Revert "remove false BUG_ON() from run_posix_cpu_timers()"

This reverts commit 3de463c7.

Roland has another patch that allows us to leave the BUG_ON() in place
by just making sure that the condition it tests for really is always
true.

That goes in next.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/exit.c

@@ -825,6 +825,14 @@ fastcall NORET_TYPE void do_exit(long code)
 
 	tsk->flags |= PF_EXITING;
 
+	/*
+	 * Make sure we don't try to process any timer firings
+	 * while we are already exiting.
+	 */
+ 	tsk->it_virt_expires = cputime_zero;
+ 	tsk->it_prof_expires = cputime_zero;
+	tsk->it_sched_expires = 0;
+
 	if (unlikely(in_atomic()))
 		printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
 				current->comm, current->pid,

kernel/posix-cpu-timers.c

@@ -1293,30 +1293,30 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 
 #undef	UNEXPIRED
 
+	BUG_ON(tsk->exit_state);
+
 	/*
 	 * Double-check with locks held.
 	 */
 	read_lock(&tasklist_lock);
-	if (likely(tsk->signal != NULL)) {
-		spin_lock(&tsk->sighand->siglock);
+	spin_lock(&tsk->sighand->siglock);
 
-		/*
-		 * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
-		 * all the timers that are firing, and put them on the firing list.
-		 */
-		check_thread_timers(tsk, &firing);
-		check_process_timers(tsk, &firing);
+	/*
+	 * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
+	 * all the timers that are firing, and put them on the firing list.
+	 */
+	check_thread_timers(tsk, &firing);
+	check_process_timers(tsk, &firing);
 
-		/*
-		 * We must release these locks before taking any timer's lock.
-		 * There is a potential race with timer deletion here, as the
-		 * siglock now protects our private firing list.  We have set
-		 * the firing flag in each timer, so that a deletion attempt
-		 * that gets the timer lock before we do will give it up and
-		 * spin until we've taken care of that timer below.
-		 */
-		spin_unlock(&tsk->sighand->siglock);
-	}
+	/*
+	 * We must release these locks before taking any timer's lock.
+	 * There is a potential race with timer deletion here, as the
+	 * siglock now protects our private firing list.  We have set
+	 * the firing flag in each timer, so that a deletion attempt
+	 * that gets the timer lock before we do will give it up and
+	 * spin until we've taken care of that timer below.
+	 */
+	spin_unlock(&tsk->sighand->siglock);
 	read_unlock(&tasklist_lock);
 
 	/*