Re: [PATCH net-next 5/9] sunvnet: add memory barrier before check for tx enable

On Fri, 2017-02-03 at 13:20 -0800, Shannon Nelson wrote:

On 2/3/2017 9:56 AM, Eric Dumazet wrote:

quoted

On Fri, 2017-02-03 at 09:42 -0800, Shannon Nelson wrote:

quoted

In order to allow the underlying LDC and outstanding memory operations
to potentially catch up with the driver's Tx requests, add a memory
barrier before checking again for available tx descriptors.

Signed-off-by: Shannon Nelson <redacted>
---
 drivers/net/ethernet/sun/sunvnet_common.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/drivers/net/ethernet/sun/sunvnet_common.c b/drivers/net/ethernet/sun/sunvnet_common.c
index 5d0d386..98e758e 100644
--- a/drivers/net/ethernet/sun/sunvnet_common.c
+++ b/drivers/net/ethernet/sun/sunvnet_common.c

@@ -1467,6 +1467,7 @@ ldc_start_done:
 	dr->prod = (dr->prod + 1) & (VNET_TX_RING_SIZE - 1);
 	if (unlikely(vnet_tx_dring_avail(dr) < 1)) {
 		netif_tx_stop_queue(txq);
+		dma_wmb();

This does not look right.

I believe you need smp_rmb() here.

Well, it probably should be dma_rmb(), since regardless of the number of 
cores we think we have, we're communicating with a peer ldom that has 
its own core(s).  Either way, on sparc they all seem to boil down to the 
same bit of asm, but using the "rmb" part makes more logical sense. 
I'll respin with dma_rmb().

Transmit completion might happen on another cpu, regardless of ldom.

Therefore you need smp_rmb() here ( like mellanox/mlx4/en_tx.c) , or
even smp_mb() as bnx2x does.

dma_rmb() is never used in this context.