From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754237AbcDTNRn (ORCPT ); Wed, 20 Apr 2016 09:17:43 -0400 Received: from merlin.infradead.org ([205.233.59.134]:38051 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753796AbcDTNRl (ORCPT ); Wed, 20 Apr 2016 09:17:41 -0400 Date: Wed, 20 Apr 2016 15:17:37 +0200 From: Peter Zijlstra To: xlpang@redhat.com Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Juri Lelli , Ingo Molnar , Steven Rostedt Subject: Re: [PATCH v3 5/6] sched/deadline/rtmutex: Fix unprotected PI access in enqueue_task_dl() Message-ID: <20160420131737.GB3430@twins.programming.kicks-ass.net> References: <1460633827-345-1-git-send-email-xlpang@redhat.com> <1460633827-345-6-git-send-email-xlpang@redhat.com> <20160414153111.GC2975@worktop.cust.blueprintrf.com> <57104ADB.20402@redhat.com> <57104FA0.4090509@redhat.com> <20160420122552.GY3430@twins.programming.kicks-ass.net> <57177D70.10504@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57177D70.10504@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 20, 2016 at 09:00:32PM +0800, Xunlei Pang wrote: > > But what happens? How is it changed when it is blocked? > > The top waiter's policy can be changed by other tasks through sched_setattr() syscall during it was blocked. > I created another thread doing the following thing: > while (1) { > change the waiter to cfs > do something > change the waiter to deadline > } Indeed; so why didn't you say that? That is the single most important thing in the Changelog -- the _actual_ problem, and you left it out. I'm not quite sure how to go fix that best, but copying the state is not right. That precludes being able to change the state. There's two (obvious but) rather ugly solutions: - delay the __sched_setscheduler() call until such a time that the task is no longer the top waiter. - deboost + __sched_setscheduler() + boost Both have a different set of problems, but both keep the p->pi_task pointer and its state 'stable'.