Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754086AbcLPRPn (ORCPT ); Fri, 16 Dec 2016 12:15:43 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:32976 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751448AbcLPRPg (ORCPT ); Fri, 16 Dec 2016 12:15:36 -0500 Date: Fri, 16 Dec 2016 18:15:24 +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: <20161216171524.GU3107@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: 1454 Lines: 47 On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai H?hnle wrote: > The concern about picking up a handoff that we didn't request is real, > though it cannot happen in the first iteration. Perhaps this __mutex_trylock > can be moved to the end of the loop? See below... > >>@@ -728,7 +800,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > >> * or we must see its unlock and acquire. > >> */ > >> if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) || > >>- __mutex_trylock(lock, first)) > >>+ __mutex_trylock(lock, use_ww_ctx || first)) > >> break; > >> > >> spin_lock_mutex(&lock->wait_lock, flags); > > Change this code to: > > acquired = first && > mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, > &waiter); > spin_lock_mutex(&lock->wait_lock, flags); > > if (acquired || > __mutex_trylock(lock, use_ww_ctx || first)) > break; goto acquired; will work lots better. > } > > This changes the trylock to always be under the wait_lock, but we previously > had that at the beginning of the loop anyway. > It also removes back-to-back > calls to __mutex_trylock when going through the loop; Yeah, I had that explicitly. It allows taking the mutex when mutex_unlock() is still holding the wait_lock. > and for the first > iteration, there is a __mutex_trylock under wait_lock already before adding > ourselves to the wait list. Correct.