Re: [PATCH 4/6] cgroups: forbid pre_destroy callback to fail

Michal Hocko
2012-10-19T13:32:49Z

On Thu 18-10-12 15:41:48, Tejun Heo wrote:
> Hello, Michal.
>
> On Wed, Oct 17, 2012 at 03:30:46PM +0200, Michal Hocko wrote:
> > Now that mem_cgroup_pre_destroy callback doesn't fail finally we can
> > safely move on and forbit all the callbacks to fail. The last missing
> > piece is moving cgroup_call_pre_destroy after cgroup_clear_css_refs so
> > that css_tryget fails so no new charges for the memcg can happen.
> > The callbacks are also called from within cgroup_lock to guarantee that
> > no new tasks show up. We could theoretically call them outside of the
> > lock but then we have to move after CGRP_REMOVED flag is set.
> >
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
>
> So, the plan is to do something like the following once memcg is
> ready.
>
> http://thread.gmane.org/gmane.linux.kernel.containers/22559/focus=75251
>
> Note that the patch is broken in a couple places but it does show the
> general direction. I'd prefer if patch #3 simply makes pre_destroy()
> return 0 and drop __DEPRECATED_clear_css_refs from mem_cgroup_subsys.

We can still fail inn #3 without this patch becasuse there are is no
guarantee that a new task is attached to the group. And I wanted to keep
memcg and generic cgroup parts separated.
--
Michal Hocko
SUSE Labs