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.
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