Drivers: hv: vmbus: Fix a bug in hv_need_to_signal()

As part of updating the vmbus protocol, the function hv_need_to_signal()
was introduced. This functions helps optimize signalling from guest to
host. The newly added memory barrier is needed to ensure that we correctly
decide when to signal the host.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Reported-by: Olaf Hering <olh@suse.de>
Cc: Stable <stable@vger.kernel.org>  (V3.8+)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/ring_buffer.c

@@ -71,6 +71,7 @@ u32 hv_end_read(struct hv_ring_buffer_info *rbi)
 
 static bool hv_need_to_signal(u32 old_write, struct hv_ring_buffer_info *rbi)
 {
+	smp_mb();
 	if (rbi->ring_buffer->interrupt_mask)
 		return false;