From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx118.postini.com [74.125.245.118]) by kanga.kvack.org (Postfix) with SMTP id 458176B002B for ; Wed, 10 Oct 2012 07:24:52 -0400 (EDT) Date: Wed, 10 Oct 2012 13:24:47 +0200 From: Michal Hocko Subject: Re: [PATCH v4 08/14] res_counter: return amount of charges after res_counter_uncharge Message-ID: <20121010112446.GE23011@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> <20121009153506.GD7655@dhcp22.suse.cz> <507539EB.90006@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <507539EB.90006@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 Wed 10-10-12 13:03:39, Glauber Costa wrote: > On 10/09/2012 07:35 PM, Michal Hocko wrote: > > 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 > > > > Sorry Michal, I didn't realize you were talking about > res_counter_uncharge_until. I could have been more specific. > I don't really need res_counter_uncharge_until to return anything, so I > can just remove that if you prefer, keeping just the main > res_counter_uncharge. > > However, I still can't make sense of your concern. > > The return value will return the value of the counter passed as a > parameter to the function: > > r = res_counter_uncharge_locked(c, val); > if (c == counter) > ret = r; Dohh. I have no idea where I took ret += r from. Sorry about the noise. > So when you call res_counter_uncharge_until(D, whatever, x), you will > see zero here as a result, and when you call > res_counter_uncharge_until(D, whatever, y) you will see 0 here as well. > > A doesn't get involved with that. You are right. -- 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