[TG3]: Fix tx race condition

Fix a subtle race condition between tg3_start_xmit() and tg3_tx()
discovered by Herbert Xu <herbert@gondor.apana.org.au>:

CPU0					CPU1
tg3_start_xmit()
	if (tx_ring_full) {
		tx_lock
					tg3_tx()
						if (!netif_queue_stopped)
		netif_stop_queue()
		if (!tx_ring_full)
						update_tx_ring 
			netif_wake_queue()
		tx_unlock
	}

Even though tx_ring is updated before the if statement in tg3_tx() in
program order, it can be re-ordered by the CPU as shown above.  This
scenario can cause the tx queue to be stopped forever if tg3_tx() has
just freed up the entire tx_ring.  The possibility of this happening
should be very rare though.

The following changes are made:

1. Add memory barrier to fix the above race condition.

2. Eliminate the private tx_lock altogether and rely solely on
netif_tx_lock.  This eliminates one spinlock in tg3_start_xmit()
when the ring is full.

3. Because of 2, use netif_tx_lock in tg3_tx() before calling
netif_wake_queue().

4. Change TX_BUFFS_AVAIL to an inline function with a memory barrier.
Herbert and David suggested using the memory barrier instead of
volatile.

5. Check for the full wake queue condition before getting
netif_tx_lock in tg3_tx().  This reduces the number of unnecessary
spinlocks when the tx ring is full in a steady-state condition.

6. Update version to 3.65.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 31 insertions, 26 deletions

diff --git a/drivers/net/tg3.c b/drivers/net/tg3.c
index 0afbed6753fa..eafabb253f08 100644
--- a/drivers/net/tg3.c
+++ b/drivers/net/tg3.c
@@ -68,8 +68,8 @@
 
 #define DRV_MODULE_NAME         "tg3"
 #define PFX DRV_MODULE_NAME     ": "
-#define DRV_MODULE_VERSION      "3.64"
-#define DRV_MODULE_RELDATE      "July 31, 2006"
+#define DRV_MODULE_VERSION      "3.65"
+#define DRV_MODULE_RELDATE      "August 07, 2006"
 
 #define TG3_DEF_MAC_MODE        0
 #define TG3_DEF_RX_MODE         0
@@ -123,9 +123,6 @@
                                    TG3_RX_RCB_RING_SIZE(tp))
 #define TG3_TX_RING_BYTES       (sizeof(struct tg3_tx_buffer_desc) * \
                                  TG3_TX_RING_SIZE)
-#define TX_BUFFS_AVAIL(TP)                                              \
-        ((TP)->tx_pending -                                             \
-         (((TP)->tx_prod - (TP)->tx_cons) & (TG3_TX_RING_SIZE - 1)))
 #define NEXT_TX(N)              (((N) + 1) & (TG3_TX_RING_SIZE - 1))
 
 #define RX_PKT_BUF_SZ           (1536 + tp->rx_offset + 64)
@@ -2987,6 +2984,13 @@ static void tg3_tx_recover(struct tg3 *tp)
         spin_unlock(&tp->lock);
 }
 
+static inline u32 tg3_tx_avail(struct tg3 *tp)
+{
+        smp_mb();
+        return (tp->tx_pending -
+                ((tp->tx_prod - tp->tx_cons) & (TG3_TX_RING_SIZE - 1)));
+}
+
 /* Tigon3 never reports partial packet sends.  So we do not
  * need special logic to handle SKBs that have not had all
  * of their frags sent yet, like SunGEM does.
@@ -3038,12 +3042,20 @@ static void tg3_tx(struct tg3 *tp)
 
         tp->tx_cons = sw_idx;
 
-        if (unlikely(netif_queue_stopped(tp->dev))) {
-                spin_lock(&tp->tx_lock);
+        /* Need to make the tx_cons update visible to tg3_start_xmit()
+         * before checking for netif_queue_stopped().  Without the
+         * memory barrier, there is a small possibility that tg3_start_xmit()
+         * will miss it and cause the queue to be stopped forever.
+         */
+        smp_mb();
+
+        if (unlikely(netif_queue_stopped(tp->dev) &&
+                     (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH))) {
+                netif_tx_lock(tp->dev);
                 if (netif_queue_stopped(tp->dev) &&
-                    (TX_BUFFS_AVAIL(tp) > TG3_TX_WAKEUP_THRESH))
+                    (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH))
                         netif_wake_queue(tp->dev);
-                spin_unlock(&tp->tx_lock);
+                netif_tx_unlock(tp->dev);
         }
 }
 
