Char/Misc fix for 4.16-rc7

This is a single hyperv bugfix for 4.16-rc7.
 
 It resolves an issue with the ring-buffer signaling to resolve reported
 problems.
 
 It's been in linux-next for a while now with no reported issues.
 
 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Merge tag 'char-misc-4.16-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc

Pull hyperv fix from Greg KH:
 "This is a single hyperv bugfix for 4.16-rc7.

  It resolves an issue with the ring-buffer signaling to resolve
  reported problems.

  It's been in linux-next for a while now with no reported issues"

* tag 'char-misc-4.16-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc:
  Drivers: hv: vmbus: Fix ring buffer signaling

drivers/hv/ring_buffer.c

@@ -417,13 +417,24 @@ __hv_pkt_iter_next(struct vmbus_channel *channel,
 }
 EXPORT_SYMBOL_GPL(__hv_pkt_iter_next);
 
+/* How many bytes were read in this iterator cycle */
+static u32 hv_pkt_iter_bytes_read(const struct hv_ring_buffer_info *rbi,
+					u32 start_read_index)
+{
+	if (rbi->priv_read_index >= start_read_index)
+		return rbi->priv_read_index - start_read_index;
+	else
+		return rbi->ring_datasize - start_read_index +
+			rbi->priv_read_index;
+}
+
 /*
  * Update host ring buffer after iterating over packets.
  */
 void hv_pkt_iter_close(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
-	u32 orig_write_sz = hv_get_bytes_to_write(rbi);
+	u32 curr_write_sz, pending_sz, bytes_read, start_read_index;
 
 	/*
 	 * Make sure all reads are done before we update the read index since
@@ -431,8 +442,12 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 * is updated.
 	 */
 	virt_rmb();
+	start_read_index = rbi->ring_buffer->read_index;
 	rbi->ring_buffer->read_index = rbi->priv_read_index;
 
+	if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
+		return;
+
 	/*
 	 * Issue a full memory barrier before making the signaling decision.
 	 * Here is the reason for having this barrier:
@@ -446,26 +461,29 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 */
 	virt_mb();
 
-	/* If host has disabled notifications then skip */
+	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
-	if (rbi->ring_buffer->interrupt_mask)
+	if (!pending_sz)
 		return;
 
-	if (rbi->ring_buffer->feature_bits.feat_pending_send_sz) {
+	/*
-		u32 pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
+	 * Ensure the read of write_index in hv_get_bytes_to_write()
+	 * happens after the read of pending_send_sz.
+	 */
+	virt_rmb();
+	curr_write_sz = hv_get_bytes_to_write(rbi);
+	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);
 
 	/*
 	 * If there was space before we began iteration,
-		 * then host was not blocked. Also handles case where
+	 * then host was not blocked.
-		 * pending_sz is zero then host has nothing pending
-		 * and does not need to be signaled.
 	 */
-		if (orig_write_sz > pending_sz)
+
+	if (curr_write_sz - bytes_read > pending_sz)
 		return;
 
 	/* If pending write will not fit, don't give false hope. */
-		if (hv_get_bytes_to_write(rbi) < pending_sz)
+	if (curr_write_sz <= pending_sz)
 		return;
-	}
 
 	vmbus_setevent(channel);
 }