net: Fix race condition in store_rps_map

There is a race condition in store_rps_map that allows jump label
count in rps_needed to go below zero. This can happen when
concurrently attempting to set and a clear map.

Scenario:

1. rps_needed count is zero
2. New map is assigned by setting thread, but rps_needed count _not_ yet
incremented (rps_needed count still zero)
2. Map is cleared by second thread, old_map set to that just assigned
3. Second thread performs static_key_slow_dec, rps_needed count now goes
negative

Fix is to increment or decrement rps_needed under the spinlock.

Signed-off-by: Tom Herbert <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/net-sysfs.c

···
 	old_map = rcu_dereference_protected(queue->rps_map,
 					    lockdep_is_held(&rps_map_lock));
 	rcu_assign_pointer(queue->rps_map, map);
-	spin_unlock(&rps_map_lock);
 
 	if (map)
 		static_key_slow_inc(&rps_needed);
-	if (old_map) {
-		kfree_rcu(old_map, rcu);
+	if (old_map)
 		static_key_slow_dec(&rps_needed);
-	}
+
+	spin_unlock(&rps_map_lock);
+
+	if (old_map)
+		kfree_rcu(old_map, rcu);
+
 	free_cpumask_var(mask);
 	return len;
 }