i.e: spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->lock); This code is confusing enough. Thanks, tglx -- 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/