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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,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 B48C5C282C8 for ; Mon, 28 Jan 2019 13:58:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8574B20989 for ; Mon, 28 Jan 2019 13:58:13 +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="WY88YdWO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726958AbfA1N6L (ORCPT ); Mon, 28 Jan 2019 08:58:11 -0500 Received: from merlin.infradead.org ([205.233.59.134]:36820 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726735AbfA1N6L (ORCPT ); Mon, 28 Jan 2019 08:58:11 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=CbSKuq8gbOVSeLddKe0qvIRvpEXxj8L97la8Qvn8jgo=; b=WY88YdWO8Y8JmXcrY81y+HAPs G3HyBzyOtTvYMy3+V4ycOvgdjitPJwXFi1VokoGoiEPSmq1PZ7J9nHwkEIs3TiKPMyjRLviKPY/PI A0Yx9niNkrX32YHvV/MdlBvYlWsyszyVWHbU9MpHOL3CfZuqZ6v+QezOmOUbhYgeggOP1cjI4RHCX 7adDwvPvL+EPKdAd52QOzl2qpnIXdAmMK/mRMYJGhObXY9p5uBFVrqEGCDqq87iplBOaBjl8iKSMI eBBUgglYhWe/WS0EE9ztxPbxzEnM5gMmwuBLl8vBBD1a6JOwjZ5x4hO5+tnU6KBLjAXaSqVGEfNnm Avk/DTWWg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1go7QN-0007PX-35; Mon, 28 Jan 2019 13:58:07 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D0C07201EC174; Mon, 28 Jan 2019 14:58:04 +0100 (CET) Date: Mon, 28 Jan 2019 14:58:04 +0100 From: Peter Zijlstra To: Heiko Carstens Cc: Thomas Gleixner , Ingo Molnar , Martin Schwidefsky , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Stefan Liebler Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered Message-ID: <20190128135804.GB28878@hirez.programming.kicks-ass.net> References: <20181127081115.GB3625@osiris> <20181129112321.GB3449@osiris> <20190128134410.GA28485@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190128134410.GA28485@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 28, 2019 at 02:44:10PM +0100, Peter Zijlstra wrote: > On Thu, Nov 29, 2018 at 12:23:21PM +0100, Heiko Carstens wrote: > > > And indeed, if I run only this test case in an endless loop and do > > some parallel work (like kernel compile) it currently seems to be > > possible to reproduce the warning: > > > > while true; do time ./testrun.sh nptl/tst-robustpi8 --direct ; done > > > > within the build directory of glibc (2.28). > > Right; so that reproduces for me. > > After staring at all that for a while; trying to remember how it all > worked (or supposed to work rather), I became suspiscous of commit: > > 56222b212e8e ("futex: Drop hb->lock before enqueueing on the rtmutex") > > And indeed, when I revert that; the above reproducer no longer works (as > in, it no longer triggers in minutes and has -- so far -- held up for an > hour+ or so). > > That patch in particular allows futex_unlock_pi() to 'start' early: > > > futex_lock_pi() futex_unlock_pi() > lock hb > queue > lock wait_lock > unlock hb > lock hb > futex_top_waiter > get_pi_state > lock wait_lock > rt_mutex_proxy_start // fail > unlock wait_lock > // acquired wait_lock unlock hb > wake_futex_pi() > rt_mutex_next_owner() // whoops, no waiter > WARN and simply removing that WARN, would allow futex_unlock_pi() to spin on retry until the futex_lock_pi() CPU comes around to doing the lock hb below: > lock hb > unqueue_me_pi Which seems undesirable from a determinsm POV.