linux-kernel - Re: [PATCH 2/7] freezer: add missing mb's to freezer_count() and freezer_should

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Message-ID: <20121023153919.GA16201@redhat.com>
Date:	Tue, 23 Oct 2012 17:39:19 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	rjw@...k.pl, linux-kernel@...r.kernel.org, lizefan@...wei.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/7] freezer: add missing mb's to freezer_count() and
	freezer_should_skip()

Hi Tejun,

On 10/22, Tejun Heo wrote:
>
> On Mon, Oct 22, 2012 at 07:44:04PM +0200, Oleg Nesterov wrote:
>
> > We probably have another similar race. If ptrace_stop()->may_ptrace_stop()
> > returns false, the task does
> >
> > 	__set_current_state(TASK_RUNNING);
> > 	// no mb in between
> > 	try_to_freeze();
> >
> > And this can race with task_is_stopped_or_traced() check in the same way.
> > (of course this is only theoretical).
> >
> > do_signal_stop() is probably fine, we can rely on ->siglock.
>
> Hmm....  Guess we should drop __ from set_current_state.

Yes.

Or we can change ptrace_stop() and do_signal_stop() to use freezer_do_not_count/
freezer_count and remove task_is_stopped_or_traced() from update_if_frozen()
and try_to_freeze_tasks(). But this means that do_signal_stop() will call
try_to_freeze() twice, unless we add __freezer_count() which only clears
PF_FREEZER_SKIP.

> I wonder
> whether we should just add mb to freezing()?  What do you think?

Yes, I thought about this too. I just do not know what would be better.

Oleg.

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