Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Nov 14, 2019 at 08:33:40PM +0100, Michal Hocko wrote:
> > It is useful for controlling admissions of new userspace visible uses
> > - e.g. a tracepoint shouldn't be allowed to be attached to a cgroup
> > which has already been deleted.
> 
> I am not sure I understand. Roman says that the cgroup can get offline
> right after the function returns. How is "already deleted" different
> from "just deleted"? I thought that the state is preserved at least
> while the rcu lock is held but my memory is dim here.

It's the same difference as between "opening a file and deleting it"
and "deleting a file and opening it".  We shoud allow the former while
not allowing the latter.

> > We're just using it too liberally.
> 
> Can we get a doc update to be explicit about sensible usecases so that
> others can be dropped accordingly? 

Yeah, we should audit and convert the uses and update the doc.

Thanks.

-- 
tejun