[PATCH 1/3] check_process_timers: fix possible lockup

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

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 patch 2/3) and the 'if (PF_EXITING)'
check in arm_timer() is racy (see the patch 3/3).

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.17-rc6/kernel/posix-cpu-timers.c~1_CPT	2006-06-15 17:59:15.000000000 +0400
+++ 2.6.17-rc6/kernel/posix-cpu-timers.c	2006-06-15 18:01:57.000000000 +0400
@@ -1173,6 +1173,9 @@ static void check_process_timers(struct 
 		}
 		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 
 					      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);
 	}
 }