Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S939456AbcKXMTg (ORCPT ); Thu, 24 Nov 2016 07:19:36 -0500 Received: from merlin.infradead.org ([205.233.59.134]:50760 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934732AbcKXMTe (ORCPT ); Thu, 24 Nov 2016 07:19:34 -0500 Date: Thu, 24 Nov 2016 12:56:01 +0100 From: Peter Zijlstra To: Daniel Vetter Cc: Nicolai =?iso-8859-1?Q?H=E4hnle?= , Nicolai =?iso-8859-1?Q?H=E4hnle?= , Linux Kernel Mailing List , stable , Ingo Molnar , dri-devel , Maarten Lankhorst Subject: Re: [PATCH 1/4] locking/ww_mutex: Fix a deadlock affecting ww_mutexes Message-ID: <20161124115601.GR3207@twins.programming.kicks-ass.net> References: <1479900325-28358-1-git-send-email-nhaehnle@gmail.com> <20161123140336.GU3092@twins.programming.kicks-ass.net> <20161123142525.ns2pkyp4bo2sa5z2@phenom.ffwll.local> <20161124114007.GE3092@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1387 Lines: 28 On Thu, Nov 24, 2016 at 12:52:25PM +0100, Daniel Vetter wrote: > On Thu, Nov 24, 2016 at 12:40 PM, Peter Zijlstra wrote: > > > >> I do believe we can win a bit by keeping the wait list sorted, if we also > >> make sure that waiters don't add themselves in the first place if they see > >> that a deadlock situation cannot be avoided. > >> > >> I will probably want to extend struct mutex_waiter with ww_mutex-specific > >> fields to facilitate this (i.e. ctx pointer, perhaps stamp as well to reduce > >> pointer-chasing). That should be fine since it lives on the stack. > > > > Right, shouldn't be a problem I think. > > > > The only 'problem' I can see with using that is that its possible to mix > > ww and !ww waiters through ww_mutex_lock(.ctx = NULL). This makes the > > list order somewhat tricky. > > > > Ideally we'd remove that feature, although I see its actually used quite > > a bit :/ > > I guess we could create a small fake acquire_ctx for single-lock > paths. That way callers still don't need to deal with having an > explicit ctx, but we can assume the timestamp (for ensuring fairness) > is available for all cases. Otherwise there's indeed a problem with > correctly (well fairly) interleaving ctx and non-ctx lockers I think. Actually tried that, but we need a ww_class to get a stamp from, and ww_mutex_lock() doesn't have one of those..