From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753244AbcFOHZJ (ORCPT ); Wed, 15 Jun 2016 03:25:09 -0400 Received: from foss.arm.com ([217.140.101.70]:34831 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751842AbcFOHZH (ORCPT ); Wed, 15 Jun 2016 03:25:07 -0400 Date: Wed, 15 Jun 2016 08:25:07 +0100 From: Juri Lelli To: Peter Zijlstra Cc: mingo@kernel.org, tglx@linutronix.de, rostedt@goodmis.org, xlpang@redhat.com, linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com, jdesfossez@efficios.com, bristot@redhat.com Subject: Re: [RFC][PATCH 8/8] rtmutex: Fix PI chain order integrity Message-ID: <20160615072507.GS5981@e106622-lin> References: <20160607195635.710022345@infradead.org> <20160607200216.117270606@infradead.org> <20160614173908.GQ5981@e106622-lin> <20160614194401.GL30921@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160614194401.GL30921@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/06/16 21:44, Peter Zijlstra wrote: > On Tue, Jun 14, 2016 at 06:39:08PM +0100, Juri Lelli wrote: > > On 07/06/16 21:56, Peter Zijlstra wrote: > > > rt_mutex_waiter::prio is a copy of task_struct::prio which is updated > > > during the PI chain walk, such that the PI chain order isn't messed up > > > by (asynchronous) task state updates. > > > > > > Currently rt_mutex_waiter_less() uses task state for deadline tasks; > > > this is broken, since the task state can, as said above, change > > > asynchronously, causing the RB tree order to change without actual > > > tree update -> FAIL. > > > > > > Fix this by also copying the deadline into the rt_mutex_waiter state > > > and updating it along with its prio field. > > > > > > Ideally we would also force PI chain updates whenever DL tasks update > > > their deadline parameter, but for first approximation this is less > > > broken than it was. > > > > > > > The patch looks OK to me. However, I'm failing to see when we can update > > dl.deadline of a waiter asynchronously. Since a waiter is blocked, we > > can't really change his dl.deadline by calling setscheduler on him, as > > the update would operate on dl.dl_deadline. The new values will start to > > be used as soon as it gets unblocked. The situation seems different for > > RT tasks, for which priority change takes effect immediately. > > > > What am I missing? :-) > > Ah, I missed the dl_deadline vs deadline thing. Still, with optimistic > spinning the waiter could hit its throttle/refresh path, right? And then > that would update deadline. > I guess it's not that likely, but yes it could potentially happen that a waiter is optimistically spinning, depletes its runtime, gets throttled and then replenished when still spinning. Maybe it doesn't really make sense continuing spinning in this situation, but I guess things get really complicated. :-/ Anyway, as said, I think this patch is OK. Maybe we want to add a comment just to remember what situation can cause an issue if we don't do this? Patch changelog would be OK as well for such a comment IMHO. Thanks, - Juri