From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754116Ab2GSL0a (ORCPT ); Thu, 19 Jul 2012 07:26:30 -0400 Received: from e28smtp06.in.ibm.com ([122.248.162.6]:56712 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752761Ab2GSL00 (ORCPT ); Thu, 19 Jul 2012 07:26:26 -0400 From: "Aneesh Kumar K.V" To: Kamezawa Hiroyuki Cc: Li Zefan , Andrew Morton , linux-mm@kvack.org, mhocko@suse.cz, linux-kernel@vger.kernel.org Subject: Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback In-Reply-To: <5007E0A2.70906@jp.fujitsu.com> References: <1342589649-15066-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20120718142628.76bf78b3.akpm@linux-foundation.org> <87hat4794l.fsf@skywalker.in.ibm.com> <5007B034.4030909@huawei.com> <87wr20f5pj.fsf@skywalker.in.ibm.com> <5007E0A2.70906@jp.fujitsu.com> User-Agent: Notmuch/0.13.2+63~g548a9bf (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu) Date: Thu, 19 Jul 2012 16:56:18 +0530 Message-ID: <87r4s8f0v9.fsf@skywalker.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii x-cbid: 12071911-9574-0000-0000-000003A2C750 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Kamezawa Hiroyuki writes: >>>>> >>>>> We test RES_USAGE before taking hugetlb_lock. What prevents some other >>>>> thread from increasing RES_USAGE after that test? >>>>> >>>>> After walking the list we test RES_USAGE after dropping hugetlb_lock. >>>>> What prevents another thread from incrementing RES_USAGE before that >>>>> test, triggering the BUG? >>>> >>>> IIUC core cgroup will prevent a new task getting added to the cgroup >>>> when we are in pre_destroy. Since we already check that the cgroup doesn't >>>> have any task, the RES_USAGE cannot increase in pre_destroy. >>>> >>> >>> >>> You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve >>> the lock after that, so a task can be attached to the cgroup in this interval. >>> >> >> But that means rmdir can be racy right ? What happens if the task got >> added, allocated few pages and then moved out ? We still would have task >> count 0 but few pages, which we missed to to move to parent cgroup. >> > > That's a problem even if it's verrrry unlikely. > I'd like to look into it and fix the race in cgroup layer. > But I'm sorry I'm a bit busy in these days... > How about moving that mutex_unlock(&cgroup_mutex) to memcg callback ? That can be a patch for 3.5 ? -aneesh