Re: [PATCH] cgroup: replace global percpu_rwsem with signal_struct->group_rwsem when writing cgroup.procs/threads

Hello, Michal.

On Wed, Sep 03, 2025 at 10:03:39PM +0200, Michal Koutný wrote:

On Wed, Sep 03, 2025 at 06:53:36AM -1000, Tejun Heo [off-list ref] wrote:

quoted

If you use CLONE_INTO_CGROUP, cgroup migration doesn't just become cold. It
disappears completely and CLONE_INTO_CGROUP doesn't need any global locks
from cgroup side.

CLONE_INTO_CGROUP uses cgroup_mutex and threadgroup rwsem like regular
migration, no? Its effect is atomicity wrt clone.
Or, Tejum, what do you mean that it disappears? (I think we cannot give
up cgroup_mutex as it ensures synchronization of possible parent's
migration.)

Sorry, I was confused. We no longer need to write lock threadgroup rwsem
when CLONE_INTO_CGROUP'ing into an empty cgroup. We do still need
cgroup_mutex.

  671c11f0619e ("cgroup: Elide write-locking threadgroup_rwsem when updating csses on an empty subtree")

Thanks.

-- 
tejun