posix-cpu-timers: Stop disabling timers on mt-exec

The reasons why the extra posix_cpu_timers_exit_group() invocation has been
added are not entirely clear from the commit message.  Today all that
posix_cpu_timers_exit_group() does is stop timers that are tracking the
task from firing.  Every other operation on those timers is still allowed.

The practical implication of this is posix_cpu_timer_del() which could
not get the siglock after the thread group leader has exited (because
sighand == NULL) would be able to run successfully because the timer
was already dequeued.

With that locking issue fixed there is no point in disabling all of the
timers.  So remove this ``tempoary'' hack.

Fixes: e0a70217 ("posix-cpu-timers: workaround to suppress the problems with mt exec")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/87o8tityzs.fsf@x220.int.ebiederm.org

kernel/exit.c

@@ -103,17 +103,8 @@ static void __exit_signal(struct task_struct *tsk)
 
 #ifdef CONFIG_POSIX_TIMERS
 	posix_cpu_timers_exit(tsk);
-	if (group_dead) {
+	if (group_dead)
 		posix_cpu_timers_exit_group(tsk);
-	} else {
-		/*
-		 * This can only happen if the caller is de_thread().
-		 * FIXME: this is the temporary hack, we should teach
-		 * posix-cpu-timers to handle this case correctly.
-		 */
-		if (unlikely(has_group_leader_pid(tsk)))
-			posix_cpu_timers_exit_group(tsk);
-	}
 #endif
 
 	if (group_dead) {