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, URIBL_BLOCKED,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 670D4C04ABB for ; Tue, 11 Sep 2018 11:41:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0780720866 for ; Tue, 11 Sep 2018 11:41:14 +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="oF7Hh/tx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0780720866 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 S1726818AbeIKQkJ (ORCPT ); Tue, 11 Sep 2018 12:40:09 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:59368 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726563AbeIKQkJ (ORCPT ); Tue, 11 Sep 2018 12:40:09 -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=1ZsO7SKBqI5QaDuFbOuMPbxnyoB3l5ADrwEIywG/Dl8=; b=oF7Hh/txgX931uVtJuJt8P9G9 BSPp0Cl1Vs4u9uSGQjv9i1qlObZRU0eR7lh1jLYYINJEQJ1ZBm3Yi026Uedy5vLsZme3YXJnePUr5 vhbCuMBrgUhBHc3gDOxKa3u6E+XDD1YNLEKN+UwMgX1j1qA/9DZf0D/Z3njRpcSFgzt4lTwpn4k9z NC87LbpA0bO7CfWe37YnFGoOVPSCaGJFg8eze8dlCkj2JVMiTE18zduI1cxqIRY7wQQQNCLOF6UwD jJXiPhZ4FBn/3UxgHq4lJIkrNGq0wX6X17GHfKmZXcpUG0OD7SQlOPgbkqdiiF++KsFAkCMI0d9EZ sjnzgnZyw==; 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 1fzh2E-0007Qw-27; Tue, 11 Sep 2018 11:40:46 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 7F8292057F833; Tue, 11 Sep 2018 13:40:43 +0200 (CEST) Date: Tue, 11 Sep 2018 13:40:43 +0200 From: Peter Zijlstra To: Dmitry Safonov 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" Subject: Re: [PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader Message-ID: <20180911114043.GR24106@hirez.programming.kicks-ass.net> References: <20180911014821.26286-1-dima@arista.com> <20180911014821.26286-3-dima@arista.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180911014821.26286-3-dima@arista.com> 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 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.intel.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. > put_task_struct(tsk); > }