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Subject[PATCH] Re: Q: natsemi.c spinlocks
Andrew Morton wrote:
>
> Manfred wrote:
> >
> > Hi Jeff, Tjeerd,
> >
> > I spotted the spin_lock in natsemi.c, and I think it's bogus.
> >
> > The "simultaneous interrupt entry" is a bug in some 2.0 and 2.1 kernel
> > (even Alan didn't remember it exactly when I asked him), thus a sane
> > driver can assume that an interrupt handler is never reentered.
> >
> > Donald often uses dev->interrupt to hide other races, but I don't see
> > anything in this driver (tx_timeout and netdev_timer are both trivial)
>
> Hi, Manfed.
>
> I think you're right. 2.4's interrupt handling prevents
> simultaneous entry of the same ISR.
>
> However, natsemi.c's spinlock needs to be retained, and
> extended into start_tx(), because this driver has
> a race which has cropped up in a few others:
>
> Current code:
>
> start_tx()
> {
> ...
> if (np->cur_tx - np->dirty_tx >= TX_QUEUE_LEN - 1) {
> /* WINDOW HERE */
> np->tx_full = 1;
> netif_stop_queue(dev);
> }
> ...
> }
>
> If the ring is currently full and an interrupt comes in
> at the indicated window and reaps ALL the packets in the
> ring, the driver ends up in state `tx_full = 1' and tramsmit
> disabled, but with no outstanding transmit interrupts.
>
> It's screwed. You need another interrupt so tx_full
> can be cleared and the queue can be restarted, but you can't
> *get* another interrupt because there are no Tx packets outstanding.
>
> It's very unlikely to happen with this particular driver
> because it's also polling the transmit queue within
> receive interrupts. Receiving a packet will clear
> the condition.
>
> If you were madly hosing out UDP packets and receiving nothing
> then this could occur. It was certainly triggerable in 3c59x.c,
> which doesn't test the Tx queue state in Rx interrupts.
>
> I currently have natsemi.c lying in pieces on my garage floor,
> so I'll put this locking in if it's OK with everyone?

(entire message quoted, as it was from 12/23/2000)

Attached is a patch against 2.4.1-pre9, which includes the changes I
would prefer. Comments?

The Tx locking is a bit conservative -- I think Donald suggested it
could be removed completely -- but I would prefer to have something I am
100% certain will work, and then test the driver without locking under
stress conditions to make sure no race or other bug exists.

Jeff


--
Jeff Garzik | "You see, in this world there's two kinds of
Building 1024 | people, my friend: Those with loaded guns
MandrakeSoft | and those who dig. You dig." --BlondieIndex: linux_2_4/drivers/net/natsemi.c
diff -u linux_2_4/drivers/net/natsemi.c:1.1.1.6 linux_2_4/drivers/net/natsemi.c:1.1.1.6.24.1
--- linux_2_4/drivers/net/natsemi.c:1.1.1.6 Mon Dec 11 19:23:42 2000
+++ linux_2_4/drivers/net/natsemi.c Sun Jan 21 06:39:02 2001
@@ -26,6 +26,11 @@
- Bug fixes and better intr performance (Tjeerd)
Version 1.0.2:
- Now reads correct MAC address from eeprom
+ Version 1.0.3:
+ - Eliminate redundant priv->tx_full flag
+ - Call netif_start_queue from dev->tx_timeout
+ - wmb() in start_tx() to flush data
+ - Update Tx locking

*/

@@ -35,7 +40,7 @@
static const char version2[] =
" http://www.scyld.com/network/natsemi.html\n";
static const char version3[] =
-" (unofficial 2.4.x kernel port, version 1.0.2, October 6, 2000 Jeff Garzik, Tjeerd Mulder)\n";
+" (unofficial 2.4.x kernel port, version 1.0.3, January 21, 2001 Jeff Garzik, Tjeerd Mulder)\n";
/* Updated to recommendations in pci-skeleton v2.03. */

/* Automatically extracted configuration info:
@@ -187,13 +192,14 @@

The send packet thread has partial control over the Tx ring and 'dev->tbusy'
flag. It sets the tbusy flag whenever it's queuing a Tx packet. If the next
-queue slot is empty, it clears the tbusy flag when finished otherwise it sets
-the 'lp->tx_full' flag.
+queue slot is empty, it clears the tbusy flag when finished. Under 2.4, the
+"tbusy flag" is now controlled by netif_{start,stop,wake}_queue() and tested
+by netif_queue_stopped().

