Re: [PATCH] Additional/catchup RCU signal fixes for -mm

[Oleg Nesterov <oleg@tv-sign.ru>]

"Paul E. McKenney" wrote:
> 
> > If !thread_group_leader() does exec de_thread() calls release_task(->group_leader)
> > before calling exit_itimers(). This means that send_group_sigqueue() which
> > always has p == ->group_leader parameter can oops here.
> 
> But in that case, __exit_sighand(->group_leader) would have been called,
> so ->sighand would be NULL.

Yes, that is why (I think) oops can happen.

> And none of this can change while we are holding
> tasklist_lock.

Yes, but de_thread()->release_task(->group_leader) can take tasklist_lock
before us.

> If we don't want to be hitting the exec()ed task with a signal, the
> thing to do would be to drop the signal, as in the attached patch.
> I believe that this is an acceptable approach, since had the timer
> fired slightly later, it would have been disabled, right?
> 
> Thoughts?
> 
>                                                 Thanx, Paul
> 
> Signed-off-by: <paulmck@us.ibm.com>
> 
> diff -urpNa -X dontdiff linux-2.6.14-mm0-fix-2/kernel/signal.c linux-2.6.14-mm0-fix-3/kernel/signal.c
> --- linux-2.6.14-mm0-fix-2/kernel/signal.c      2005-11-05 15:05:38.000000000 -0800
> +++ linux-2.6.14-mm0-fix-3/kernel/signal.c      2005-11-05 16:27:52.000000000 -0800
> @@ -1481,6 +1481,10 @@ send_group_sigqueue(int sig, struct sigq
>         read_lock(&tasklist_lock);
>         while (p->group_leader != p)
>                 p = p->group_leader;
> +       if (p->sighand == NULL) {
> +               ret = 1;

Oh, I think there is another problem here. I'll post a separate
message.

Oleg.