Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2014/6/25 5:01, Tejun Heo wrote:
> 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?
> 

unfortunately that won't help. I think what you suggest is:

cgroup_mount()
{
	mutex_lock();
	lookup_cgroup_root();
	mutex_unlock();
	kernfs_mount();
}

cgroup_kill_sb()
{
	mutex_lock();
	percpu_ref_kill();
	mutex_Unlock();
	kernfs_kill_sb();
}

See, we may still be destroying the superblock after we've succeeded
in getting the refcnt of cgroup root.