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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,URIBL_BLOCKED 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 22336C04ABB for ; Tue, 11 Sep 2018 12:49:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A21AD20833 for ; Tue, 11 Sep 2018 12:49:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=arista.com header.i=@arista.com header.b="bb2QW5JC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A21AD20833 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=arista.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 S1727842AbeIKRsN (ORCPT ); Tue, 11 Sep 2018 13:48:13 -0400 Received: from mail-ed1-f66.google.com ([209.85.208.66]:41634 "EHLO mail-ed1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727685AbeIKRsN (ORCPT ); Tue, 11 Sep 2018 13:48:13 -0400 Received: by mail-ed1-f66.google.com with SMTP id f38-v6so19113918edd.8 for ; Tue, 11 Sep 2018 05:49:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=arista.com; s=googlenew; h=message-id:subject:from:to:cc:date:in-reply-to:references :mime-version:content-transfer-encoding; bh=/nHJ/v7W1DSFpBMS8EBbbFqk0cpjhvjbNdPU/+LgkS0=; b=bb2QW5JC5N/vafUbYxI+QP1ED7XAyqKyfOdK2GzfCEHCtCVYuRVpEW9NySDT7JT76s 8v8mDEDpCiG4//h67UA2LrYqh7MtlWmVozJZUTImJ4GwXDwn3omHI+VxwaCqyNQpYG7B BAN/os50gA/yO6z452PC+vSS2TQnEaNnnjREAZBMJFKzCF9ZrAsPfU98Sq12P0QasdzL 32Bt5IKql0o44y9DDGnoztN4NChPJXa7rgLL6ZlrKBNrdJAMedOljW0QgCnGaGgC/Obx jowrcOUweiCb5beJ0KZnT91kLNvFOKzF9LpDXXwZ9+jHRIKzxh0opK0oY1Pj5/MsMl2C mSyQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:message-id:subject:from:to:cc:date:in-reply-to :references:mime-version:content-transfer-encoding; bh=/nHJ/v7W1DSFpBMS8EBbbFqk0cpjhvjbNdPU/+LgkS0=; b=DjTQfUqMAM8is4X98oq6YKJCZVr0H93bgGUtAJwiih3PM93UdhudxZBBtT6JSqjah9 X1yl79rPtzvlZKTj9G1w2eYsqtRtwLEXyLPb3JpwdJ7jtKy8M8I3RjEwLLWLs6TWzAYT hwm2A5RUx1Cpc+3rg1f8BslM01Z38t7vg5SfFPEFIcEXFvwhtDqpeUfybuxeBDhF3d5S /CaO1OsMPsXIA4yspcI+IPWnAKUoq67WodZcK1m4doqphcEVP+dcLslNpMrwUvQ9CrRn QuWNGcA6DcaHPwqsY4Gz9xpDOeDjLK/tTZgLTvhHHNtfz6D89EMhHqCOKWGVMVMV5vjt X3ag== X-Gm-Message-State: APzg51AzPxSVi91dt6+Vk9uv0DoQZtcP7j48u+3HbxOSdWosBKXE2Udq +It62R5QgOSp4h60HqZDYll5CQ== X-Google-Smtp-Source: ANB0Vda2qbSpPGSp0aid5rVZqmFJw9AAhsHt0eA6Sw7L2aLwWV2UAW+fv+IGsBcY6bri3E5LdUbVnw== X-Received: by 2002:a50:b1d1:: with SMTP id n17-v6mr28065812edd.52.1536670140557; Tue, 11 Sep 2018 05:49:00 -0700 (PDT) Received: from dhcp.ire.aristanetworks.com ([217.173.96.166]) by smtp.gmail.com with ESMTPSA id n15-v6sm11010049edn.44.2018.09.11.05.48.59 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 11 Sep 2018 05:48:59 -0700 (PDT) Message-ID: <1536670138.2710.20.camel@arista.com> Subject: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader From: Dmitry Safonov To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Dmitry Safonov <0x7f454c46@gmail.com>, Daniel Axtens , Dmitry Vyukov , Michael Neuling , Mikulas Patocka , Nathan March , Pasi =?ISO-8859-1?Q?K=E4rkk=E4inen?= , Peter Hurley , "Rong, Chen" , Sergey Senozhatsky , Tan Xiaojun , Tetsuo Handa , stable@vger.kernel.org, Greg Kroah-Hartman , Jiri Slaby , "Paul E. McKenney" Date: Tue, 11 Sep 2018 13:48:58 +0100 In-Reply-To: <20180911114043.GR24106@hirez.programming.kicks-ass.net> References: <20180911014821.26286-1-dima@arista.com> <20180911014821.26286-3-dima@arista.com> <20180911114043.GR24106@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.24.6 (3.24.6-1.fc26) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2018-09-11 at 13:40 +0200, Peter Zijlstra wrote: > On Tue, Sep 11, 2018 at 02:48:17AM +0100, Dmitry Safonov wrote: > > There is a couple of reports about lockup in ldsem_down_read() > > without > > anyone holding write end of ldisc semaphore: > > lkml.kernel.org/r/<20171121132855.ajdv4k6swzhvktl6@wfg-t540p.sh.int > > el.com> > > lkml.kernel.org/r/<20180907045041.GF1110@shao2-debian> > > > > They all looked like a missed wake up. > > I wasn't lucky enough to reproduce it, but it seems like reader on > > another CPU can miss waiter->task update and schedule again, > > resulting > > in indefinite (MAX_SCHEDULE_TIMEOUT) sleep. > > > > Make sure waked up reader will see waiter->task == NULL. > > --- a/drivers/tty/tty_ldsem.c > > +++ b/drivers/tty/tty_ldsem.c > > @@ -118,6 +118,8 @@ static void __ldsem_wake_readers(struct > > ld_semaphore *sem) > > tsk = waiter->task; > > smp_mb(); > > waiter->task = NULL; > > + /* Make sure down_read_failed() will see !waiter- > > >task update */ > > + smp_wmb(); > > wake_up_process(tsk); > > This is 'wrong', wake_up_process() should imply sufficient for this > to > already be true. Yeah, thanks. It was stupid of me not to check that.. Saw the smoke that would describe the reports and made too long-going conjectures. Need more covfefe and staring into that code. > > > put_task_struct(tsk); > > } -- Thanks, Dmitry