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[BNX2]: Fix tx race condition.

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Fix a subtle race condition between bnx2_start_xmit() and bnx2_tx_int()
similar to the one in tg3 discovered by Herbert Xu:

CPU0					CPU1
bnx2_start_xmit()
	if (tx_ring_full) {
		tx_lock
					bnx2_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 bnx2_tx_int() 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 bnx2_tx_int() has
just freed up the entire tx_ring.  The possibility of this happening
should be very rare though.

The following changes are made, very much identical to the tg3 fix:

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 bnx2_start_xmit()
when the ring is full.

3. Because of 2, use netif_tx_lock in bnx2_tx_int() before calling
netif_wake_queue().

4. Add memory barrier to bnx2_tx_avail().

5. Add bp->tx_wake_thresh which is set to half the tx ring size.

6. 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.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Michael Chan
2006-08-15 01:39:10 -07:00
committed by David S. Miller
parent fb33f82568
commit 2f8af120a1
2 changed files with 24 additions and 23 deletions
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12
drivers/net/bnx2.h
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@@ -3890,10 +3890,6 @@ struct bnx2 {
u32 tx_prod_bseq __attribute__((aligned(L1_CACHE_BYTES)));
u16 tx_prod;
struct tx_bd *tx_desc_ring;
struct sw_bd *tx_buf_ring;
int tx_ring_size;
u16 tx_cons __attribute__((aligned(L1_CACHE_BYTES)));
u16 hw_tx_cons;
@@ -3916,9 +3912,11 @@ struct bnx2 {
struct sw_bd *rx_buf_ring;
struct rx_bd *rx_desc_ring[MAX_RX_RINGS];
/* Only used to synchronize netif_stop_queue/wake_queue when tx */
/* ring is full */
spinlock_t tx_lock;
/* TX constants */
struct tx_bd *tx_desc_ring;
struct sw_bd *tx_buf_ring;
int tx_ring_size;
u32 tx_wake_thresh;
/* End of fields used in the performance code paths. */