Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757650AbbGGOai (ORCPT ); Tue, 7 Jul 2015 10:30:38 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:57779 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757275AbbGGOa2 (ORCPT ); Tue, 7 Jul 2015 10:30:28 -0400 Message-ID: <559BE27E.6060901@hp.com> Date: Tue, 07 Jul 2015 10:30:22 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Will Deacon CC: Peter Zijlstra , 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 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> <20150707114918.GG23879@arm.com> In-Reply-To: <20150707114918.GG23879@arm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1808 Lines: 36 On 07/07/2015 07:49 AM, Will Deacon wrote: > 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 > _QW_WAITING and _QW_LOCKED has different semantics and are necessary for the proper handshake between readers and writer. We set _QW_WAITING when readers own the lock and the writer is waiting for the readers to go away. The _QW_WAITING flag will force new readers to go to queuing while the writer is waiting. We set _QW_LOCKED when a writer own the lock and it can only be set atomically when no reader is present. Without the intermediate _QW_WAITING step, a continuous stream of incoming readers (which make the reader count never 0) could deny a writer from getting the lock indefinitely. Cheers, Longman -- 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/