ath9k: fix rx descriptor related race condition

commit e96542e5 upstream.

Similar to a race condition that exists in the tx path, the hardware
might re-read the 'next' pointer of a descriptor of the last completed
frame. This only affects non-EDMA (pre-AR93xx) devices.

To deal with this race, defer clearing and re-linking a completed rx
descriptor until the next one has been processed.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/wireless/ath/ath9k/ath9k.h

@@ -77,10 +77,6 @@ struct ath_config {
 		       sizeof(struct ath_buf_state));		\
 	} while (0)
 
-#define ATH_RXBUF_RESET(_bf) do {		\
-		(_bf)->bf_stale = false;	\
-	} while (0)
-
 /**
  * enum buffer_type - Buffer type flags
  *
@@ -308,6 +304,7 @@ struct ath_rx {
 	struct ath_buf *rx_bufptr;
 	struct ath_rx_edma rx_edma[ATH9K_RX_QUEUE_MAX];
 
+	struct ath_buf *buf_hold;
 	struct sk_buff *frag;
 };
 

drivers/net/wireless/ath/ath9k/recv.c

@@ -78,8 +78,6 @@ static void ath_rx_buf_link(struct ath_softc *sc, struct ath_buf *bf)
 	struct ath_desc *ds;
 	struct sk_buff *skb;
 
-	ATH_RXBUF_RESET(bf);
-
 	ds = bf->bf_desc;
 	ds->ds_link = 0; /* link to null */
 	ds->ds_data = bf->bf_buf_addr;
@@ -106,6 +104,14 @@ static void ath_rx_buf_link(struct ath_softc *sc, struct ath_buf *bf)
 	sc->rx.rxlink = &ds->ds_link;
 }
 
+static void ath_rx_buf_relink(struct ath_softc *sc, struct ath_buf *bf)
+{
+	if (sc->rx.buf_hold)
+		ath_rx_buf_link(sc, sc->rx.buf_hold);
+
+	sc->rx.buf_hold = bf;
+}
+
 static void ath_setdefantenna(struct ath_softc *sc, u32 antenna)
 {
 	/* XXX block beacon interrupts */
@@ -153,7 +159,6 @@ static bool ath_rx_edma_buf_link(struct ath_softc *sc,
 
 	skb = bf->bf_mpdu;
 
-	ATH_RXBUF_RESET(bf);
 	memset(skb->data, 0, ah->caps.rx_status_len);
 	dma_sync_single_for_device(sc->dev, bf->bf_buf_addr,
 				ah->caps.rx_status_len, DMA_TO_DEVICE);
@@ -492,6 +497,7 @@ int ath_startrecv(struct ath_softc *sc)
 	if (list_empty(&sc->rx.rxbuf))
 		goto start_recv;
 
+	sc->rx.buf_hold = NULL;
 	sc->rx.rxlink = NULL;
 	list_for_each_entry_safe(bf, tbf, &sc->rx.rxbuf, list) {
 		ath_rx_buf_link(sc, bf);
@@ -742,6 +748,9 @@ static struct ath_buf *ath_get_next_rx_buf(struct ath_softc *sc,
 	}
 
 	bf = list_first_entry(&sc->rx.rxbuf, struct ath_buf, list);
+	if (bf == sc->rx.buf_hold)
+		return NULL;
+
 	ds = bf->bf_desc;
 
 	/*
@@ -1974,7 +1983,7 @@ int ath_rx_tasklet(struct ath_softc *sc, int flush, bool hp)
 		if (edma) {
 			ath_rx_edma_buf_link(sc, qtype);
 		} else {
-			ath_rx_buf_link(sc, bf);
+			ath_rx_buf_relink(sc, bf);
 			ath9k_hw_rxena(ah);
 		}
 	} while (1);