Revert "Drivers: hv: vmbus: Eliminate the spin lock on the read path"

BugLink: http://bugs.launchpad.net/bugs/1583357

This reverts commit ebade2fc.

Favor upstream
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/hv/ring_buffer.c

@@ -390,6 +390,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
 	u32 bytes_avail_toread;
 	u32 next_read_location = 0;
 	u64 prev_indices = 0;
+	unsigned long flags;
 	struct vmpacket_descriptor desc;
 	u32 offset;
 	u32 packetlen;
@@ -398,6 +399,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
 	if (buflen <= 0)
 		return -EINVAL;
 
+	spin_lock_irqsave(&inring_info->ring_lock, flags);
 
 	*buffer_actual_len = 0;
 	*requestid = 0;
@@ -412,7 +414,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
 		 * No error is set when there is even no header, drivers are
 		 * supposed to analyze buffer_actual_len.
 		 */
-		return ret;
+		goto out_unlock;
 	}
 
 	next_read_location = hv_get_next_read_location(inring_info);
@@ -425,11 +427,15 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
 	*buffer_actual_len = packetlen;
 	*requestid = desc.trans_id;
 
-	if (bytes_avail_toread < packetlen + offset)
-		return -EAGAIN;
+	if (bytes_avail_toread < packetlen + offset) {
+		ret = -EAGAIN;
+		goto out_unlock;
+	}
 
-	if (packetlen > buflen)
-		return -ENOBUFS;
+	if (packetlen > buflen) {
+		ret = -ENOBUFS;
+		goto out_unlock;
+	}
 
 	next_read_location =
 		hv_get_next_readlocation_withoffset(inring_info, offset);
@@ -456,5 +462,7 @@ int hv_ringbuffer_read(struct hv_ring_buffer_info *inring_info,
 
 	*signal = hv_need_to_signal_on_read(inring_info);
 
+out_unlock:
+	spin_unlock_irqrestore(&inring_info->ring_lock, flags);
 	return ret;
 }