Subject: Re: [PATCH net] amd-xgbe: Use wmb before updating current descriptor count

From: Tom Lendacky <[email-masked]>
Date: Fri, 23 Oct 2015 11:57:27 -0500

On 10/23/2015 11:29 AM, Alexander Duyck wrote:
> On 10/21/2015 01:37 PM, Tom Lendacky wrote:
>> The code currently uses the lightweight dma_wmb barrier before updating
>> the current descriptor count. Under heavy load, the Tx cleanup routine
>> was seeing the updated current descriptor count before the updated
>> descriptor information. As a result, the Tx descriptor was being cleaned
>> up before it was used because it was not "owned" by the hardware yet,
>> resulting in a Tx queue hang.
>>
>> Using the wmb barrier insures that the descriptor is updated before the
>> descriptor counter preventing the Tx queue hang. For extra insurance,
>> the Tx cleanup routine is changed to grab the current decriptor count on
>> entry and uses that initial value in the processing loop rather than
>> trying to chase the current value.
>>
>> Signed-off-by: Tom Lendacky <[email-masked]>
>> Tested-by: Christoffer Dall <[email-masked]>
>
> This shouldn't be using wmb() or dma_wmb().  It looks like you are just
> using the barriers to synchronize between CPUs.  If that is the case you
> should probably be using smp_wmb()/smp_rmb().
>
>> ---
>>   drivers/net/ethernet/amd/xgbe/xgbe-dev.c |    2 +-
>>   drivers/net/ethernet/amd/xgbe/xgbe-drv.c |    4 +++-
>>   2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
>> b/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
>> index a4473d8..e9ab8b9 100644
>> --- a/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
>> +++ b/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
>> @@ -1595,7 +1595,7 @@ static void xgbe_dev_xmit(struct xgbe_channel
>> *channel)
>>                     packet->rdesc_count, 1);
>>       /* Make sure ownership is written to the descriptor */
>> -    dma_wmb();
>> +    wmb();
>>       ring->cur = cur_index + 1;
>>       if (!packet->skb->xmit_more ||
>
> If anything you could probably just replace it with an smp_wmb() since
> the device doesn't ever read the ring->cur value.  Using a wmb() here is
> just doing unnecessary harm to  your performance as you aren't dealing
> with multiple memory domains.
>
>> diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
>> b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
>> index aae9d5e..d2b77d9 100644
>> --- a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
>> +++ b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
>> @@ -1807,6 +1807,7 @@ static int xgbe_tx_poll(struct xgbe_channel
>> *channel)
>>       struct netdev_queue *txq;
>>       int processed = 0;
>>       unsigned int tx_packets = 0, tx_bytes = 0;
>> +    unsigned int cur;
>>       DBGPR("-->xgbe_tx_poll\n");
>> @@ -1814,10 +1815,11 @@ static int xgbe_tx_poll(struct xgbe_channel
>> *channel)
>>       if (!ring)
>>           return 0;
>> +    cur = ring->cur;
>>       txq = netdev_get_tx_queue(netdev, channel->queue_index);
>>       while ((processed < XGBE_TX_DESC_MAX_PROC) &&
>> -           (ring->dirty != ring->cur)) {
>> +           (ring->dirty != cur)) {
>>           rdata = XGBE_GET_DESC_DATA(ring, ring->dirty);
>>           rdesc = rdata->rdesc;
>>
>
> I believe your bug is down here.  You likely need an smp_rmb() between
> the read of ring->cur and rdata.  Otherwise there is nothing to prevent
> the compiler from reordering this so that it reads the descriptor data
> ahead of ring->cur.
>
> Moving the read out of the loop likely resolves the issue as it becomes
> harder for the compiler to optimize the read versus the code in the
> loop.  You may want to just try reverting this patch and starting the
> loop with a smp_rmb() here and replace the dma_wmb() with an smp_wmb()
> above and you should see the race between the interrupt routine and the
> transmit path resolved as I believe the current fix is still a bit racy.

Alex, thanks for the advice, it makes a lot of sense. I'll test your
suggested fix.

David, if this is indeed the proper fix would you want to me to send a
new patch based on this old patch or a new patch based on you having
reverted the old patch?

Thanks,
Tom

> - Alex