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Subject[PATCH 4.14 029/101] Drivers: hv: vmbus: Fix ring buffer signaling
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Kelley <mhkelley@outlook.com>

commit 655296c8bbeffcf020558c4455305d597a73bde1 upstream.

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")

Cc: Stable <stable@vger.kernel.org> # 4.14 and above
Signed-off-by: Michael Kelley <mhkelley@outlook.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/hv/ring_buffer.c | 54 +++++++++++++++++++++++++++++++----------------
1 file changed, 36 insertions(+), 18 deletions(-)

--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -394,13 +394,24 @@ __hv_pkt_iter_next(struct vmbus_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
@@ -408,8 +419,12 @@ void hv_pkt_iter_close(struct vmbus_chan
* 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:
@@ -423,26 +438,29 @@ void hv_pkt_iter_close(struct vmbus_chan
*/
virt_mb();

- /* If host has disabled notifications then skip */
- if (rbi->ring_buffer->interrupt_mask)
+ pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
+ 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.
+ */

- /*
- * 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)
- return;
-
- /* If pending write will not fit, don't give false hope. */
- if (hv_get_bytes_to_write(rbi) < pending_sz)
- return;
- }
+ if (curr_write_sz - bytes_read > pending_sz)
+ return;
+
+ /* If pending write will not fit, don't give false hope. */
+ if (curr_write_sz <= pending_sz)
+ return;

vmbus_setevent(channel);
}

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