On Thu 09-07-15 17:32:47, Vladimir Davydov wrote: > On Thu, Jul 09, 2015 at 04:13:21PM +0200, Michal Hocko wrote: > > On Wed 08-07-15 20:43:31, Vladimir Davydov wrote: > > > On Wed, Jul 08, 2015 at 02:27:52PM +0200, Michal Hocko wrote: > > [...] > > > > @@ -1091,12 +1079,14 @@ bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg) > > > > task_unlock(p); > > > > } else { > > > > /* > > > > - * All threads may have already detached their mm's, but the oom > > > > - * killer still needs to detect if they have already been oom > > > > - * killed to prevent needlessly killing additional tasks. > > > > + * All threads have already detached their mm's but we should > > > > + * still be able to at least guess the original memcg from the > > > > + * task_css. These two will match most of the time but there are > > > > + * corner cases where task->mm and task_css refer to a different > > > > + * cgroups. > > > > */ > > > > rcu_read_lock(); > > > > - task_memcg = mem_cgroup_from_task(task); > > > > + task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id)); > > > > css_get(&task_memcg->css); > > > > > > I wonder why it's safe to call css_get here. > > > > What do you mean by safe? Memcg cannot go away because we are under rcu > > lock. > > No, it can't, but css->refcnt can reach zero while we are here, can't > it? If it happens, css->refcnt.release will be called twice, which will > have very bad consequences. I think it's OK to call css_tryget{_online} > from an RCU read-side section, but not css_get. Am I missing something? OK, now I see what you mean. This is a good question indeed. This code has been like that for quite a while and I took it for granted. I have to think about it some more. Anyway the patch doesn't change the behavior here. -- 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>