drivers: hv: switch to use mb() instead of smp_mb()

commit 35848f68 upstream.

Even if guest were compiled without SMP support, it could not assume that host
wasn't. So switch to use mb() instead of smp_mb() to force memory barriers for
UP guest.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2:
 - Drop changes to functions that don't exist here
 - hv_ringbuffer_write() has only a write memory barrier]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[hq: Backported to 3.4:
 - Add the change in hv_ringbuffer_read]
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/ring_buffer.c

@@ -383,7 +383,7 @@ int hv_ringbuffer_write(struct hv_ring_buffer_info *outring_info,
 					     sizeof(u64));
 
 	/* Make sure we flush all writes before updating the writeIndex */
-	smp_wmb();
+	wmb();
 
 	/* Now, update the write location */
 	hv_set_next_write_location(outring_info, next_write_location);
@@ -485,7 +485,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info, void *buffer,
 	/* Make sure all reads are done before we update the read index since */
 	/* the writer may start writing to the read area once the read index */
 	/*is updated */
-	smp_mb();
+	mb();
 
 	/* Update the read index */
 	hv_set_next_read_location(inring_info, next_read_location);

drivers/hv/vmbus_drv.c

@@ -466,7 +466,7 @@ static void vmbus_on_msg_dpc(unsigned long data)
 		 * will not deliver any more messages since there is
 		 * no empty slot
 		 */
-		smp_mb();
+		mb();
 
 		if (msg->header.message_flags.msg_pending) {
 			/*