From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758632AbcGKMMd (ORCPT ); Mon, 11 Jul 2016 08:12:33 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54216 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752062AbcGKMM0 (ORCPT ); Mon, 11 Jul 2016 08:12:26 -0400 Reply-To: xlpang@redhat.com Subject: Re: [PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair References: <146608183552.21905.15924473394414832071.stgit@buzz> <57835735.6020906@redhat.com> <57835BFD.90201@redhat.com> To: Wanpeng Li , xlpang@redhat.com, Konstantin Khlebnikov Cc: Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org" , stable@vger.kernel.org From: Xunlei Pang Message-ID: <57838D28.4090003@redhat.com> Date: Mon, 11 Jul 2016 20:12:24 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 11 Jul 2016 12:12:26 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016/07/11 at 17:54, Wanpeng Li wrote: > Hi Konstantin, Xunlei, > 2016-07-11 16:42 GMT+08:00 Xunlei Pang : >> On 2016/07/11 at 16:22, Xunlei Pang wrote: >>> On 2016/07/11 at 15:25, Wanpeng Li wrote: >>>> 2016-06-16 20:57 GMT+08:00 Konstantin Khlebnikov : >>>>> Hierarchy could be already throttled at this point. Throttled next >>>>> buddy could trigger null pointer dereference in pick_next_task_fair(). >>>> There is cfs_rq->next check in pick_next_entity(), so how can null >>>> pointer dereference happen? >>> I guess it's the following code leading to a NULL se returned: >> s/NULL/empty-entity cfs_rq se/ >> >>> pick_next_entity(): >>> if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) > ^^^^^^^^^^^^^ > I think this will return false. With the wrong throttled_hierarchy(), I think this can happen. But after we have the corrected throttled_hierarchy() patch, I can't see how it is possible. dequeue_task_fair(): if (task_sleep && parent_entity(se)) set_next_buddy(parent_entity(se)); How does dequeue_task_fair() with DEQUEUE_SLEEP set(true task_sleep) happen to a throttled hierarchy? IOW, a task belongs to a throttled hierarchy is running? Maybe Konstantin knows the reason. Regards, Xunlei