Re: [perfmon2] [RFC][PATCH] cgroup: Fix cgroup_subsys::exit callback

[Paul M. <me...@go...>]

On Mon, Feb 7, 2011 at 8:10 AM, Peter Zijlstra <pe...@in...> wrote:
>
> Make the ::exit method act like ::attach, it is after all very nearly
> the same thing.

The major difference between attach and exit is that the former is
only triggered in response to user cgroup-management action, whereas
the latter is triggered whenever a task exits, even if cgroups aren't
set up.


>  void cgroup_exit(struct task_struct *tsk, int run_callbacks)
>  {
> -       int i;
>        struct css_set *cg;
> +       int i;
>
> -       if (run_callbacks && need_forkexit_callback) {
> -               /*
> -                * modular subsystems can't use callbacks, so no need to lock
> -                * the subsys array
> -                */
> -               for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> -                       struct cgroup_subsys *ss = subsys[i];
> -                       if (ss->exit)
> -                               ss->exit(ss, tsk);
> -               }
> -       }
> +       mutex_lock(&cgroup_mutex);

NACK - cgroup_mutex is way too heavy to take in the task exit path.
We'll need to find some other way to fix this if it's really needed.
task->alloc_lock is also normally a valid thing to synchronize against
cgroup moves, but I'd have to look at the exit path to see if it's
still valid there.

Paul