Re: [PATCH 1/1] freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()

[Oleg Nesterov <oleg@redhat.com>]

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.