linux-kernel - Re: [PATCH v2 1/1] freezer: change ptrace_stop/do_signal_stop to use freezable

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Date:	Fri, 26 Oct 2012 10:52:58 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 v2 1/1] freezer: change ptrace_stop/do_signal_stop to use
 freezable_schedule()

On Fri, Oct 26, 2012 at 07:46:06PM +0200, Oleg Nesterov wrote:
> try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
> to ensure that a task doing STOPPED/TRACED -> RUNNING transition
> can't escape freezing. This mostly works, but ptrace_stop() does
> not necessarily call schedule(), it can change task->state back to
> RUNNING and check freezing() without any lock/barrier in between.
> 
> We could add the necessary barrier, but this patch changes
> ptrace_stop() and do_signal_stop() to use freezable_schedule().
> This fixes the race, freezer_count() and freezer_should_skip()
> carefully avoid the race.
> 
> And this simplifies the code, try_to_freeze_tasks/update_if_frozen
> no longer need to use task_is_stopped_or_traced() checks with the
> non trivial assumptions. We can rely on the mechanism which was
> specially designed to mark the sleeping task as "frozen enough".
> 
> v2: As Tejun pointed out, we can also change get_signal_to_deliver()
> and move try_to_freeze() up before 'relock' label.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Looks good to me. :)

 Acked-by: Tejun Heo <tj@...nel.org>

Rafael, sorry that this one doesn't have pm cc'd but can you please
pick up this one too?

Thanks a lot.

-- 
tejun
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