Re: [PATCH 7/8] cgroups: Add a task counter subsystem

Frederic Weisbecker
2011-08-09T18:34:11Z

On Tue, Aug 09, 2011 at 08:19:47PM +0200, Oleg Nesterov wrote:
> On 08/09, Frederic Weisbecker wrote:
> >
> > On Tue, Aug 09, 2011 at 07:57:08PM +0200, Oleg Nesterov wrote:
> > > > > cgroup_attach_proc() is different, it calls cgroup_task_migrate()
> > > > > after ->attach_task(). Cough.
> > > >
> > > > That's bad. I need to fix that.
> > > >
> > > > So if it returns -ESRCH, I shall not call attach_task() on it
> > > > but cancel_attach_task().
> > >
> > > Afaics this can't help, or I misunderstood. probably attach_task()
> > > can check PF_EXITING...
> >
> > But cgroup_task_migrate() checks that already before migrating
> > cgroups (checks PF_EXITING under task_lock() so that it's
> > synchronized against cgroup_exit())
>
> Yes, but how this can help?
>
> ->attach_task() is called before cgroup_task_migrate(). Suppose
> that it exits before ->attach_task(). In this case we shouldn't
> uncharge the old cgroup, it was already uncharged by cgroup_exit.

Yeah that's why in cgroup_attach_proc() we need to call cgroup_task_migrate()
before ->attach_task(), as it's done in cgroup_attach_task(). And also call
cancel_attach_task() if it fails there with -ESRCH without cancelling
the whole group attachment.