Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

[Paul E. McKenney]

On Tue, Feb 12, 2013 at 12:25:26PM -0500, Johannes Weiner wrote:
> On Tue, Feb 12, 2013 at 08:10:51AM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 12, 2013 at 04:43:30PM +0100, Michal Hocko wrote:
> > > On Tue 12-02-13 10:10:02, Johannes Weiner wrote:
> > > > On Tue, Feb 12, 2013 at 10:54:19AM +0100, Michal Hocko wrote:
> > > > > On Mon 11-02-13 17:39:43, Johannes Weiner wrote:
> > > > > > On Mon, Feb 11, 2013 at 10:27:56PM +0100, Michal Hocko wrote:
> > > > > > > On Mon 11-02-13 14:58:24, Johannes Weiner wrote:
> > > > > > > > That way, if the dead count gives the go-ahead, you KNOW that the
> > > > > > > > position cache is valid, because it has been updated first.
> > > > > > > 
> > > > > > > OK, you are right. We can live without css_tryget because dead_count is
> > > > > > > either OK which means that css would be alive at least this rcu period
> > > > > > > (and RCU walk would be safe as well) or it is incremented which means
> > > > > > > that we have started css_offline already and then css is dead already.
> > > > > > > So css_tryget can be dropped.
> > > > > > 
> > > > > > Not quite :)
> > > > > > 
> > > > > > The dead_count check is for completed destructions,
> > > > > 
> > > > > Not quite :P. dead_count is incremented in css_offline callback which is
> > > > > called before the cgroup core releases its last reference and unlinks
> > > > > the group from the siblinks. css_tryget would already fail at this stage
> > > > > because CSS_DEACT_BIAS is in place at that time but this doesn't break
> > > > > RCU walk. So I think we are safe even without css_get.
> > > > 
> > > > But you drop the RCU lock before you return.
> > > >
> > > > dead_count IS incremented for every destruction, but it's not reliable
> > > > for concurrent ones, is what I meant.  Again, if there is a dead_count
> > > > mismatch, your pointer might be dangling, easy case.  However, even if
> > > > there is no mismatch, you could still race with a destruction that has
> > > > marked the object dead, and then frees it once you drop the RCU lock,
> > > > so you need try_get() to check if the object is dead, or you could
> > > > return a pointer to freed or soon to be freed memory.
> > > 
> > > Wait a moment. But what prevents from the following race?
> > > 
> > > rcu_read_lock()
> > > 						mem_cgroup_css_offline(memcg)
> > > 						root->dead_count++
> > > iter->last_dead_count = root->dead_count
> > > iter->last_visited = memcg
> > > 						// final
> > > 						css_put(memcg);
> > > // last_visited is still valid
> > > rcu_read_unlock()
> > > [...]
> > > // next iteration
> > > rcu_read_lock()
> > > iter->last_dead_count == root->dead_count
> > > // KABOOM
> > > 
> > > The race window between dead_count++ and css_put is quite big but that
> > > is not important because that css_put can happen anytime before we start
> > > the next iteration and take rcu_read_lock.
> > 
> > The usual approach is to make sure that there is a grace period (either
> > synchronize_rcu() or call_rcu()) between the time that the data is
> > made inaccessible to readers (this would be mem_cgroup_css_offline()?)
> > and the time it is freed (css_put(), correct?).
> 
> Absolutely!  And there is a synchronize_rcu() in between those two
> operations.
> 
> However, we want to keep a weak reference to the cgroup after we drop
> the rcu read-side lock, so rcu alone is not enough for us to guarantee
> object life time.  We still have to carefully detect any concurrent
> offlinings in order to validate the weak reference next time around.

That would make things more interesting.  ;-)

Exactly who or what holds the weak reference?  And the idea is that if
you attempt to use the weak reference beforehand, the css_put() does not
actually free it, but if you attempt to use it afterwards, you get some
sort of failure indication?

							Thanx, Paul

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