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

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Message-ID: <20121025173433.GA7650@redhat.com>
Date:	Thu, 25 Oct 2012 19:34:33 +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 1/1] freezer: change ptrace_stop/do_signal_stop to use
	freezable_schedule()

On 10/25, Tejun Heo wrote:
>
> Hello, Oleg.
>
> On Thu, Oct 25, 2012 at 06:39:59PM +0200, Oleg Nesterov wrote:
> > Change ptrace_stop() and do_signal_stop() to use freezable_schedule()
> > rather than rely on subsequent try_to_freeze().
> >
> > This allows to remove the task_is_stopped_or_traced() checks from
> > try_to_freeze_tasks() and update_if_frozen(), and this fixes the
> > unlikely race with ptrace_stop(). If the tracee does not schedule()
> > it can miss a freezing condition.
>
> I think it would be great if the description is more detailed.  This
> code path always makes my head spin and I think we can definitely use
> some more guiding in understanding this dang thing. :)

Do you mean describe the race in more details? OK, will do and resend
tomorrow.

> > @@ -2092,7 +2085,7 @@ static bool do_signal_stop(int signr)
> >  		}
> >
> >  		/* Now we don't run again until woken by SIGCONT or SIGKILL */
> > -		schedule();
> > +		freezable_schedule();
>
> This makes me wonder whether we still need try_to_freeze() in
> get_signal_to_deliver() right after the relock: label.  Freezer no
> longer treats STOPPED/TRACED special and both sleeping sites in signal
> deliver path are marked freezable_schedule().  We shouldn't need the
> explicit try_to_freeze(), right?

OOPS.

I'd say this doesn't really matter but yes we can move it up,
get_signal_to_deliver() will be called again.

But! the comment above try_to_freeze() becomes misleading with
this patch, so this really needs v2.

Thanks.

Oleg.

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