[PATCH net-next] net: dlink: add synchronization for stats update

[Moon Yeounsu <yyyynoom@gmail.com>]

There are two paths that call `get_stats()`:
    1. From user space via the `ip` command
    2. From interrupt context via `rio_interrupt()`

Case 1 is synchronized by `rtnl_lock()`, so it is safe.
However, the two cases above are not synchronized with each other.
Therefore, `spin_lock` is needed to protect `get_stats()` as it can be
preempted by an interrupt. In this context, `spin_lock_irq()` is required
(using `spin_lock_bh()` may result in a deadlock).

While `spin_lock` protects `get_stats()`, it does not protect updates to
`dev->stats.tx_errors` and `dev->stats.collisions`, which may be
concurrently modified by the interrupt handler and user space.
By using temporary variables in `np->tx_errors` and `np->collisions`,
we can safely update `dev->stats` without additional locking.

Tested-on: D-Link DGE-550T Rev-A3
Signed-off-by: Moon Yeounsu <yyyynoom@gmail.com>
---
Question:
	This might be a bit off-topic, but I don’t fully understand why a single global
	`rtnl_lock` is used for synchronization. While I may not be fully aware of the 
	design rationale, it seems somewhat suboptimal. I believe it could be improved.
---
 drivers/net/ethernet/dlink/dl2k.c | 11 +++++++++--
 drivers/net/ethernet/dlink/dl2k.h |  5 +++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/dlink/dl2k.c b/drivers/net/ethernet/dlink/dl2k.c
index d0ea92607870..2d929a83e101 100644
--- a/drivers/net/ethernet/dlink/dl2k.c
+++ b/drivers/net/ethernet/dlink/dl2k.c
@@ -865,7 +865,7 @@ tx_error (struct net_device *dev, int tx_status)
 	frame_id = (tx_status & 0xffff0000);
 	printk (KERN_ERR "%s: Transmit error, TxStatus %4.4x, FrameId %d.\n",
 		dev->name, tx_status, frame_id);
-	dev->stats.tx_errors++;
+	np->tx_errors++;
 	/* Ttransmit Underrun */
 	if (tx_status & 0x10) {
 		dev->stats.tx_fifo_errors++;
@@ -904,7 +904,7 @@ tx_error (struct net_device *dev, int tx_status)
 	}
 	/* Maximum Collisions */
 	if (tx_status & 0x08)
-		dev->stats.collisions++;
+		np->collisions++;
 	/* Restart the Tx */
 	dw32(MACCtrl, dr16(MACCtrl) | TxEnable);
 }
@@ -1074,6 +1074,7 @@ get_stats (struct net_device *dev)
 #endif
 	unsigned int stat_reg;
 
+	spin_lock_irq(&np->stats_lock);
 	/* All statistics registers need to be acknowledged,
 	   else statistic overflow could cause problems */
 
@@ -1085,6 +1086,7 @@ get_stats (struct net_device *dev)
 	dev->stats.multicast = dr32(McstFramesRcvdOk);
 	dev->stats.collisions += dr32(SingleColFrames)
 			     +  dr32(MultiColFrames);
+	dev->stats.collisions += np->collisions;
 
 	/* detailed tx errors */
 	stat_reg = dr16(FramesAbortXSColls);
@@ -1095,6 +1097,8 @@ get_stats (struct net_device *dev)
 	dev->stats.tx_carrier_errors += stat_reg;
 	dev->stats.tx_errors += stat_reg;
 
+	dev->stats.tx_errors += np->tx_errors;
+
 	/* Clear all other statistic register. */
 	dr32(McstOctetXmtOk);
 	dr16(BcstFramesXmtdOk);
@@ -1123,6 +1127,9 @@ get_stats (struct net_device *dev)
 	dr16(TCPCheckSumErrors);
 	dr16(UDPCheckSumErrors);
 	dr16(IPCheckSumErrors);
+
+	spin_unlock_irq(&np->stats_lock);
+
 	return &dev->stats;
 }
 
diff --git a/drivers/net/ethernet/dlink/dl2k.h b/drivers/net/ethernet/dlink/dl2k.h
index 195dc6cfd895..dc8755a69b73 100644
--- a/drivers/net/ethernet/dlink/dl2k.h
+++ b/drivers/net/ethernet/dlink/dl2k.h
@@ -372,6 +372,8 @@ struct netdev_private {
 	struct pci_dev *pdev;
 	void __iomem *ioaddr;
 	void __iomem *eeprom_addr;
+	// To ensure synchronization when stats are updated.
+	spinlock_t stats_lock;
 	spinlock_t tx_lock;
 	spinlock_t rx_lock;
 	unsigned int rx_buf_sz;		/* Based on MTU+slack. */
@@ -401,6 +403,9 @@ struct netdev_private {
 	u16 negotiate;		/* Negotiated media */
 	int phy_addr;		/* PHY addresses. */
 	u16 led_mode;		/* LED mode read from EEPROM (IP1000A only) */
+
+	u64 collisions;
+	u64 tx_errors;
 };
 
 /* The station address location in the EEPROM. */
-- 
2.49.0