Re: [PATCH v2] cgroup: protect modifications to cgroup->idr with cgroup_mutex

[Li Zefan <lizefan@huawei.com>]

On 2014/2/12 14:37, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, Feb 12, 2014 at 02:28:53PM +0800, Li Zefan wrote:
>> v2:
>> - Don't call deactivate_super() inside cgroup_mutex, as cgroup_kill_sb()
>> will be called if sb refcnt reaches 0. I don't think this can happen,
>> as cgroup_create() is called through vfs, so vfs should guarantee the
>> superblock won't disappear. Still better not depend on it even my guess
>> is probably correct.
> 
> If the deadlock can't actually happen, I don't really care either way
> as the code goes away after kernfs conversion anyway.  I've already
> applied v1, so if you think this change is important, can you send an
> incremental patch?
> 

I'm fine to stick with V1.

I'm pretty confident it's safe, as we can increment sb refcnt without
any checking or locking (even cgroup_mutex as the comment says) in
cgroup_create().