linux-kernel - Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill

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Message-ID: <20140624210119.GC14909@htj.dyndns.org>
Date:	Tue, 24 Jun 2014 17:01:19 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and
 cgroup_kill_sb()

Hello, Li.

On Tue, Jun 24, 2014 at 09:22:00AM +0800, Li Zefan wrote:
> > Ah, right.  Gees, I'm really hating the fact that we have ->mount but
> > not ->umount.  However, can't we make it a bit simpler by just
> > introducing a mutex protecting looking up and refing up an existing
> > root and a sb going away?  The only problem is that the refcnt being
> > killed isn't atomic w.r.t. new live ref coming up, right?  Why not
> > just add a mutex around them so that they can't race?
> 
> Well, kill_sb() is called with sb->s_umount held, while kernfs_mount()
> returned with sb->s_umount held, so adding a mutex will lead to ABBA
> deadlock.

Hmmm?  Why does that matter?  The only region in cgroup_mount() which
needs to be put inside such mutex would be root lookup, no?

Thanks.

-- 
tejun
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