Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757107AbbGGLta (ORCPT ); Tue, 7 Jul 2015 07:49:30 -0400 Received: from foss.arm.com ([217.140.101.70]:35239 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756521AbbGGLtW (ORCPT ); Tue, 7 Jul 2015 07:49:22 -0400 Date: Tue, 7 Jul 2015 12:49:18 +0100 From: Will Deacon To: Peter Zijlstra Cc: Waiman Long , Ingo Molnar , Arnd Bergmann , Thomas Gleixner , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency Message-ID: <20150707114918.GG23879@arm.com> References: <1436197386-58635-1-git-send-email-Waiman.Long@hp.com> <1436197386-58635-3-git-send-email-Waiman.Long@hp.com> <20150706182353.GC1607@arm.com> <559ADBCD.6020803@hp.com> <20150707091711.GA23879@arm.com> <20150707111731.GQ3644@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150707111731.GQ3644@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1157 Lines: 26 On Tue, Jul 07, 2015 at 12:17:31PM +0100, Peter Zijlstra wrote: > On Tue, Jul 07, 2015 at 10:17:11AM +0100, Will Deacon wrote: > > > > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg > > > > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire) > > > > rmode until it hits zero? > > > > > > No, this is how we make the lock fair so that an incoming streams of > > > later readers won't block a writer from getting the lock. > > > > But won't those readers effectively see that the lock is held for write > > (because we set wmode to _QW_LOCKED before the existing reader had drained) > > and therefore fall down the slow-path and get held up on the spinlock? > > Yes, that's the entire point. Once there's a writer pending, new readers > should queue too. Agreed. My point was that we can achieve the same result without a separate _QW_WAITING flag afaict. Will -- 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/