[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>