Re: [RFC][PATCH] cgroup_threadgroup_rwsem - affects scalability and OOM

On Wed, Aug 10, 2016 at 12:26 AM, Tejun Heo [off-list ref] wrote:

Hello, Balbir.

On Tue, Aug 09, 2016 at 05:02:47PM +1000, Balbir Singh wrote:

quoted

quoted

Hmm? Where does mem_cgroup_iter grab cgroup_mutex?  cgroup_mutex nests
outside cgroup_threadgroup_rwsem or most other mutexes for that matter
and isn't exposed from cgroup core.

I based my theory on the code

mem_cgroup_iter -> css_next_descendant_pre which asserts

cgroup_assert_mutex_or_rcu_locked(),

although you are right, we hold RCU lock while calling css_* routines.

That's "or".  The iterator can be called either with RCU lock or
cgroup_mutex.  cgroup core may use it under cgroup_mutex.  Everyone
else uses it with rcu.

Thanks.

Hi, Tejun

Thanks agreed! Could you please consider queuing the fix after review.

Balbir Singh.