[PATCH] check_process_timers: fix possible lockup

If the local timer interrupt happens just after do_exit() sets PF_EXITING
(and before it clears ->it_xxx_expires) run_posix_cpu_timers() will call
check_process_timers() with tasklist_lock + ->siglock held and

	check_process_timers:

		t = tsk;
		do {
			....

			do {
				t = next_thread(t);
			} while (unlikely(t->flags & PF_EXITING));
		} while (t != tsk);

the outer loop will never stop.

Actually, the window is bigger.  Another process can attach the timer
after ->it_xxx_expires was cleared (see the next commit) and the 'if
(PF_EXITING)' check in arm_timer() is racy (see the one after that).
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

kernel/posix-cpu-timers.c

@@ -1173,6 +1173,9 @@ static void check_process_timers(struct task_struct *tsk,
 		}
 		t = tsk;
 		do {
+			if (unlikely(t->flags & PF_EXITING))
+				continue;
+
 			ticks = cputime_add(cputime_add(t->utime, t->stime),
 					    prof_left);
 			if (!cputime_eq(prof_expires, cputime_zero) &&
@@ -1193,11 +1196,7 @@ static void check_process_timers(struct task_struct *tsk,
 					      t->it_sched_expires > sched)) {
 				t->it_sched_expires = sched;
 			}
-
-			do {
-				t = next_thread(t);
-			} while (unlikely(t->flags & PF_EXITING));
-		} while (t != tsk);
+		} while ((t = next_thread(t)) != tsk);
 	}
 }