From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx189.postini.com [74.125.245.189]) by kanga.kvack.org (Postfix) with SMTP id E8AE96B0044 for ; Tue, 9 Oct 2012 11:35:09 -0400 (EDT) Date: Tue, 9 Oct 2012 17:35:06 +0200 From: Michal Hocko Subject: Re: [PATCH v4 08/14] res_counter: return amount of charges after res_counter_uncharge Message-ID: <20121009153506.GD7655@dhcp22.suse.cz> References: <1349690780-15988-1-git-send-email-glommer@parallels.com> <1349690780-15988-9-git-send-email-glommer@parallels.com> <20121009150845.GC7655@dhcp22.suse.cz> <50743F71.7090409@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50743F71.7090409@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: To: Glauber Costa Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mel Gorman , Suleiman Souhlal , Tejun Heo , cgroups@vger.kernel.org, kamezawa.hiroyu@jp.fujitsu.com, Johannes Weiner , Greg Thelen , devel@openvz.org, Frederic Weisbecker 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org