On 07/16/2015 05:59 PM, Michal Hocko wrote: > On Thu 16-07-15 16:34:08, Nikolay Borisov wrote: >> Hello, >> >> I'd like to ask what are the locking rules when using >> mem_cgroup_from_task(current)? Currently I'm doing this under >> rcu_read_lock which I believe is sufficient. However, I've seen patches >> where reference is obtained via mem_cgroup_from_task and then >> css_tryget_online is used on the resulting cgroup? > > RCU will guarantee that the memcg will not go away. The rest depends on > what you want to do with it. If you want to use it outside of RCU you > have to take a reference. And then it depends what the memcg is used > for - some operations can be done also on the offline memcg. > > Btw. mem_cgroup_from_task is not the proper interface for you. You > really want to do > memcg = get_mem_cgroup_from_mm(current->mm) > [...] > css_put(&memcg) Unfortunately this function is static, do you think there might be any value of a patch that exposes it upstream? > >> Looking at the context of css_tryget_online it seems it will only >> succeed if the cgroup isn't being terminated (cgroup_destroy_locked >> isn't being invoked). Judging from this then if a css_tryget_online >> succeeds this means the caller can be sure they are working with a live >> cgroup, however, what happens if the process of acuiqring a reference on >> css is skipped AND the caller is under RCU read lock? > > The memcg will not get freed. It still might be offline. > >> They are >> guaranteed to succeed, but after the rcu read lock is released the >> cgroup might go away ? > > Yes it might go away. > >> Essentially my use case is to obtain a reference to a memcg for the >> current process and query some counter values IOW - just reading from >> the memcg. Do I need to acquire a CSS reference in this case? > > I would strongly recommend using get_mem_cgroup_from_mm as shown above. > Thanks for the info ! -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
