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

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Date:	Tue, 9 Oct 2012 19:14:57 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
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 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.

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