On Tue, Aug 09, 2016 at 05:00:59PM +0800, Zefan Li wrote: > > This almost stalls the system, this patch moves the threadgroup_change_begin > > from before cgroup_fork() to just before cgroup_canfork(). Ideally we shouldn't > > have to worry about threadgroup changes till the task is actually added to > > the threadgroup. This avoids having to call reclaim with cgroup_threadgroup_rwsem > > held. > > > > There are other theoretical issues with this semaphore > > > > systemd can do > > > > 1. cgroup_mutex (cgroup_kn_lock_live) > > 2. cgroup_threadgroup_rwsem (W) (__cgroup_procs_write) > > > > and other threads can go > > > > 1. cgroup_threadgroup_rwsem (R) (copy_process) > > 2. mem_cgroup_iter (as a part of reclaim) (cgroup_mutex -- rcu lock or cgroup_mutex) > > > > However, I've not examined them in too much detail or looked at lockdep > > wait chains for those paths. > > > > I am sure there is a good reason for placing cgroup_threadgroup_rwsem > > where it is today and I might be missing something. I am also surprised > > no-one else has run into it so far. > > > > Comments? > > > > We used to use cgroup_threadgroup_rwsem for syncronization between threads > in the same threadgroup, but now it has evolved to ensure atomic operations > across multi processes. > Yes and it seems incorrect > For example, I'm trying to fix a race. See https://lkml.org/lkml/2016/8/8/900 > > And the fix kind of relies on the fact that cgroup_post_fork() is placed > inside the read section of cgroup_threadgroup_rwsem, so that cpuset_fork() > won't race with cgroup migration. > My patch retains that behaviour, before ss->fork() is called we hold the cgroup_threadgroup_rwsem, in fact it is held prior to ss->can_fork() Balbir -- 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>