From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v8 0/7] freezer for cgroup v2
Date: Tue, 5 Mar 2019 09:27:32 -0800
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To: Oleg Nesterov <oleg@redhat.com>
Cc: Roman Gushchin <guro@fb.com>, Roman Gushchin <guroan@gmail.com>, Kernel Team <Kernel-team@fb.com>, "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>

Hello, Oleg.

Sorry about the delay.

On Mon, Feb 25, 2019 at 04:57:25PM +0100, Oleg Nesterov wrote:
> > As long as the task is
> > guaranteed to be trapped by signal stop afterwards (and they are), we
> > likely can use them the same way.  The only thing to be careful about
> > would be ensuring that we don't end up flipping group level frozen
> > state inbetween.  Would something like that work?
> 
> I have no idea because I do not understand what exactly do you mean ;)

Heh, sorry about that.  What I meant was that we can consider a task
which is blocked in vfork wait as already frozen and that if we do so
we need to be careful so that frozen state doesn't do a flip between
vfork wait ending and the task getting parked again in a jobctl stop.

> However. Thinking more about this, I am not sure my concerns were valid.
> Yes, cg freezer can "hang" if it races with vfork(). But probably we should
> blame vfork(), not freezer.

I think we'd need to cover that ground regardless of where blame lies.
It's weird if freezing doesn't complete cuz one of the tasks messed up
while vforking.

> The problem is, even ^Z can "hang" if the foreground process does vfork()
> and the new child stops before exit/exec. Now I recall that I even tried
> to make a patch to fix this using ERESTART_RESTARTBLOCK, but had some nasty
> problems with blocked signals...

Ugh... yeah, these wait non-interruptible wait sites which can be
exposed to userspace are nasty.  They end up adding a unique wait
state visible to userspace which comes with a bunch of corner cases.

> de_thread() should use freezable_schedule() in TASK_KILLABLE too. Currently
> it doesn't, but only because we have other (much more serious) problems with
> cred_guard_mutex/exec. However, this is is fine wrt cg freezer, other threads
> can't be frozen exactly because it is killable.
> 
> Anything else does freezer_do_not_count() in TASK_KILLABLE and waits for
> another freezable process?

Can't find any.  Hopefully, that was it?

> So it seems I have to take my words back, perhaps we can forget about
> freezable_schedule/etc.

I think it'd be great to be able to handle these if at all possible.

Thanks.

-- 
tejun