From mboxrd@z Thu Jan 1 00:00:00 1970 From: peterz@infradead.org (Peter Zijlstra) Date: Thu, 20 Sep 2018 18:22:16 +0200 Subject: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath In-Reply-To: <20180920160832.GZ24124@hirez.programming.kicks-ass.net> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-3-git-send-email-will.deacon@arm.com> <20180409105835.GC23134@arm.com> <20180409145409.GA9661@arm.com> <20180409155420.GB4082@hirez.programming.kicks-ass.net> <20180409171959.GB9661@arm.com> <20180920160832.GZ24124@hirez.programming.kicks-ass.net> Message-ID: <20180920162216.GR24142@hirez.programming.kicks-ass.net> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Sep 20, 2018 at 06:08:32PM +0200, Peter Zijlstra wrote: > Another approach might be to use something like: > > val = xchg_relaxed(&lock->locked_pending, _Q_PENDING_VAL | _Q_LOCKED_VAL); > val |= atomic_read_acquire(&lock->val) & _Q_TAIL_MASK; > > combined with something like: > > /* 0,0,0 -> 0,1,1 - we won trylock */ > if (!(val & _Q_LOCKED_MASK)) { That one doesn't actually work... let me think about this more. > clear_pending(lock); > return; > } > > /* 0,0,1 -> 0,1,1 - we won pending */ > if (!(val & ~_Q_LOCKED_MASK)) { > ... > } > > /* *,0,1 -> *,1,1 - we won pending, but there's queueing */ > if (!(val & _Q_PENDING_VAL)) > clear_pending(lock); > > ... > > > Hmmm? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8CA45ECE566 for ; Thu, 20 Sep 2018 16:22:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2D94F2147D for ; Thu, 20 Sep 2018 16:22:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="oVISmaLe" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2D94F2147D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728547AbeITWGj (ORCPT ); Thu, 20 Sep 2018 18:06:39 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:49926 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725876AbeITWGi (ORCPT ); Thu, 20 Sep 2018 18:06:38 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=xJ3yBp77MqWtiwJKjICMtjEDV065Wj+aVcTDyX7S1Jw=; b=oVISmaLe+PC7Tx6igS7AEJIpQ EPZv249C2/S9XO2QI73yhkjLdzPylL6AfU/SaXTCm9+HtZSqQ9KZLNCj7VqmO726Mci3iFHi6hSxE L81aKJw/2vI9L0YrWo/WZL2TIsdeh53JIDhlWzs7zloJPl32iti/2Q2uJ9vGDAY9/L9hZaQrvSDXX GmNqMSlg9WoxU7SJ3ePZtej1oS1PRd23azyl6Fm6ADL4Bq+wI2xfwzGHY9lOLDsmDemP97kJv4Pny wxxnlDXdJVzedWcWIUiZbBsvmYmlN/3NAP2dHvNrDrQG9Ei/NamM9koO2rfhSluo1St+0ZoRT5JO4 KSS4TtdTA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1g31ie-0008HS-Kz; Thu, 20 Sep 2018 16:22:20 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0D0902024E44B; Thu, 20 Sep 2018 18:22:17 +0200 (CEST) Date: Thu, 20 Sep 2018 18:22:16 +0200 From: Peter Zijlstra To: Will Deacon Cc: Waiman Long , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@kernel.org, boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com, catalin.marinas@arm.com Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Message-ID: <20180920162216.GR24142@hirez.programming.kicks-ass.net> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-3-git-send-email-will.deacon@arm.com> <20180409105835.GC23134@arm.com> <20180409145409.GA9661@arm.com> <20180409155420.GB4082@hirez.programming.kicks-ass.net> <20180409171959.GB9661@arm.com> <20180920160832.GZ24124@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180920160832.GZ24124@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 20, 2018 at 06:08:32PM +0200, Peter Zijlstra wrote: > Another approach might be to use something like: > > val = xchg_relaxed(&lock->locked_pending, _Q_PENDING_VAL | _Q_LOCKED_VAL); > val |= atomic_read_acquire(&lock->val) & _Q_TAIL_MASK; > > combined with something like: > > /* 0,0,0 -> 0,1,1 - we won trylock */ > if (!(val & _Q_LOCKED_MASK)) { That one doesn't actually work... let me think about this more. > clear_pending(lock); > return; > } > > /* 0,0,1 -> 0,1,1 - we won pending */ > if (!(val & ~_Q_LOCKED_MASK)) { > ... > } > > /* *,0,1 -> *,1,1 - we won pending, but there's queueing */ > if (!(val & _Q_PENDING_VAL)) > clear_pending(lock); > > ... > > > Hmmm?