Re: [PATCH 2/2] kernel: rerun task_work while freezing in get_signal()

[Pavel Begunkov]

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