linux-kernel - Re: [PATCH cgroup/for-6.19] cgroup: Fix sleeping from invalid context warning on PREEMPT

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Message-ID: <aQzg9kcnCsdRQiB4@slm.duckdns.org>
Date: Thu, 6 Nov 2025 07:55:02 -1000
From: Tejun Heo <tj@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Calvin Owens <calvin@...nvd.org>, linux-kernel@...r.kernel.org,
	Dan Schatzberg <dschatzberg@...a.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH cgroup/for-6.19] cgroup: Fix sleeping from invalid
 context warning on PREEMPT_RT

Hello,

On Thu, Nov 06, 2025 at 06:46:14PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-11-06 07:37:16 [-1000], Tejun Heo wrote:
> > Will switch to IRQ_WORK_LAZY_INIT.
> 
> Quick question: Since it is not important at all, would it work to have
> it in task's RCU callback, __put_task_struct()?

It doesn't have to run right away but it better run in some definite time
frame because at this point the task is not visible from userspace otherwise
(it doesn't have a pid) but are still pinning the cgroup, so we're in this
limbo state where reading cgroup.procs should return empty (there may be a
bug here right now. I think the code will try to deref NULL pid pointer) but
the cgroup is not empty. This window is not really broken in itself because
cgroup empty state is tracked and notified separately. However, task_struct
can be pinned and can linger for indefinite amount of time after being dead,
and that would become an actual problem.

So, to add a bit of qualitifier, while it's okay to run it with some amount
of delay that's not very significant to human perception, we definitely
don't want to allow delaying it indefinitely.

Thanks.

-- 
tejun