From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757143AbbGGLRu (ORCPT ); Tue, 7 Jul 2015 07:17:50 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:36276 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755776AbbGGLRm (ORCPT ); Tue, 7 Jul 2015 07:17:42 -0400 Date: Tue, 7 Jul 2015 13:17:31 +0200 From: Peter Zijlstra To: Will Deacon 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: <20150707111731.GQ3644@twins.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150707091711.GA23879@arm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.