linux-kernel - Re: [PATCH 3/4] cgroup: fix locking in cgroup_cfts

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Message-ID: <52E87780.1040002@huawei.com>
Date:	Wed, 29 Jan 2014 11:37:36 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH 3/4] cgroup: fix locking in cgroup_cfts_commit()

On 2014/1/28 23:32, Tejun Heo wrote:
> cgroup_cfts_commit() walks the cgroup hierarchy that the target
> subsystem is attached to and tries to apply the file changes.  Due to
> the convolution with inode locking, it can't keep cgroup_mutex locked
> while iterating.  It currently holds only RCU read lock around the
> actual iteration and then pins the found cgroup using dget().
> 
> Unfortunately, this is incorrect.  Although the iteration does check
> cgroup_is_dead() before invoking dget(), there's nothing which
> prevents the dentry from going away inbetween.  Note that this is
> different from the usual css iterations where css_tryget() is used to
> pin the css - css_tryget() tests whether the css can be pinned and
> fails if not.
> 
> The problem can be solved by simply holding cgroup_mutex instead of
> RCU read lock around the iteration, which actually reduces LOC.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: stable@...r.kernel.org

Good catch!

Acked-by: Li Zefan <lizefan@...wei.com>
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