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

[Tejun Heo]

On Fri, Nov 15, 2019 at 06:40:31PM +0100, Michal Hocko wrote:
> I am sorry but I do not follow. How can css_tryget_online provide the
> same semantic when the css can go offline right after the tryget call
> returns so it is effectivelly undistinguishable from the case when the
> css was already online before the call was made. Or is my understanding
> of what Roman's said earlier in the thread?

It's *exactly* the same semantics as opening a file and deleting it
and keeping using it vs. deleting a file and then trying to open it.
You can't give out a new reference when the entity's visibility is
expected to be gone already and that's the exactly the condition that
tryget_online avoids.  I don't know how to better explain it if the
file analogy doesn't work.

Thanks.

-- 
tejun