[PATCH] CPU Scheduler: fix potential error in runqueue nr_uninterruptible count

Problem:

In the function try_to_wake_up(), when the runqueue's nr_uninterruptible
field is decremented it's possible (on SMP systems) that the pointer no
longer points to the runqueue that the task being woken was on when it went
to sleep.  This would cause the wrong runqueue's field to be decremented
and the correct one tp remain unchanged.

Fix:

Save a pointer to the old runqueue at the beginning of the function and use
it when decrementing nr_uninterruptible.
Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -981,14 +981,14 @@ static int try_to_wake_up(task_t * p, unsigned int state, int sync)
 	int cpu, this_cpu, success = 0;
 	unsigned long flags;
 	long old_state;
-	runqueue_t *rq;
+	runqueue_t *rq, *old_rq;
 #ifdef CONFIG_SMP
 	unsigned long load, this_load;
 	struct sched_domain *sd;
 	int new_cpu;
 #endif
 
-	rq = task_rq_lock(p, &flags);
+	old_rq = rq = task_rq_lock(p, &flags);
 	schedstat_inc(rq, ttwu_cnt);
 	old_state = p->state;
 	if (!(old_state & state))
@@ -1083,7 +1083,7 @@ static int try_to_wake_up(task_t * p, unsigned int state, int sync)
 out_activate:
 #endif /* CONFIG_SMP */
 	if (old_state == TASK_UNINTERRUPTIBLE) {
-		rq->nr_uninterruptible--;
+		old_rq->nr_uninterruptible--;
 		/*
 		 * Tasks on involuntary sleep don't earn
 		 * sleep_avg beyond just interactive state.