Re: Access rules for current->memcg

[Nikolay Borisov <n.borisov-/eCPMmvKun9pLGFMi4vTTA@public.gmane.org>]

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 !