The interrupt handler has exclusive control over the Rx ring and records stats
from the Tx ring. After reaping the stats, it marks the Tx queue entry as
-empty by incrementing the dirty_tx mark. Iff the 'lp->tx_full' flag is set, it
-clears both the tx_full and tbusy flags.
+empty by incrementing the dirty_tx mark. Iff Tx queueing is stopped and Tx
+entries were reaped, the Tx queue is started and scheduled.

IV. Notes

@@ -319,7 +325,6 @@
unsigned int cur_rx, dirty_rx; /* Producer/consumer ring indices */
unsigned int cur_tx, dirty_tx;
unsigned int rx_buf_sz; /* Based on MTU+slack. */
- unsigned int tx_full:1; /* The Tx queue is full. */
/* These values are keep track of the transceiver/media in use. */
unsigned int full_duplex:1; /* Full-duplex operation requested. */
unsigned int duplex_lock:1;
@@ -697,7 +702,7 @@

dev->trans_start = jiffies;
np->stats.tx_errors++;
- return;
+ netif_start_queue(dev);
}


@@ -707,7 +712,6 @@
struct netdev_private *np = (struct netdev_private *)dev->priv;
int i;

- np->tx_full = 0;
np->cur_rx = np->cur_tx = 0;
np->dirty_rx = np->dirty_tx = 0;

@@ -763,11 +767,13 @@
np->cur_tx++;

/* StrongARM: Explicitly cache flush np->tx_ring and skb->data,skb->len. */
+ wmb();

- if (np->cur_tx - np->dirty_tx >= TX_QUEUE_LEN - 1) {
- np->tx_full = 1;
+ spin_lock_irq(&np->lock);
+ if (np->cur_tx - np->dirty_tx >= TX_QUEUE_LEN - 1)
netif_stop_queue(dev);
- }
+ spin_unlock_irq(&np->lock);
+
/* Wake the potentially-idle transmit channel. */
writel(TxOn, dev->base_addr + ChipCmd);

@@ -798,9 +804,7 @@
#endif

ioaddr = dev->base_addr;
- np = (struct netdev_private *)dev->priv;
-
- spin_lock(&np->lock);
+ np = dev->priv;

do {
u32 intr_status = readl(ioaddr + IntrStatus);
@@ -818,6 +822,8 @@
if (intr_status & (IntrRxDone | IntrRxErr | IntrRxIdle | IntrRxOverrun))
netdev_rx(dev);

+ spin_lock(&np->lock);
+
for (; np->cur_tx - np->dirty_tx > 0; np->dirty_tx++) {
int entry = np->dirty_tx % TX_RING_SIZE;
if (np->tx_ring[entry].cmd_status & cpu_to_le32(DescOwn))
@@ -839,13 +845,14 @@
dev_kfree_skb_irq(np->tx_skbuff[entry]);
np->tx_skbuff[entry] = 0;
}
- if (np->tx_full
+ if (netif_queue_stopped(dev)
&& np->cur_tx - np->dirty_tx < TX_QUEUE_LEN - 4) {
/* The ring is no longer full, wake queue. */
- np->tx_full = 0;
netif_wake_queue(dev);
}

+ spin_unlock(&np->lock);
+
/* Abnormal error summary/uncommon events handlers. */
if (intr_status & IntrAbnormalSummary)
netdev_error(dev, intr_status);
@@ -873,8 +880,6 @@
}
}
#endif
-
- spin_unlock(&np->lock);
}

/* This routine is logically part of the interrupt handler, but separated
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