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

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>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/ring_buffer.c

@@ -32,7 +32,7 @@
 void hv_begin_read(struct hv_ring_buffer_info *rbi)
 {
 	rbi->ring_buffer->interrupt_mask = 1;
-	smp_mb();
+	mb();
 }
 
 u32 hv_end_read(struct hv_ring_buffer_info *rbi)
@@ -41,7 +41,7 @@ u32 hv_end_read(struct hv_ring_buffer_info *rbi)
 	u32 write;
 
 	rbi->ring_buffer->interrupt_mask = 0;
-	smp_mb();
+	mb();
 
 	/*
 	 * Now check to see if the ring buffer is still empty.
@@ -71,7 +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();
+	mb();
 	if (rbi->ring_buffer->interrupt_mask)
 		return false;
 
@@ -442,7 +442,7 @@ int hv_ringbuffer_write(struct hv_ring_buffer_info *outring_info,
 					     sizeof(u64));
 
 	/* Issue a full memory barrier before updating the write index */
-	smp_mb();
+	mb();
 
 	/* Now, update the write location */
 	hv_set_next_write_location(outring_info, next_write_location);
@@ -549,7 +549,7 @@ 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

@@ -434,7 +434,7 @@ 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) {
 			/*