linux-kernel - Re: [PATCH 2/2] kernel: rerun task_work while freezing in get

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Message-ID: <20240709190743.GB3892@redhat.com>
Date: Tue, 9 Jul 2024 21:07:43 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	Tycho Andersen <tandersen@...flix.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Julian Orth <ju.orth@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] kernel: rerun task_work while freezing in
 get_signal()

Hi Tejun,

Thanks for looking at this, can you review this V2 patch from Pavel?
To me it makes sense even without 1/2 which I didn't even bother to
read. At least as a simple workaround for now.

On 07/09, Tejun Heo wrote:
>
> Hello,
>
> On Tue, Jul 09, 2024 at 03:05:21PM +0100, Pavel Begunkov wrote:
> > > -----------------------------------------------------------------------
> > > Either way I have no idea whether a cgroup_task_frozen() task should
> > > react to task_work_add(TWA_SIGNAL) or not.
> > >
> > > Documentation/admin-guide/cgroup-v2.rst says
> > >
> > > 	Writing "1" to the file causes freezing of the cgroup and all
> > > 	descendant cgroups. This means that all belonging processes will
> > > 	be stopped and will not run until the cgroup will be explicitly
> > > 	unfrozen.
> > >
> > > AFAICS this is not accurate, they can run but can't return to user-mode.
> > > So I guess task_work_run() is fine.
> >
> > IIUC it's a user facing doc, so maybe it's accurate enough from that
> > perspective. But I do agree that the semantics around task_work is
> > not exactly clear.
>
> A good correctness test for cgroup freezer is whether it'd be safe to
> snapshot and restore the tasks in the cgroup while frozen.

Well, I don't really understand what can snapshot/restore actually mean...

I forgot everything about cgroup freezer and I am already sleeping, but even
if we forget about task_work_add/TIF_NOTIFY_SIGNAL/etc, afaics ptrace can
change the state of cgroup_task_frozen() task between snapshot and restore ?

Oleg.