Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752713AbbFWC6B (ORCPT ); Mon, 22 Jun 2015 22:58:01 -0400 Received: from g2t2353.austin.hp.com ([15.217.128.52]:20567 "EHLO g2t2353.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751731AbbFWC5x (ORCPT ); Mon, 22 Jun 2015 22:57:53 -0400 Message-ID: <5588CB2C.108@hp.com> Date: Mon, 22 Jun 2015 22:57:48 -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 , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch Subject: Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING References: <1434729002-57724-1-git-send-email-Waiman.Long@hp.com> <1434729002-57724-4-git-send-email-Waiman.Long@hp.com> <20150622162123.GI1583@arm.com> In-Reply-To: <20150622162123.GI1583@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: 1492 Lines: 41 On 06/22/2015 12:21 PM, Will Deacon wrote: > Hi Waiman, > > On Fri, Jun 19, 2015 at 04:50:02PM +0100, Waiman Long wrote: >> The current cmpxchg() loop in setting the _QW_WAITING flag for writers >> in queue_write_lock_slowpath() will contend with incoming readers >> causing possibly extra cmpxchg() operations that are wasteful. This >> patch changes the code to do a byte cmpxchg() to eliminate contention >> with new readers. > [...] > >> diff --git a/arch/x86/include/asm/qrwlock.h b/arch/x86/include/asm/qrwlock.h >> index a8810bf..5678b0a 100644 >> --- a/arch/x86/include/asm/qrwlock.h >> +++ b/arch/x86/include/asm/qrwlock.h >> @@ -7,8 +7,7 @@ >> #define queued_write_unlock queued_write_unlock >> static inline void queued_write_unlock(struct qrwlock *lock) >> { >> - barrier(); >> - ACCESS_ONCE(*(u8 *)&lock->cnts) = 0; >> + smp_store_release(&lock->wmode, 0); >> } >> #endif > I reckon you could actually use this in the asm-generic header and remove > the x86 arch version altogether. Most architectures support single-copy > atomic byte access and those that don't (alpha?) can just not use qrwlock > (or override write_unlock with atomic_sub). > > I already have a patch making this change, so I'm happy either way. Yes, I am aware of that. If you have a patch to make that change, I am fine with that too. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/