Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753563AbYJUJ3l (ORCPT ); Tue, 21 Oct 2008 05:29:41 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751953AbYJUJ3b (ORCPT ); Tue, 21 Oct 2008 05:29:31 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:62306 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751929AbYJUJ3a (ORCPT ); Tue, 21 Oct 2008 05:29:30 -0400 Message-ID: <48FDA0F3.2070306@cn.fujitsu.com> Date: Tue, 21 Oct 2008 17:29:23 +0800 From: Li Zefan User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Cedric Le Goater CC: Andrew Morton , Linux Containers , LKML Subject: Re: [PATCH 2/4] freezer_cg: remove redundant check in freezer_can_attach() References: <48FD26F5.1070809@cn.fujitsu.com> <48FD273F.3040505@cn.fujitsu.com> <48FD85A2.8090605@fr.ibm.com> In-Reply-To: <48FD85A2.8090605@fr.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2615 Lines: 79 Cedric Le Goater wrote: > Li Zefan wrote: >> It is sufficient to check if @task is frozen, and no need to check if >> the original freezer is frozen. > > hmm, a task being frozen does not mean that its freezer cgroup is > frozen. If a task has being frozen, then can_attach() returns -EBUSY at once. If a task isn't frozen, then we have orig_freezer->state == THAWED. So we need to check the state of the task only. > So the extra check seems valid but looking at the comment : > > /* > * The call to cgroup_lock() in the freezer.state write method prevents > * a write to that file racing against an attach, and hence the > * can_attach() result will remain valid until the attach completes. > */ > static int freezer_can_attach(struct cgroup_subsys *ss, > > how do we know that the task_freezer(task), which is not protected by > the cgroup_lock(), is not going to change its state to CGROUP_FROZEN > it looks racy. > Since freezer_can_attach() is called by cgroup core with cgroup_lock held, there is no race with task attaching and state changing. > C. > >> Signed-off-by: Li Zefan >> --- >> kernel/cgroup_freezer.c | 16 +++++++--------- >> 1 files changed, 7 insertions(+), 9 deletions(-) >> >> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c >> index 7f54d1c..6fadafe 100644 >> --- a/kernel/cgroup_freezer.c >> +++ b/kernel/cgroup_freezer.c >> @@ -162,9 +162,13 @@ static int freezer_can_attach(struct cgroup_subsys *ss, >> struct task_struct *task) >> { >> struct freezer *freezer; >> - int retval; >> >> - /* Anything frozen can't move or be moved to/from */ >> + /* >> + * Anything frozen can't move or be moved to/from. >> + * >> + * Since orig_freezer->state == FROZEN means that @task has been >> + * frozen, so it's sufficient to check the latter condition. >> + */ >> >> if (is_task_frozen_enough(task)) >> return -EBUSY; >> @@ -173,13 +177,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss, >> if (freezer->state == CGROUP_FROZEN) >> return -EBUSY; >> >> - retval = 0; >> - task_lock(task); >> - freezer = task_freezer(task); >> - if (freezer->state == CGROUP_FROZEN) >> - retval = -EBUSY; >> - task_unlock(task); >> - return retval; >> + return 0; >> } >> >> static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task) > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/