@@ -3795,7 +3807,7 @@ static int tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
          * interrupt.  Furthermore, IRQ processing runs lockless so we have
          * no IRQ context deadlocks to worry about either.  Rejoice!
          */
-        if (unlikely(TX_BUFFS_AVAIL(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
+        if (unlikely(tg3_tx_avail(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
                 if (!netif_queue_stopped(dev)) {
                         netif_stop_queue(dev);
 
@@ -3891,12 +3903,10 @@ static int tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
         tw32_tx_mbox((MAILBOX_SNDHOST_PROD_IDX_0 + TG3_64BIT_REG_LOW), entry);
 
         tp->tx_prod = entry;
-        if (unlikely(TX_BUFFS_AVAIL(tp) <= (MAX_SKB_FRAGS + 1))) {
-                spin_lock(&tp->tx_lock);
+        if (unlikely(tg3_tx_avail(tp) <= (MAX_SKB_FRAGS + 1))) {
                 netif_stop_queue(dev);
-                if (TX_BUFFS_AVAIL(tp) > TG3_TX_WAKEUP_THRESH)
+                if (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH)
                         netif_wake_queue(tp->dev);
-                spin_unlock(&tp->tx_lock);
         }
 
 out_unlock:
@@ -3918,7 +3928,7 @@ static int tg3_tso_bug(struct tg3 *tp, struct sk_buff *skb)
         struct sk_buff *segs, *nskb;
 
         /* Estimate the number of fragments in the worst case */
-        if (unlikely(TX_BUFFS_AVAIL(tp) <= (skb_shinfo(skb)->gso_segs * 3))) {
+        if (unlikely(tg3_tx_avail(tp) <= (skb_shinfo(skb)->gso_segs * 3))) {
                 netif_stop_queue(tp->dev);
                 return NETDEV_TX_BUSY;
         }
@@ -3958,7 +3968,7 @@ static int tg3_start_xmit_dma_bug(struct sk_buff *skb, struct net_device *dev)
          * interrupt.  Furthermore, IRQ processing runs lockless so we have
          * no IRQ context deadlocks to worry about either.  Rejoice!
          */
-        if (unlikely(TX_BUFFS_AVAIL(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
+        if (unlikely(tg3_tx_avail(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
                 if (!netif_queue_stopped(dev)) {
                         netif_stop_queue(dev);
 
@@ -4108,12 +4118,10 @@ static int tg3_start_xmit_dma_bug(struct sk_buff *skb, struct net_device *dev)
         tw32_tx_mbox((MAILBOX_SNDHOST_PROD_IDX_0 + TG3_64BIT_REG_LOW), entry);
 
         tp->tx_prod = entry;
-        if (unlikely(TX_BUFFS_AVAIL(tp) <= (MAX_SKB_FRAGS + 1))) {
-                spin_lock(&tp->tx_lock);
+        if (unlikely(tg3_tx_avail(tp) <= (MAX_SKB_FRAGS + 1))) {
                 netif_stop_queue(dev);
-                if (TX_BUFFS_AVAIL(tp) > TG3_TX_WAKEUP_THRESH)
+                if (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH)
                         netif_wake_queue(tp->dev);
-                spin_unlock(&tp->tx_lock);
         }
 
 out_unlock:
@@ -11472,7 +11480,6 @@ static int __devinit tg3_init_one(struct pci_dev *pdev,
         tp->grc_mode |= GRC_MODE_BSWAP_NONFRM_DATA;
 #endif
         spin_lock_init(&tp->lock);
-        spin_lock_init(&tp->tx_lock);
         spin_lock_init(&tp->indirect_lock);
         INIT_WORK(&tp->reset_task, tg3_reset_task, tp);
 
diff --git a/drivers/net/tg3.h b/drivers/net/tg3.h
index ba2c98711c88..3ecf356cfb08 100644
--- a/drivers/net/tg3.h
+++ b/drivers/net/tg3.h
@@ -2079,9 +2079,9 @@ struct tg3 {
          * lock: Held during reset, PHY access, timer, and when
          *       updating tg3_flags and tg3_flags2.
          *
-         * tx_lock: Held during tg3_start_xmit and tg3_tx only
-         *          when calling netif_[start|stop]_queue.
-         *          tg3_start_xmit is protected by netif_tx_lock.
+         * netif_tx_lock: Held during tg3_start_xmit. tg3_tx holds
+         *                netif_tx_lock when it needs to call
+         *                netif_wake_queue.
          *
          * Both of these locks are to be held with BH safety.
          *
@@ -2118,8 +2118,6 @@ struct tg3 {
         u32                             tx_cons;
         u32                             tx_pending;
 
-        spinlock_t                      tx_lock;
-
         struct tg3_tx_buffer_desc       *tx_ring;
         struct tx_ring_info             *tx_buffers;
         dma_addr_t                      tx_desc_mapping;