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@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>