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

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Date:	Thu, 25 Oct 2012 10:36:32 -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 1/1] freezer: change ptrace_stop/do_signal_stop to use
 freezable_schedule()

Hello,

On Thu, Oct 25, 2012 at 07:34:33PM +0200, Oleg Nesterov wrote:
> > 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.

Yeah and maybe explain briefly how schedule_freezable() gets us out of
the trouble.

> > > @@ -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.

Right, we can't remove it.  That's our main freezing point for
userland tasks.

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

But, yeah, I think we should move it above relock: and update the
comment to explain that that's the usual freezing site.

Thanks.

-- 
tejun
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