Reorder atomic operations dealing with sc_tx_avail

The atomic decrement of the number of available descriptors must
be performed before sending the RNDIS message to the hypervisor
so that packet transmit completion that may execute in parallel
won't increase the value past the maximum.

1 files changed, 9 insertions, 9 deletions

diff --git a/sys/dev/pv/if_hvn.c b/sys/dev/pv/if_hvn.c
index 42e70180b80..14cb3421037 100644
--- a/sys/dev/pv/if_hvn.c
+++ b/sys/dev/pv/if_hvn.c
@@ -120,7 +120,7 @@ TAILQ_HEAD(rndis_queue, rndis_cmd);
 
 struct hvn_tx_desc {
 	uint32_t			 txd_id;
-	int				 txd_ready;
+	volatile int			 txd_ready;
 	struct vmbus_gpa		 txd_sgl[HVN_TX_FRAGS + 1];
 	int				 txd_nsge;
 	struct mbuf			*txd_buf;
@@ -485,9 +485,6 @@ hvn_start(struct ifnet *ifp)
 		}
 
 		sc->sc_tx_next++;
-		sc->sc_tx_next %= HVN_TX_DESC;
-
-		atomic_dec_int(&sc->sc_tx_avail);
 
 		ifp->if_opackets++;
 	}
@@ -524,12 +521,11 @@ hvn_encap(struct hvn_softc *sc, struct mbuf *m, struct hvn_tx_desc **txd0)
 	size_t pktlen;
 	int i, rv;
 
-	txd = &sc->sc_tx_desc[sc->sc_tx_next];
-	while (!txd->txd_ready) {
+	do {
+		txd = &sc->sc_tx_desc[sc->sc_tx_next % HVN_TX_DESC];
 		sc->sc_tx_next++;
-		sc->sc_tx_next %= HVN_TX_DESC;
-		txd = &sc->sc_tx_desc[sc->sc_tx_next];
-	}
+	} while (!txd->txd_ready);
+	txd->txd_ready = 0;
 
 	pkt = txd->txd_req;
 	memset(pkt, 0, sizeof(*pkt));
@@ -602,6 +598,9 @@ hvn_encap(struct hvn_softc *sc, struct mbuf *m, struct hvn_tx_desc **txd0)
 	}
 
 	*txd0 = txd;
+
+	atomic_dec_int(&sc->sc_tx_avail);
+
 	return (0);
 }
 
@@ -614,6 +613,7 @@ hvn_decap(struct hvn_softc *sc, struct hvn_tx_desc *txd)
 	txd->txd_buf = NULL;
 	txd->txd_nsge = 0;
 	txd->txd_ready = 1;
+	atomic_inc_int(&sc->sc_tx_avail);
 }
 
 int