[PATCH 4/8] cgroup: deactivate CSS's and mark cgroup dead before invoking ->pre_destroy()

[Kamezawa Hiroyuki]

(2012/10/31 13:22), Tejun Heo wrote:
> Because ->pre_destroy() could fail and can't be called under
> cgroup_mutex, cgroup destruction did something very ugly.
> 
>    1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise.
> 
>    2. Release cgroup_mutex and call ->pre_destroy().
> 
>    3. Re-grab cgroup_mutex and verify it can still be destroyed; fail
>       otherwise.
> 
>    4. Continue destroying.
> 
> In addition to being ugly, it has been always broken in various ways.
> For example, memcg ->pre_destroy() expects the cgroup to be inactive
> after it's done but tasks can be attached and detached between #2 and
> #3 and the conditions that memcg verified in ->pre_destroy() might no
> longer hold by the time control reaches #3.
> 
> Now that ->pre_destroy() is no longer allowed to fail.  We can switch
> to the following.
> 
>    1. Grab cgroup_mutex and fail if it can't be destroyed; fail
>       otherwise.
> 
>    2. Deactivate CSS's and mark the cgroup removed thus preventing any
>       further operations which can invalidate the verification from #1.
> 
>    3. Release cgroup_mutex and call ->pre_destroy().
> 
>    4. Re-grab cgroup_mutex and continue destroying.
> 
> After this change, controllers can safely assume that ->pre_destroy()
> will only be called only once for a given cgroup and, once
> ->pre_destroy() is called, the cgroup will stay dormant till it's
> destroyed.
> 
> Signed-off-by: Tejun Heo <tj at kernel.org>


Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>

Ok, cgroup_lock_live_group() will work synchronously against attach_task().
I welcome this.