linux-kernel - Re: [PATCH sched_ext/for-6.18] sched_ext: Use cgroup_lock/unlock() to synchronize against cgroup operations

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Message-ID: <aLirwysOBpAGubIy@slm.duckdns.org>
Date: Wed, 3 Sep 2025 10:57:39 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	sched-ext@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH sched_ext/for-6.18] sched_ext: Use cgroup_lock/unlock()
 to synchronize against cgroup operations

On Wed, Sep 03, 2025 at 10:48:09PM +0200, Andrea Righi wrote:
> On Wed, Sep 03, 2025 at 06:39:46AM -1000, Tejun Heo wrote:
> > Hello,
> > 
> > On Wed, Sep 03, 2025 at 02:44:58PM +0200, Andrea Righi wrote:
> > > >  static void scx_cgroup_lock(void)
> > > >  {
> > > > -	percpu_down_write(&scx_cgroup_rwsem);
> > > > +	percpu_down_write(&scx_cgroup_ops_rwsem);
> > > > +	cgroup_lock();
> > > >  }
> > > 
> > > Shouldn't we acquire cgroup_lock() before scx_cgroup_ops_rwsem to avoid
> > > a potential AB-BA deadlock?
> > 
> > There's no existing ordering between the two locks, so any order should be
> > safe. The reason why I put it in this particular order is because any
> > cgroup_lock() holder has no reason to grab ops_rwsem now or in the future
> > while the opposite direction is still unlikely but theoretically more
> > possible.
> 
> Isn't scx_group_set_weight() called with cgroup_mutex held? In this case
> the order is cgroup_lock() -> scx_cgroup_ops_rwsem, or am I missing
> something?

Oh, no, cgroup_lock() is only held for operations that change the cgroup
hierarchy - cgroup creation, deletion, controller enable/disable, task
migration and so on. Writes to control knobs doesn't acquire cgroup_lock().

Thanks.

-- 
tejun