posix-timers: always do get_task_struct(timer->it_process)

Change the code to get/put timer->it_process regardless of
SIGEV_THREAD_ID.  This streamlines the create/destroy paths and allows us
to simplify the usage of exit_itimers() in de_thread().

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

@@ -540,11 +540,10 @@ sys_timer_create(const clockid_t which_clock,
 			 */
 			spin_lock_irqsave(&process->sighand->siglock, flags);
 			if (!(process->flags & PF_EXITING)) {
+				get_task_struct(process);
 				new_timer->it_process = process;
 				list_add(&new_timer->list,
 					 &process->signal->posix_timers);
-				if (new_timer->it_sigev_notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
-					get_task_struct(process);
 				spin_unlock_irqrestore(&process->sighand->siglock, flags);
 			} else {
 				spin_unlock_irqrestore(&process->sighand->siglock, flags);
@@ -561,6 +560,7 @@ sys_timer_create(const clockid_t which_clock,
 		new_timer->it_sigev_signo = SIGALRM;
 		new_timer->it_sigev_value.sival_int = new_timer->it_id;
 		process = current->group_leader;
+		get_task_struct(process);
 		spin_lock_irqsave(&process->sighand->siglock, flags);
 		new_timer->it_process = process;
 		list_add(&new_timer->list, &process->signal->posix_timers);
@@ -853,8 +853,7 @@ retry_delete:
 	 * This keeps any tasks waiting on the spin lock from thinking
 	 * they got something (see the lock code above).
 	 */
-	if (timer->it_sigev_notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
-		put_task_struct(timer->it_process);
+	put_task_struct(timer->it_process);
 	timer->it_process = NULL;
 
 	unlock_timer(timer, flags);
@@ -881,8 +880,7 @@ retry_delete:
 	 * This keeps any tasks waiting on the spin lock from thinking
 	 * they got something (see the lock code above).
 	 */
-	if (timer->it_sigev_notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
-		put_task_struct(timer->it_process);
+	put_task_struct(timer->it_process);
 	timer->it_process = NULL;
 
 	unlock_timer(timer, flags);