Re: [PATCH] cgroup: fix to break the while loop in cgroup_attach_task() correctly

[Tejun Heo <tj@kernel.org>]

On Sat, Oct 12, 2013 at 10:59:17AM +0800, Li Zefan wrote:
> From: Anjana V Kumar <anjanavk12@gmail.com>
> 
> Both Anjana and Eunki reported a stall in the while_each_thread loop
> in cgroup_attach_task().
> 
> It's because, when we attach a single thread to a cgroup, if the cgroup
> is exiting or is already in that cgroup, we won't break the loop.
> 
> If the task is already in the cgroup, the bug can lead to another thread
> being attached to the cgroup unexpectedly:
> 
>   # echo 5207 > tasks
>   # cat tasks
>   5207
>   # echo 5207 > tasks
>   # cat tasks
>   5207
>   5215
> 
> What's worse, if the task to be attached isn't the leader of the thread
> group, we might never exit the loop, hence cpu stall. Thanks for Oleg's
> analysis.
> 
> This bug was introduced by commit 081aa458c38ba576bdd4265fc807fa95b48b9e79
> ("cgroup: consolidate cgroup_attach_task() and cgroup_attach_proc()")
> 
> Cc: <stable@vger.kernel.org> # 3.9+
> Reported-by: Eunki Kim <eunki_kim@samsung.com>
> Reported-by: Anjana V Kumar <anjanavk12@gmail.com>
> Signed-off-by: Anjana V Kumar <anjanavk12@gmail.com>
> [ lizf: - fixed the first continue, pointed out by Oleg,
>         - rewrote changelog. ]
> Signed-off-by: Li Zefan <lizefan@huawei.com>

Applied to cgroup/for-3.12-fixes.

Thanks.

-- 
tejun