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

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Message-ID: <20130212161051.GQ2666@linux.vnet.ibm.com>
Date:	Tue, 12 Feb 2013 08:10:51 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ying Han <yinghan@...gle.com>, Tejun Heo <htejun@...il.com>,
	Glauber Costa <glommer@...allels.com>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

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?).

							Thanx, Paul

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