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

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Date:   Fri, 03 Feb 2017 14:11:59 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Shannon Nelson <shannon.nelson@...cle.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 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:
> > On Fri, 2017-02-03 at 09:42 -0800, Shannon Nelson wrote:
> >> 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 <shannon.nelson@...cle.com>
> >> ---
> >>  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.