linux-kernel - Re: [PATCH v8 0/7] freezer for cgroup v2

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Date:   Mon, 25 Feb 2019 16:57:25 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Roman Gushchin <guro@...com>, Roman Gushchin <guroan@...il.com>,
        Kernel Team <Kernel-team@...com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 0/7] freezer for cgroup v2

On 02/22, Tejun Heo wrote:
>
> > So I think it too should somehow interact with freezable_schedule/etc.
>
> You mean freezer_do_not_count(), right?

yes,

> 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 ;)

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.

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...

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?

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

Oleg.