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

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Message-ID: <7e05b179-90ec-449b-86a8-796f4a12180a@huaweicloud.com>
Date: Thu, 4 Sep 2025 09:40:12 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Tejun Heo <tj@...nel.org>, 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



On 2025/9/4 4:45, Tejun Heo wrote:
> 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.
> 

I'm still a bit confused. Commit 671c11f0619e ("cgroup: Elide write-locking threadgroup_rwsem when
updating csses on an empty subtree") only applies to CSS updates. However, cloning with
CLONE_INTO_CGROUP still requires acquiring the threadgroup_rwsem.

cgroup_can_fork
  cgroup_css_set_fork
    	if (kargs->flags & CLONE_INTO_CGROUP)
		cgroup_lock();
	cgroup_threadgroup_change_begin(current);

-- 
Best regards,
Ridong