From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and
 cgroup_kill_sb()
Date: Tue, 24 Jun 2014 17:01:19 -0400
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To: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
Cc: LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>

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