Re: [PATCH 3/3] memcg: simplify mem_cgroup_reclaim_iter

[Tejun Heo <tj@kernel.org>]

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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