posix-timers: don't switch to ->group_leader if ->it_process dies

posix_timer_event() drops SIGEV_THREAD_ID and switches to ->group_leader
if send_sigqueue() fails.

This is not very useful and doesn't work reliably.  send_sigqueue() can
only fail if ->it_process is dead.  But it can die before it dequeues the
SI_TIMER signal, in that case the timer stops anyway.

Remove this code.  I guess it was needed a long ago to ensure that the
timer is not destroyed when when its creator thread dies.

Q: perhaps it makes sense to change sys_timer_settime() to return an error
if ->it_process is dead?

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: mingo@elte.hu
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/posix-timers.c

@@ -298,6 +298,7 @@ void do_schedule_next_timer(struct siginfo *info)
 
 int posix_timer_event(struct k_itimer *timr, int si_private)
 {
+	int shared, ret;
 	/*
 	 * FIXME: if ->sigq is queued we can race with
 	 * dequeue_signal()->do_schedule_next_timer().
@@ -316,20 +317,10 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
 	timr->sigq->info.si_tid = timr->it_id;
 	timr->sigq->info.si_value = timr->it_sigev_value;
 
-	if (timr->it_sigev_notify & SIGEV_THREAD_ID) {
-		struct task_struct *leader;
-		int ret = send_sigqueue(timr->sigq, timr->it_process, 0);
-
-		if (likely(ret >= 0))
-			return ret;
-
-		timr->it_sigev_notify = SIGEV_SIGNAL;
-		leader = timr->it_process->group_leader;
-		put_task_struct(timr->it_process);
-		timr->it_process = leader;
-	}
-
-	return send_sigqueue(timr->sigq, timr->it_process, 1);
+	shared = !(timr->it_sigev_notify & SIGEV_THREAD_ID);
+	ret = send_sigqueue(timr->sigq, timr->it_process, shared);
+	/* If we failed to send the signal the timer stops. */
+	return ret > 0;
 }
 EXPORT_SYMBOL_GPL(posix_timer_event);