Re: [PATCH] bna: Add synchronization for tx ring.

[David Miller]

From: Benjamin Poirier 
Date: Mon,  7 Nov 2016 17:57:56 +0800

> We received two reports of BUG_ON in bnad_txcmpl_process() where
> hw_consumer_index appeared to be ahead of producer_index. Out of order
> write/read of these variables could explain these reports.
> 
> bnad_start_xmit(), as a producer of tx descriptors, has a few memory
> barriers sprinkled around writes to producer_index and the device's
> doorbell but they're not paired with anything in bnad_txcmpl_process(), a
> consumer.
> 
> Since we are synchronizing with a device, we must use mandatory barriers,
> not smp_*. Also, I didn't see the purpose of the last smp_mb() in
> bnad_start_xmit().
> 
> Signed-off-by: Benjamin Poirier 

Applied.

[PATCH] bna: Add synchronization for tx ring.

[Benjamin Poirier]

We received two reports of BUG_ON in bnad_txcmpl_process() where
hw_consumer_index appeared to be ahead of producer_index. Out of order
write/read of these variables could explain these reports.

bnad_start_xmit(), as a producer of tx descriptors, has a few memory
barriers sprinkled around writes to producer_index and the device's
doorbell but they're not paired with anything in bnad_txcmpl_process(), a
consumer.

Since we are synchronizing with a device, we must use mandatory barriers,
not smp_*. Also, I didn't see the purpose of the last smp_mb() in
bnad_start_xmit().

Signed-off-by: Benjamin Poirier 
---
 drivers/net/ethernet/brocade/bna/bnad.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/brocade/bna/bnad.c 
b/drivers/net/ethernet/brocade/bna/bnad.c
index f9df4b5a..f42f672 100644
--- a/drivers/net/ethernet/brocade/bna/bnad.c
+++ b/drivers/net/ethernet/brocade/bna/bnad.c
@@ -177,6 +177,7 @@ bnad_txcmpl_process(struct bnad *bnad, struct bna_tcb *tcb)
return 0;
 
hw_cons = *(tcb->hw_consumer_index);
+   rmb();
cons = tcb->consumer_index;
q_depth = tcb->q_depth;
 
@@ -3094,7 +3095,7 @@ bnad_start_xmit(struct sk_buff *skb, struct net_device 
*netdev)
BNA_QE_INDX_INC(prod, q_depth);
tcb->producer_index = prod;
 
-   smp_mb();
+   wmb();
 
if (unlikely(!test_bit(BNAD_TXQ_TX_STARTED, >flags)))
return NETDEV_TX_OK;
@@ -3102,7 +3103,6 @@ bnad_start_xmit(struct sk_buff *skb, struct net_device 
*netdev)
skb_tx_timestamp(skb);
 
bna_txq_prod_indx_doorbell(tcb);
-   smp_mb();
 
return NETDEV_TX_OK;
 }
-- 
2.9.3