[PATCH] powerpc/cell spidernet refine locking

The transmit side of the spider ethernet driver currently
places locks around some very large chunks of code. This
results in a fair amount of lock contention is some cases.
This patch makes the locks much more fine-grained, protecting
only the cirtical sections. One lock is used to protect
three locations: the queue head and tail pointers, and the
queue low-watermark location.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: James K Lewis <jklewis@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

1 files changed, 43 insertions, 52 deletions

diff --git a/drivers/net/spider_net.c b/drivers/net/spider_net.c
index e429abc1e947..f8d7d0d91a6d 100644
--- a/drivers/net/spider_net.c
+++ b/drivers/net/spider_net.c
@@ -646,8 +646,9 @@ static int
 spider_net_prepare_tx_descr(struct spider_net_card *card,
                             struct sk_buff *skb)
 {
-        struct spider_net_descr *descr = card->tx_chain.head;
+        struct spider_net_descr *descr;
         dma_addr_t buf;
+        unsigned long flags;
         int length;
 
         length = skb->len;
@@ -666,6 +667,10 @@ spider_net_prepare_tx_descr(struct spider_net_card *card,
                 return -ENOMEM;
         }
 
+        spin_lock_irqsave(&card->tx_chain.lock, flags);
+        descr = card->tx_chain.head;
+        card->tx_chain.head = descr->next;
+
         descr->buf_addr = buf;
         descr->buf_size = length;
         descr->next_descr_addr = 0;
@@ -674,6 +679,8 @@ spider_net_prepare_tx_descr(struct spider_net_card *card,
 
         descr->dmac_cmd_status =
                         SPIDER_NET_DESCR_CARDOWNED | SPIDER_NET_DMAC_NOCS;
+        spin_unlock_irqrestore(&card->tx_chain.lock, flags);
+
         if (skb->protocol == htons(ETH_P_IP))
                 switch (skb->nh.iph->protocol) {
                 case IPPROTO_TCP:
@@ -691,42 +698,17 @@ spider_net_prepare_tx_descr(struct spider_net_card *card,
         return 0;
 }
 
-/**
- * spider_net_release_tx_descr - processes a used tx descriptor
- * @card: card structure
- * @descr: descriptor to release
- *
- * releases a used tx descriptor (unmapping, freeing of skb)
- */
-static inline void
-spider_net_release_tx_descr(struct spider_net_card *card)
-{
-        struct spider_net_descr *descr = card->tx_chain.tail;
-        struct sk_buff *skb;
-        unsigned int len;
-
-        card->tx_chain.tail = card->tx_chain.tail->next;
-        descr->dmac_cmd_status |= SPIDER_NET_DESCR_NOT_IN_USE;
-
-        /* unmap the skb */
-        skb = descr->skb;
-        if (!skb)
-                return;
-        len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;
-        pci_unmap_single(card->pdev, descr->buf_addr, len,
-                        PCI_DMA_TODEVICE);
-        dev_kfree_skb(skb);
-}
-
 static void
 spider_net_set_low_watermark(struct spider_net_card *card)
 {
+        unsigned long flags;
         int status;
         int cnt=0;
         int i;
         struct spider_net_descr *descr = card->tx_chain.tail;
 
-        /* Measure the length of the queue. */
+        /* Measure the length of the queue. Measurement does not
+         * need to be precise -- does not need a lock. */
         while (descr != card->tx_chain.head) {
                 status = descr->dmac_cmd_status & SPIDER_NET_DESCR_NOT_IN_USE;
                 if (status == SPIDER_NET_DESCR_NOT_IN_USE)
@@ -746,11 +728,13 @@ spider_net_set_low_watermark(struct spider_net_card *card)
                 descr = descr->next;
 
         /* Set the new watermark, clear the old watermark */
+        spin_lock_irqsave(&card->tx_chain.lock, flags);
         descr->dmac_cmd_status |= SPIDER_NET_DESCR_TXDESFLG;
         if (card->low_watermark && card->low_watermark != descr)
                 card->low_watermark->dmac_cmd_status =
                      card->low_watermark->dmac_cmd_status & ~SPIDER_NET_DESCR_TXDESFLG;
         card->low_watermark = descr;
+        spin_unlock_irqrestore(&card->tx_chain.lock, flags);
 }
 
