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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 3094EC48BD6 for ; Wed, 26 Jun 2019 22:47:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 07B30217D4 for ; Wed, 26 Jun 2019 22:47:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726497AbfFZWrj (ORCPT ); Wed, 26 Jun 2019 18:47:39 -0400 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:33891 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726354AbfFZWrj (ORCPT ); Wed, 26 Jun 2019 18:47:39 -0400 Received: from callcc.thunk.org (guestnat-104-133-0-109.corp.google.com [104.133.0.109] (may be forged)) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id x5QMl9xf027338 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 26 Jun 2019 18:47:10 -0400 Received: by callcc.thunk.org (Postfix, from userid 15806) id 9C2EC42002E; Wed, 26 Jun 2019 18:47:09 -0400 (EDT) Date: Wed, 26 Jun 2019 18:47:09 -0400 From: "Theodore Ts'o" To: syzbot , adilger.kernel@dilger.ca, davem@davemloft.net, eladr@mellanox.com, idosch@mellanox.com, jiri@mellanox.com, john.stultz@linaro.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com, tglx@linutronix.de Subject: Re: INFO: rcu detected stall in ext4_write_checks Message-ID: <20190626224709.GH3116@mit.edu> Mail-Followup-To: Theodore Ts'o , syzbot , adilger.kernel@dilger.ca, davem@davemloft.net, eladr@mellanox.com, idosch@mellanox.com, jiri@mellanox.com, john.stultz@linaro.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com, tglx@linutronix.de References: <000000000000d3f34b058c3d5a4f@google.com> <20190626184251.GE3116@mit.edu> <20190626210351.GF3116@mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190626210351.GF3116@mit.edu> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org More details about what is going on. First, it requires root, because one of that is required is using sched_setattr (which is enough to shoot yourself in the foot): sched_setattr(0, {size=0, sched_policy=0x6 /* SCHED_??? */, sched_flags=0, sched_nice=0, sched_priority=0, sched_runtime=2251799813724439, sched_deadline=4611686018427453437, sched_period=0}, 0) = 0 This is setting the scheduler policy to be SCHED_DEADLINE, with a runtime parameter of 2251799.813724439 seconds (or 26 days) and a deadline of 4611686018.427453437 seconds (or 146 *years*). This means a particular kernel thread can run for up to 26 **days** before it is scheduled away, and if a kernel reads gets woken up or sent a signal, no worries, it will wake up roughly seven times the interval that Rip Van Winkle spent snoozing in a cave in the Catskill Mountains (in Washington Irving's short story). We then kick off a half-dozen threads all running: sendfile(fd, fd, &pos, 0x8080fffffffe); (and since count is a ridiculously large number, this gets cut down to): sendfile(fd, fd, &pos, 2147479552); Is it any wonder that we are seeing RCU stalls? :-) - Ted