From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glauber Costa Subject: Re: [PATCH v4 08/14] res_counter: return amount of charges after res_counter_uncharge Date: Wed, 10 Oct 2012 13:03:39 +0400 Message-ID: <507539EB.90006@parallels.com> 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> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20121009153506.GD7655-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Michal Hocko Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Andrew Morton , Mel Gorman , Suleiman Souhlal , Tejun Heo , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org, Johannes Weiner , Greg Thelen , devel-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, Frederic Weisbecker 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 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; 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.