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

[Tejun Heo <tj@kernel.org>]

Hey,

On Wed, Jun 25, 2014 at 09:56:31AM +0800, Li Zefan wrote:
> > 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.

Sure, but now the decision to kill is synchronized so the other side
can interlock with it.  e.g.

cgroup_mount()
{
	mutex_lock();
	lookup_cgroup_root();
	if (root isn't killed yet)
		root->this_better_stay_alive++;
	mutex_unlock();
	kernfs_mount();
}

cgroup_kill_sb()
{
	mutex_lock();
	if (check whether root can be killed)
		percpu_ref_kill();
	mutex_unlock();
	if (the above condition was true)
		kernfs_kill_sb();
}

-- 
tejun