Re: [PATCH v3 4/6] cgroups: forbid pre_destroy callback to fail

[Michal Hocko <mhocko@suse.cz>]

On Mon 29-10-12 18:06:34, Glauber Costa wrote:
> On 10/29/2012 06:04 PM, Glauber Costa wrote:
> > On 10/26/2012 03:37 PM, Michal Hocko wrote:
> >> Now that mem_cgroup_pre_destroy callback doesn't fail (other than a race
> >> with a task attach resp. child group appears) 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.
> >> We cannot, however, move cgroup_call_pre_destroy right after because we
> >> cannot call mem_cgroup_pre_destroy with the cgroup_lock held (see
> >> 3fa59dfb cgroup: fix potential deadlock in pre_destroy) so we have to
> >> move it after the lock is released.
> >>
> > 
> > If we don't have the cgroup lock held, how safe is the following
> > statement in mem_cgroup_reparent_charges():
> > 
> > if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
> > 	return -EBUSY;
> > 
> > ?
> > 
> > IIUC, although this is not generally safe, but it would be safe here
> > because at this point we are expected to had already set the removed bit
> > in the css. If this is the case, however, this condition is impossible
> > and becomes useless - in which case you may want to remove it from Patch1.
> > 
> Which I just saw you doing in patch5... =)

Yes, I just wanted to keep this one cgroup core only to enable further
cgroup clean ups easier. Dropping the earlier in the series could
introduce regressions which I tried to avoid as much as possible.

Thanks

-- 
Michal Hocko
SUSE Labs

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