[PATCH] vmbus: fix the missed signaling in hv_signal_on_read()

[Dexuan Cui]

There is an off-by-one bug here, which can cause host-to-guest write to stall.

When cur_write_sz == pending_sz, we shouldn't signal the host because it's
meaningless: the ring mustn't be 100% full.

But when cached_write_sz == pending_sz, we must signal the host.

Fixes: 433e19cf33d3 ("Drivers: hv: vmbus: finally fix
hv_need_to_signal_on_read()")

Signed-off-by: John Starks <John.Starks@xxxxxxxxxxxxx>
Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 include/linux/hyperv.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index ba93b7e..246bc6b 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1501,11 +1501,11 @@ static inline  void hv_signal_on_read(struct vmbus_channel *channel)
 
 	cur_write_sz = hv_get_bytes_to_write(rbi);
 
-	if (cur_write_sz < pending_sz)
+	if (cur_write_sz <= pending_sz)
 		return;
 
 	cached_write_sz = hv_get_cached_bytes_to_write(rbi);
-	if (cached_write_sz < pending_sz)
+	if (cached_write_sz <= pending_sz)
 		vmbus_setevent(channel);
 
 	return;
-- 
2.7.4