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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS 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 BCB32C43441 for ; Thu, 29 Nov 2018 18:34:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88B7421104 for ; Thu, 29 Nov 2018 18:34:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 88B7421104 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1728989AbeK3Fk0 (ORCPT ); Fri, 30 Nov 2018 00:40:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38676 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725928AbeK3Fk0 (ORCPT ); Fri, 30 Nov 2018 00:40:26 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6E2D4C045776; Thu, 29 Nov 2018 18:34:07 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2AE8A17CEF; Thu, 29 Nov 2018 18:34:06 +0000 (UTC) Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil To: Will Deacon Cc: Peter Zijlstra , Yongji Xie , mingo@redhat.com, linux-kernel@vger.kernel.org, xieyongji@baidu.com, zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com, nixun@baidu.com, lilin24@baidu.com, Davidlohr Bueso References: <1543495830-2644-1-git-send-email-xieyongji@baidu.com> <20181129131232.GN2131@hirez.programming.kicks-ass.net> <5598cd71-c3c8-d6ef-eb30-777cf901a2ef@redhat.com> <20181129160627.GU2131@hirez.programming.kicks-ass.net> <8cc45695-b325-a219-8b46-d5da6ddfdd63@redhat.com> <20181129172700.GA11632@hirez.programming.kicks-ass.net> <20181129180828.GA11650@hirez.programming.kicks-ass.net> <729ceddb-dd9a-ec2a-f74e-03fa4d7e65e8@redhat.com> <20181129183103.GA4952@arm.com> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181129183103.GA4952@arm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Thu, 29 Nov 2018 18:34:07 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/29/2018 01:31 PM, Will Deacon wrote: > On Thu, Nov 29, 2018 at 01:26:34PM -0500, Waiman Long wrote: >> On 11/29/2018 01:08 PM, Peter Zijlstra wrote: >>> Hmm, I think we're missing a barrier in wake_q_add(); when cmpxchg() >>> fails we still need an smp_mb(). >>> >>> Something like so. >>> >>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>> index 3d87a28da378..69def558edf6 100644 >>> --- a/kernel/sched/core.c >>> +++ b/kernel/sched/core.c >>> @@ -400,6 +400,13 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task) >>> { >>> struct wake_q_node *node = &task->wake_q; >>> >>> + /* >>> + * Ensure, that when the below cmpxchg() fails, the corresponding >>> + * wake_up_q() will observe our prior state. >>> + * >>> + * Pairs with the smp_mb() from wake_up_q()'s wake_up_process(). >>> + */ >>> + smp_mb(); >>> /* >>> * Atomically grab the task, if ->wake_q is !nil already it means >>> * its already queued (either by us or someone else) and will get the >>> @@ -408,7 +415,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task) >>> * This cmpxchg() executes a full barrier, which pairs with the full >>> * barrier executed by the wakeup in wake_up_q(). >>> */ >>> - if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) >>> + if (cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)) >>> return; >>> >>> get_task_struct(task); >> That can be costly for x86 which will now have 2 locked instructions. >> Should we introduce a kind of special cmpxchg (e.g. cmpxchg_mb) that >> will guarantee a memory barrier whether the operation fails or not? > I thought smp_mb__before_atomic() was designed for this sort of thing? > > Will That will certainly work for x86. However, I am not sure if that will be optimal for arm and powerpc. Cheers, Longman