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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 8971AC43610 for ; Thu, 29 Nov 2018 18:49:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52B1520868 for ; Thu, 29 Nov 2018 18:49:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 52B1520868 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 S1726702AbeK3Fz7 (ORCPT ); Fri, 30 Nov 2018 00:55:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38384 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725877AbeK3Fz6 (ORCPT ); Fri, 30 Nov 2018 00:55:58 -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 9EA5181DE1; Thu, 29 Nov 2018 18:49:36 +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 38E166E3E3; Thu, 29 Nov 2018 18:49:35 +0000 (UTC) Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil To: Davidlohr Bueso , Peter Zijlstra Cc: Yongji Xie , mingo@redhat.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, xieyongji@baidu.com, zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com, nixun@baidu.com, lilin24@baidu.com, Thomas Gleixner References: <1543495830-2644-1-git-send-email-xieyongji@baidu.com> <20181129131232.GN2131@hirez.programming.kicks-ass.net> <20181129134449.GH2149@hirez.programming.kicks-ass.net> <20181129184329.ftxhtpgx4rwzoh6y@linux-r8p5> 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: <20181129184329.ftxhtpgx4rwzoh6y@linux-r8p5> 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 [10.5.110.25]); Thu, 29 Nov 2018 18:49:36 +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 01:43 PM, Davidlohr Bueso wrote: > On Thu, 29 Nov 2018, Peter Zijlstra wrote: > >> On Thu, Nov 29, 2018 at 02:12:32PM +0100, Peter Zijlstra wrote: >>> Yes, I think this is real, and worse, I think we need to go audit all >>> wake_q_add() users and document this behaviour. >>> >>> In the ideal case we'd delay the actual wakeup to the last wake_up_q(), >>> but I don't think we can easily fix that. >> >> See commit: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups"), I think >> that introduces the exact same bug. >> >> Something like the below perhaps, altough this pattern seems to want a >> wake_a_add() variant that already assumes get_task_struct(). > > So I was looking at ways to avoid the redundant reference counting, > but given how wake_q_add() and wake_up_q() are so loose I can't > see how to avoid it -- we hold reference across the calls to maintain > valid mem. > > For example, wake_q will grab reference iff the cmpxchg succeeds, > likewise it will enter the wakeup loop in wake_up_q(), and there is no > awareness of which caller had the failed cmpxchg because another wakup > was in progress. > I think it will be useful to make wake_q_add() to return the status of wake_q insertion so that we can respond appropriately if it fails. > And yes, afaict all wake_q users suffer from the same issue, so we have > to move the wake_q_add() after the condition, while explicitly doing > the task ref counting. How about adding two helpers - wake_q_enter(), wake_q_exit() that can hide all the reference counting? Thanks, Longman