On Thu, Mar 26, 2015 at 02:01:06PM +0100, Michal Hocko wrote: > On Thu 26-03-15 13:53:48, Michal Hocko wrote: > > On Wed 25-03-15 02:17:08, Johannes Weiner wrote: > > > Disabling the OOM killer needs to exclude allocators from entering, > > > not existing victims from exiting. > > > > The idea was that exit_oom_victim doesn't miss a waiter. > > > > exit_oom_victim is doing > > atomic_dec_return(&oom_victims) && oom_killer_disabled) > > > > so there is a full (implicit) memory barrier befor oom_killer_disabled > > check. The other part is trickier. oom_killer_disable does: > > oom_killer_disabled = true; > > up_write(&oom_sem); > > > > wait_event(oom_victims_wait, !atomic_read(&oom_victims)); > > > > up_write doesn't guarantee a full memory barrier AFAICS in > > Documentation/memory-barriers.txt (although the generic and x86 > > implementations seem to implement it as a full barrier) but wait_event > > implies the full memory barrier (prepare_to_wait_event does spin > > lock&unlock) before checking the condition in the slow path. This should > > be sufficient and docummented... > > > > /* > > * We do not need to hold oom_sem here because oom_killer_disable > > * guarantees that oom_killer_disabled chage is visible before > > * the waiter is put into sleep (prepare_to_wait_event) so > > * we cannot miss a wake up. > > */ > > > > in unmark_oom_victim() > > OK, I can see that the next patch removes oom_killer_disabled > completely. The dependency won't be there and so the concerns about the > memory barriers. > > Is there any reason why the ordering is done this way? It would sound > more logical to me. I honestly didn't even think about the dependency between the lock and this check. They both looked unnecessary to me and I stopped putting any more thought into it once I had convinced myself that they are. The order was chosen because the waitqueue generalization seemed like a bigger deal. One is just an unnecessary lock, but this extra check cost me quite some time debugging and seems like a much more harmful piece of code to fix. It's no problem to reorder the patches, though. -- 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>