linux-kernel - Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup

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Date:	Mon, 28 Nov 2011 18:43:37 +0300
From:	Andrew Wagin <avagin@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Matt Helsley <matthltc@...ibm.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC] cgroups: freezer -- Allow to attach a task to a frozen cgroup

>
>> > >
>> > > It's strange. A rollback can't fail. We have three situations:
>> > >
>> > > frozen -> frozen
>> > > thawed -> frozen
>> > > frozen -> thawed
>> > >
>> > > In first and second cases cancel_request can't fail.
>> > > In the third we have a problem, which may be solved if we will call
>> > > thaw_process(task) from attach_task(), we can do that, because
>> > > thaw_process() can't fail. It solves a problem, because
>> > > freezer_cancel_attach will be executed for the first and second cases
>> > > only.
>> > >
>> > > If my suggestion is correct, we can replace pr_warning on BUG_ON
>> > >
>> >
>> > Yes, the case which can fail is
>> >
>> >    frozen->(can_attach_task)->thawed
>> >      (cgroup_task_migrate failure)
>> >    thawed->(cancel_attach)->frozen
>> >
>> > and we should never fail here since otherwise we would not have
>> > a "frozen" state before. But I think placing BUG_ON is too severe
>> > here, maybe WARN_ON_ONCE(1) would fit better?
>>
>> It's true, if a task is not being executed between thaw_process() and
>> freeze_task().
>
> Hmm... But what the problem it might be if a task get executed between
> those stages even for some time?

It may become unfreezable...

>
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