ath5k: fix bf->skb==NULL panic in ath5k_tasklet_rx

Under memory pressure, we may not be able to allocate a new skb for
new packets.  If the allocation fails, ath5k_tasklet_rx will exit but
will leave a buffer in the list with a NULL skb, eventually triggering
a BUG_ON.

Extract the skb allocation from ath5k_rxbuf_setup() and change the
tasklet to allocate the next skb before accepting a packet.

Changes-licensed-under: 3-Clause-BSD
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath5k/base.c

@@ -1096,6 +1096,42 @@ ath5k_hw_to_driver_rix(struct ath5k_softc *sc, int hw_rix)
 * Buffers setup *
 \***************/
 
+static
+struct sk_buff *ath5k_rx_skb_alloc(struct ath5k_softc *sc, dma_addr_t *skb_addr)
+{
+	struct sk_buff *skb;
+	unsigned int off;
+
+	/*
+	 * Allocate buffer with headroom_needed space for the
+	 * fake physical layer header at the start.
+	 */
+	skb = dev_alloc_skb(sc->rxbufsize + sc->cachelsz - 1);
+
+	if (!skb) {
+		ATH5K_ERR(sc, "can't alloc skbuff of size %u\n",
+				sc->rxbufsize + sc->cachelsz - 1);
+		return NULL;
+	}
+	/*
+	 * Cache-line-align.  This is important (for the
+	 * 5210 at least) as not doing so causes bogus data
+	 * in rx'd frames.
+	 */
+	off = ((unsigned long)skb->data) % sc->cachelsz;
+	if (off != 0)
+		skb_reserve(skb, sc->cachelsz - off);
+
+	*skb_addr = pci_map_single(sc->pdev,
+		skb->data, sc->rxbufsize, PCI_DMA_FROMDEVICE);
+	if (unlikely(pci_dma_mapping_error(sc->pdev, *skb_addr))) {
+		ATH5K_ERR(sc, "%s: DMA mapping failed\n", __func__);
+		dev_kfree_skb(skb);
+		return NULL;
+	}
+	return skb;
+}
+
 static int
 ath5k_rxbuf_setup(struct ath5k_softc *sc, struct ath5k_buf *bf)
 {
@@ -1103,37 +1139,11 @@ ath5k_rxbuf_setup(struct ath5k_softc *sc, struct ath5k_buf *bf)
 	struct sk_buff *skb = bf->skb;
 	struct ath5k_desc *ds;
 
-	if (likely(skb == NULL)) {
-		unsigned int off;
-
-		/*
-		 * Allocate buffer with headroom_needed space for the
-		 * fake physical layer header at the start.
-		 */
-		skb = dev_alloc_skb(sc->rxbufsize + sc->cachelsz - 1);
-		if (unlikely(skb == NULL)) {
-			ATH5K_ERR(sc, "can't alloc skbuff of size %u\n",
-					sc->rxbufsize + sc->cachelsz - 1);
+	if (!skb) {
+		skb = ath5k_rx_skb_alloc(sc, &bf->skbaddr);
+		if (!skb)
 			return -ENOMEM;
-		}
-		/*
-		 * Cache-line-align.  This is important (for the
-		 * 5210 at least) as not doing so causes bogus data
-		 * in rx'd frames.
-		 */
-		off = ((unsigned long)skb->data) % sc->cachelsz;
-		if (off != 0)
-			skb_reserve(skb, sc->cachelsz - off);
-
 		bf->skb = skb;
-		bf->skbaddr = pci_map_single(sc->pdev,
-			skb->data, sc->rxbufsize, PCI_DMA_FROMDEVICE);
-		if (unlikely(pci_dma_mapping_error(sc->pdev, bf->skbaddr))) {
-			ATH5K_ERR(sc, "%s: DMA mapping failed\n", __func__);
-			dev_kfree_skb(skb);
-			bf->skb = NULL;
-			return -ENOMEM;
-		}
 	}
 
 	/*
@@ -1662,7 +1672,8 @@ ath5k_tasklet_rx(unsigned long data)
 {
 	struct ieee80211_rx_status rxs = {};
 	struct ath5k_rx_status rs = {};
-	struct sk_buff *skb;
+	struct sk_buff *skb, *next_skb;
+	dma_addr_t next_skb_addr;
 	struct ath5k_softc *sc = (void *)data;
 	struct ath5k_buf *bf, *bf_last;
 	struct ath5k_desc *ds;
@@ -1747,10 +1758,17 @@ ath5k_tasklet_rx(unsigned long data)
 				goto next;
 		}
 accept:
+		next_skb = ath5k_rx_skb_alloc(sc, &next_skb_addr);
+
+		/*
+		 * If we can't replace bf->skb with a new skb under memory
+		 * pressure, just skip this packet
+		 */
+		if (!next_skb)
+			goto next;
+
 		pci_unmap_single(sc->pdev, bf->skbaddr, sc->rxbufsize,
 				PCI_DMA_FROMDEVICE);
-		bf->skb = NULL;
-
 		skb_put(skb, rs.rs_datalen);
 
 		/* The MAC header is padded to have 32-bit boundary if the
@@ -1823,6 +1841,9 @@ ath5k_tasklet_rx(unsigned long data)
 			ath5k_check_ibss_tsf(sc, skb, &rxs);
 
 		__ieee80211_rx(sc->hw, skb, &rxs);
+
+		bf->skb = next_skb;
+		bf->skbaddr = next_skb_addr;
 next:
 		list_move_tail(&bf->list, &sc->rxbuf);
 	} while (ath5k_rxbuf_setup(sc, bf) == 0);