 /**
@@ -769,21 +753,31 @@ static int
 spider_net_release_tx_chain(struct spider_net_card *card, int brutal)
 {
         struct spider_net_descr_chain *chain = &card->tx_chain;
+        struct spider_net_descr *descr;
+        struct sk_buff *skb;
+        u32 buf_addr;
+        unsigned long flags;
         int status;
 
         spider_net_read_reg(card, SPIDER_NET_GDTDMACCNTR);
 
         while (chain->tail != chain->head) {
-                status = spider_net_get_descr_status(chain->tail);
+                spin_lock_irqsave(&chain->lock, flags);
+                descr = chain->tail;
+
+                status = spider_net_get_descr_status(descr);
                 switch (status) {
                 case SPIDER_NET_DESCR_COMPLETE:
                         card->netdev_stats.tx_packets++;
-                        card->netdev_stats.tx_bytes += chain->tail->skb->len;
+                        card->netdev_stats.tx_bytes += descr->skb->len;
                         break;
 
                 case SPIDER_NET_DESCR_CARDOWNED:
-                        if (!brutal)
+                        if (!brutal) {
+                                spin_unlock_irqrestore(&chain->lock, flags);
                                 return 1;
+                        }
+
                         /* fallthrough, if we release the descriptors
                          * brutally (then we don't care about
                          * SPIDER_NET_DESCR_CARDOWNED) */
@@ -800,12 +794,25 @@ spider_net_release_tx_chain(struct spider_net_card *card, int brutal)
 
                 default:
                         card->netdev_stats.tx_dropped++;
-                        if (!brutal)
+                        if (!brutal) {
+                                spin_unlock_irqrestore(&chain->lock, flags);
                                 return 1;
+                        }
                 }
-                spider_net_release_tx_descr(card);
-        }
 
+                chain->tail = descr->next;
+                descr->dmac_cmd_status |= SPIDER_NET_DESCR_NOT_IN_USE;
+                skb = descr->skb;
+                buf_addr = descr->buf_addr;
+                spin_unlock_irqrestore(&chain->lock, flags);
+
+                /* unmap the skb */
+                if (skb) {
+                        int len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;
+                        pci_unmap_single(card->pdev, buf_addr, len, PCI_DMA_TODEVICE);
+                        dev_kfree_skb(skb);
+                }
+        }
         return 0;
 }
 
@@ -857,27 +864,19 @@ spider_net_xmit(struct sk_buff *skb, struct net_device *netdev)
 {
         struct spider_net_card *card = netdev_priv(netdev);
         struct spider_net_descr_chain *chain = &card->tx_chain;
-        struct spider_net_descr *descr = chain->head;
-        unsigned long flags;
-
-        spin_lock_irqsave(&chain->lock, flags);
 
         spider_net_release_tx_chain(card, 0);
 
         if ((chain->head->next == chain->tail->prev) ||
-           (spider_net_get_descr_status(descr) != SPIDER_NET_DESCR_NOT_IN_USE) ||
            (spider_net_prepare_tx_descr(card, skb) != 0)) {
 
                 card->netdev_stats.tx_dropped++;
-                spin_unlock_irqrestore(&chain->lock, flags);
                 netif_stop_queue(netdev);
                 return NETDEV_TX_BUSY;
         }
 
         spider_net_set_low_watermark(card);
         spider_net_kick_tx_dma(card);
-        card->tx_chain.head = card->tx_chain.head->next;
-        spin_unlock_irqrestore(&chain->lock, flags);
         return NETDEV_TX_OK;
 }
 
@@ -893,16 +892,11 @@ spider_net_xmit(struct sk_buff *skb, struct net_device *netdev)
 static void
 spider_net_cleanup_tx_ring(struct spider_net_card *card)
 {
-        unsigned long flags;
-
-        spin_lock_irqsave(&card->tx_chain.lock, flags);
-
         if ((spider_net_release_tx_chain(card, 0) != 0) &&
             (card->netdev->flags & IFF_UP)) {
                 spider_net_kick_tx_dma(card);
                 netif_wake_queue(card->netdev);
         }
-        spin_unlock_irqrestore(&card->tx_chain.lock, flags);
 }
 
 /**
@@ -1930,10 +1924,7 @@ spider_net_stop(struct net_device *netdev)
         spider_net_disable_rxdmac(card);
 
         /* release chains */
-        if (spin_trylock(&card->tx_chain.lock)) {
-                spider_net_release_tx_chain(card, 1);
-                spin_unlock(&card->tx_chain.lock);
-        }
+        spider_net_release_tx_chain(card, 1);
 
         spider_net_free_chain(card, &card->tx_chain);
         spider_net_free_chain(card, &card->rx_chain);