[PATCH net 04/19] net/sonic: Add mutual exclusion for accessing shared state

[Finn Thain]

The netif_stop_queue() call in sonic_send_packet() races with the
netif_wake_queue() call in sonic_interrupt(). This causes issues
like "NETDEV WATCHDOG: eth0 (macsonic): transmit queue 0 timed out".
Fix this by disabling interrupts when accessing tx_skb[] and next_tx.
Update a comment to clarify the synchronization properties.

Fixes: efcce839360f ("[PATCH] macsonic/jazzsonic network drivers update")
Tested-by: Stan Johnson <userm57@xxxxxxxxx>
Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx>
---
 drivers/net/ethernet/natsemi/sonic.c | 38 +++++++++++++++++++---------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/drivers/net/ethernet/natsemi/sonic.c b/drivers/net/ethernet/natsemi/sonic.c
index 3cf84de8ad8e..dbbbc8bc72ff 100644
--- a/drivers/net/ethernet/natsemi/sonic.c
+++ b/drivers/net/ethernet/natsemi/sonic.c
@@ -242,7 +242,7 @@ static void sonic_tx_timeout(struct net_device *dev)
  *   wake the tx queue
  * Concurrently with all of this, the SONIC is potentially writing to
  * the status flags of the TDs.
- * Until some mutual exclusion is added, this code will not work with SMP. However,
+ * A spin lock is needed to make this work on SMP platforms. However,
  * MIPS Jazz machines and m68k Macs were all uni-processor machines.
  */
 
@@ -252,6 +252,7 @@ static int sonic_send_packet(struct sk_buff *skb, struct net_device *dev)
 	dma_addr_t laddr;
 	int length;
 	int entry;
+	unsigned long flags;
 
 	netif_dbg(lp, tx_queued, dev, "%s: skb=%p\n", __func__, skb);
 
@@ -273,6 +274,8 @@ static int sonic_send_packet(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_OK;
 	}
 
+	local_irq_save(flags);
+
 	entry = (lp->eol_tx + 1) & SONIC_TDS_MASK;
 
 	sonic_tda_put(dev, entry, SONIC_TD_STATUS, 0);       /* clear status */
@@ -284,10 +287,6 @@ static int sonic_send_packet(struct sk_buff *skb, struct net_device *dev)
 	sonic_tda_put(dev, entry, SONIC_TD_LINK,
 		sonic_tda_get(dev, entry, SONIC_TD_LINK) | SONIC_EOL);
 
-	/*
-	 * Must set tx_skb[entry] only after clearing status, and
-	 * before clearing EOL and before stopping queue
-	 */
 	wmb();
 	lp->tx_len[entry] = length;
 	lp->tx_laddr[entry] = laddr;
@@ -310,6 +309,8 @@ static int sonic_send_packet(struct sk_buff *skb, struct net_device *dev)
 
 	SONIC_WRITE(SONIC_CMD, SONIC_CR_TXP);
 
+	local_irq_restore(flags);
+
 	return NETDEV_TX_OK;
 }
 
@@ -322,9 +323,16 @@ static irqreturn_t sonic_interrupt(int irq, void *dev_id)
 	struct net_device *dev = dev_id;
 	struct sonic_local *lp = netdev_priv(dev);
 	int status;
+	unsigned long flags;
+
+	local_irq_save(flags);
+
+	status = SONIC_READ(SONIC_ISR) & SONIC_IMR_DEFAULT;
+	if (!status) {
+		local_irq_restore(flags);
 
-	if (!(status = SONIC_READ(SONIC_ISR) & SONIC_IMR_DEFAULT))
 		return IRQ_NONE;
+	}
 
 	do {
 		if (status & SONIC_INT_PKTRX) {
@@ -338,11 +346,12 @@ static irqreturn_t sonic_interrupt(int irq, void *dev_id)
 			int td_status;
 			int freed_some = 0;
 
-			/* At this point, cur_tx is the index of a TD that is one of:
-			 *   unallocated/freed                          (status set   & tx_skb[entry] clear)
-			 *   allocated and sent                         (status set   & tx_skb[entry] set  )
-			 *   allocated and not yet sent                 (status clear & tx_skb[entry] set  )
-			 *   still being allocated by sonic_send_packet (status clear & tx_skb[entry] clear)
+			/* The state of a Transmit Descriptor may be inferred
+			 * from { tx_skb[entry], td_status } as follows.
+			 * { clear, clear } => the TD has never been used
+			 * { set,   clear } => the TD was handed to SONIC
+			 * { set,   set   } => the TD was handed back
+			 * { clear, set   } => the TD is available for re-use
 			 */
 
 			netif_dbg(lp, intr, dev, "%s: tx done\n", __func__);
@@ -444,7 +453,12 @@ static irqreturn_t sonic_interrupt(int irq, void *dev_id)
 		/* load CAM done */
 		if (status & SONIC_INT_LCD)
 			SONIC_WRITE(SONIC_ISR, SONIC_INT_LCD); /* clear the interrupt */
-	} while((status = SONIC_READ(SONIC_ISR) & SONIC_IMR_DEFAULT));
+
+		status = SONIC_READ(SONIC_ISR) & SONIC_IMR_DEFAULT;
+	} while (status);
+
+	local_irq_restore(flags);
+
 	return IRQ_HANDLED;
 }
 
-- 
2.24.1