linux-kernel - Re: [PATCH v4 08/14] res_counter: return amount of charges after res_counter

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Message-ID: <20121009153506.GD7655@dhcp22.suse.cz>
Date:	Tue, 9 Oct 2012 17:35:06 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com,
	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>, devel@...nvz.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v4 08/14] res_counter: return amount of charges after
 res_counter_uncharge

On Tue 09-10-12 19:14:57, Glauber Costa wrote:
> On 10/09/2012 07:08 PM, Michal Hocko wrote:
> > As I have already mentioned in my previous feedback this is cetainly not
> > atomic as you the lock protects only one group in the hierarchy. How is
> > the return value from this function supposed to be used?
> 
> So, I tried to make that clearer in the updated changelog.
> 
> Only the value of the base memcg (the one passed to the function) is
> returned, and it is atomic, in the sense that it has the same semantics
> as the atomic variables: If 2 threads uncharge 4k each from a 8 k
> counter, a subsequent read can return 0 for both. The return value here
> will guarantee that only one sees the drop to 0.
> 
> This is used in the patch "kmem_accounting lifecycle management" to be
> sure that only one process will call mem_cgroup_put() in the memcg
> structure.

Yes, you are using res_counter_uncharge and its semantic makes sense.
I was refering to res_counter_uncharge_until (you removed that context
from my reply) because that one can race resulting that nobody sees 0
even though that parents get down to 0 as a result:
	 A
	 |
	 B
	/ \
      C(x)  D(y)

D and C uncharge everything.

CPU0				CPU1
ret += uncharge(D) [0]		ret += uncharge(C) [0]
ret += uncharge(B) [x-from C]
				ret += uncharge(B) [0]
				ret += uncharge(A) [y-from D]
ret += uncharge(A) [0]

ret == x			ret == y
-- 
Michal Hocko
SUSE Labs
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