[PATCH char-misc 1/1] Drivers: hv: vmbus: Fix ring buffer signaling

[Michael Kelley <mhkelley@xxxxxxxxxxx>]

Fix bugs in signaling the Hyper-V host when freeing space in the
host->guest ring buffer:

1. The interrupt_mask must not be used to determine whether to signal
   on the host->guest ring buffer
2. The ring buffer write_index must be read (via hv_get_bytes_to_write)
   *after* pending_send_sz is read in order to avoid a race condition
3. Comparisons with pending_send_sz must treat the "equals" case as
   not-enough-space
4. Don't signal if the pending_send_sz feature is not present. Older
   versions of Hyper-V that don't implement this feature will poll.

Fixes: 03bad714a161 ("vmbus: more host signalling avoidance")
Signed-off-by: Michael Kelley <mhkelley@xxxxxxxxxxx>
---
 drivers/hv/ring_buffer.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 50e0714..b64be18 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -423,7 +423,11 @@ struct vmpacket_descriptor *
 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;
+	u32 delta = rbi->ring_buffer->read_index < rbi->priv_read_index ?
+			(rbi->priv_read_index - rbi->ring_buffer->read_index) :
+			(rbi->ring_datasize - rbi->ring_buffer->read_index +
+			rbi->priv_read_index);
 
 	/*
 	 * Make sure all reads are done before we update the read index since
@@ -446,27 +450,31 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 */
 	virt_mb();
 
-	/* If host has disabled notifications then skip */
-	if (rbi->ring_buffer->interrupt_mask)
-		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);
+
+		/*
 		 * If there was space before we began iteration,
 		 * then host was not blocked. Also handles case where
 		 * 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 - delta > 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);
 	}
 
-	vmbus_setevent(channel);
 }
 EXPORT_SYMBOL_GPL(hv_pkt_iter_close);
-- 
1.8.3.1

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