On 10/25, Tejun Heo wrote:

Hello, Oleg.

On Thu, Oct 25, 2012 at 06:39:59PM +0200, Oleg Nesterov wrote:

quoted

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.

quoted

@@ -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.