Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754398AbcLPRU6 (ORCPT ); Fri, 16 Dec 2016 12:20:58 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:35981 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754069AbcLPRUs (ORCPT ); Fri, 16 Dec 2016 12:20:48 -0500 Date: Fri, 16 Dec 2016 18:20:26 +0100 From: Peter Zijlstra To: Nicolai =?iso-8859-1?Q?H=E4hnle?= Cc: linux-kernel@vger.kernel.org, Nicolai =?iso-8859-1?Q?H=E4hnle?= , Ingo Molnar , Maarten Lankhorst , Daniel Vetter , Chris Wilson , dri-devel@lists.freedesktop.org Subject: Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order Message-ID: <20161216172026.GV3107@twins.programming.kicks-ass.net> References: <1480601214-26583-1-git-send-email-nhaehnle@gmail.com> <1480601214-26583-6-git-send-email-nhaehnle@gmail.com> <20161206165544.GX3045@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit 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: 1685 Lines: 39 On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai H?hnle wrote: > >>@@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > >> spin_unlock_mutex(&lock->wait_lock, flags); > >> schedule_preempt_disabled(); > >> > >>- if (!first && __mutex_waiter_is_first(lock, &waiter)) { > >>+ if (use_ww_ctx && ww_ctx) { > >>+ /* > >>+ * Always re-check whether we're in first position. We > >>+ * don't want to spin if another task with a lower > >>+ * stamp has taken our position. > >>+ * > >>+ * We also may have to set the handoff flag again, if > >>+ * our position at the head was temporarily taken away. > >>+ */ > >>+ first = __mutex_waiter_is_first(lock, &waiter); > >>+ > >>+ if (first) > >>+ __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); > >>+ } else if (!first && __mutex_waiter_is_first(lock, &waiter)) { > >> first = true; > >> __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); > >> } > > > >So the point is that !ww_ctx entries are 'skipped' during the insertion > >and therefore, if one becomes first, it must stay first? > > Yes. Actually, it should be possible to replace all the cases of use_ww_ctx > || first with ww_ctx. Similarly, all cases of use_ww_ctx && ww_ctx could be > replaced by just ww_ctx. I'm not seeing how "use_ww_ctx || first" -> "ww_ctx" works. And while "use_ww_ctx && ww_ctx" -> "ww_ctx" is correct, it didn't work right on some older GCCs, they choked on value propagation for ww_ctx and kept emitting code even if we passed in NULL. Hence use_ww_ctx. Arnd is now looking to raise the minimum supported GCC version, so maybe we should look at that again if he gets anywhere.