Messages in this thread | | | 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 |
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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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