Re: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock()

[Roman Gushchin <guro@fb.com>]

On Fri, Aug 02, 2019 at 10:59:47AM +0200, Michal Hocko wrote:
> On Fri 02-08-19 10:04:22, Michal Hocko wrote:
> > On Thu 01-08-19 16:35:13, Roman Gushchin wrote:
> > > Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge")
> > > introduced css_tryget()/css_put() calls in drain_all_stock(),
> > > which are supposed to protect the target memory cgroup from being
> > > released during the mem_cgroup_is_descendant() call.
> > > 
> > > However, it's not completely safe. In theory, memcg can go away
> > > between reading stock->cached pointer and calling css_tryget().
> > 
> > I have to remember how is this whole thing supposed to work, it's been
> > some time since I've looked into that.
> 
> OK, I guess I remember now and I do not see how the race is possible.
> Stock cache is keeping its memcg alive because it elevates the reference
> counting for each cached charge. And that should keep the whole chain up
> to the root (of draining) alive, no? Or do I miss something, could you
> generate a sequence of events that would lead to use-after-free?

Right, but it's true when you reading a local percpu stock.
But here we read a remote stock->cached pointer, which can be cleared
by a remote concurrent drain_local_stock() execution.

In theory, it could be the last reference, and the memcg can be destroyed
remotely, so we end up trying to call css_tryget() over freed memory.

The race is theoretical, but as I wrote in the thread, I think
that it's still worth fixing, because the current code looks confusing
(and this confirms my feelings).

Thanks!