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 5C720C169C4 for ; Sun, 3 Feb 2019 16:30:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1B98A2177E for ; Sun, 3 Feb 2019 16:30:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726923AbfBCQas (ORCPT ); Sun, 3 Feb 2019 11:30:48 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:54012 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726399AbfBCQar (ORCPT ); Sun, 3 Feb 2019 11:30:47 -0500 Received: from p5492e0d8.dip0.t-ipconnect.de ([84.146.224.216] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1gqKfH-0004UY-N9; Sun, 03 Feb 2019 17:30:39 +0100 Date: Sun, 3 Feb 2019 17:30:39 +0100 (CET) From: Thomas Gleixner To: Heiko Carstens cc: Sebastian Sewior , "Paul E. McKenney" , Peter Zijlstra , Ingo Molnar , Martin Schwidefsky , LKML , linux-s390@vger.kernel.org, Stefan Liebler Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggerede In-Reply-To: <20190202112006.GB3381@osiris> Message-ID: References: <20190130233557.GA4240@linux.ibm.com> <20190131165228.GA32680@osiris> <20190131170653.spnrxsiblkssleyd@linutronix.de> <20190201161227.GG3770@osiris> <20190202091043.GA3381@osiris> <20190202112006.GB3381@osiris> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2 Feb 2019, Heiko Carstens wrote: > On Sat, Feb 02, 2019 at 11:14:27AM +0100, Thomas Gleixner wrote: > > On Sat, 2 Feb 2019, Heiko Carstens wrote: > > So after the unlock @timestamp 337.215675 the kernel does not deal with > > that futex at all until the failed lock attempt where it rightfully rejects > > the attempt due to the alleged owner being gone. > > > > So this looks more like user space doing something stupid... > > > > As we talked about the missing barriers before, I just looked at > > pthread_mutex_trylock() and that does still: > > > > if (robust) > > { > > ENQUEUE_MUTEX_PI (mutex); > > THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL); > > } > > > > So it's missing the barriers which pthread_mutex_lock() has. Grasping for > > straws obviously.... Looks more like a solid tree than a straw now. :) > Excellent! Taking a look into the disassembly of nptl/pthread_mutex_trylock.o > reveals this part: > > 140: a5 1b 00 01 oill %r1,1 > 144: e5 48 a0 f0 00 00 mvghi 240(%r10),0 <--- THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL); > 14a: e3 10 a0 e0 00 24 stg %r1,224(%r10) <--- last THREAD_SETMEM of ENQUEUE_MUTEX_PI Awesome. > I added a barrier between those two and now the code looks like this: > > 140: a5 1b 00 01 oill %r1,1 > 144: e3 10 a0 e0 00 24 stg %r1,224(%r10) > 14a: e5 48 a0 f0 00 00 mvghi 240(%r10),0 > > Looks like this was a one instruction race... Fun. JFYI, I said that I reversed the stores in glibc and on my x86 test VM it took more than _3_ days to trigger. But the good news is, that the trace looks exactly like the ones you provided. So it looks we are on the right track. > I'll try to reproduce with the patch below (sprinkling compiler > barriers just like the other files have). Looks about right. Thanks, tglx