From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754137AbcKIRud (ORCPT ); Wed, 9 Nov 2016 12:50:33 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:49653 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752634AbcKIRuc (ORCPT ); Wed, 9 Nov 2016 12:50:32 -0500 Date: Wed, 9 Nov 2016 18:50:05 +0100 From: Peter Zijlstra To: Oleg Nesterov Cc: Ingo Molnar , Linus Torvalds , Mike Galbraith , hartsjc@redhat.com, vbendel@redhat.com, vlovejoy@redhat.com, linux-kernel@vger.kernel.org Subject: Re: sched/autogroup: race if !sysctl_sched_autogroup_enabled ? Message-ID: <20161109175005.GS3142@twins.programming.kicks-ass.net> References: <20161109165933.GA26071@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161109165933.GA26071@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 09, 2016 at 05:59:33PM +0100, Oleg Nesterov wrote: > We need to ensure that autogroup/tg returned by autogroup_task_group() > can't go away if we race with autogroup_move_group(), and unless the > caller holds ->siglock we rely on fact that autogroup_move_group() > will a) see this task and b) do sched_move_task() which needs the same > same rq->lock. > > However. autogroup_move_group() skips for_each_thread/sched_move_task > if sysctl_sched_autogroup_enabled == 0. > > So. Doesn't this mean that cgroup migration to the root cgroup can race > with autogroup_move_group() and use the soon-to-be-freed autogroup->tg? Argh, its too late for this, also jet-lag. But maybe, I can sort of feel a hole here but cannot for the life of me still think. > although this is a bit off-topic. Another question is that I fail to > understand why sched_autogroup_create_attach() does autogroup_create() > and changes signal->autogroup even if !sysctl_sched_autogroup_enabled. I really cannot remember back that far, but it could be to allow flipping it back on. Then again, I don't think the fork path puts new tasks in, even if the autogroup exists.