linux-kernel - Re: [PATCH] cgroup: replace global percpu_rwsem with signal_struct->group

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Message-ID: <aLio7Z6YolSZ2lPo@slm.duckdns.org>
Date: Wed, 3 Sep 2025 10:45:33 -1000
From: Tejun Heo <tj@...nel.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: Yi Tao <escape@...ux.alibaba.com>, hannes@...xchg.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 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 <tj@...nel.org> wrote:
> > 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