Hello,
On Thu, Jun 06, 2013 at 01:50:31PM +0200, Michal Hocko wrote:
> > Also, do we need to hold a lock? It doesn't have to be completely
> > strict, so we might as well get away with something like,
> >
> > for_each_cached_pos() {
> > if (hint == me) {
> > /* simple clearing implementation, we prolly wanna push it forward */
> > cached = xchg(hint, NULL);
> > if (cached)
> > css_put(cached);
> > }
> > }
>
> This would be racy:
> mem_cgroup_iter
> rcu_read_lock
> __mem_cgroup_iter_next cgroup_destroy_locked
> css_tryget(memcg)
> atomic_add(CSS_DEACT_BIAS)
> offline_css(memcg)
> xchg(memcg, NULL)
> mem_cgroup_iter_update
> iter->last_visited = memcg
> rcy_read_unlock
>
> But if it was called from call_rcu the we should be safe AFAICS.
Oh yeah, it is racy. That's what I meant by "not having to be
completely strict". The race window is small enough and it's not like
we're messing up refcnt or may end up with use-after-free. Doing it
from RCU would make the race go away but I'm not sure whether the
extra RCU bouncing is worthwhile. I don't know. Maybe.
Thanks.
--
tejun
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