[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>

1 files changed, 4 insertions, 5 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 520f6c59948d..9d9169aa2e24 100644
--- a/kernel/posix-cpu-timers.c
+++ b/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);
         }
 }