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

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Message-ID: <d2667002-1631-4f42-8aad-a9ea56c0762b@gmail.com>
Date: Tue, 9 Jul 2024 20:26:56 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>
Cc: 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()

On 7/9/24 20:07, Oleg Nesterov wrote:
> 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.

They are kind of separate but without 1/2 this patch creates
another infinite loop, even though it's harder to hit and
is io_uring specific.

  
> 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...

CRIU, I assume. I'll try it ...

> 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 ?

... but I'm inclined to think the patch makes sense regardless,
we're replacing an infinite loop with wait-wake-execute-wait.

-- 
Pavel Begunkov