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

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Message-ID: <1486144585.21871.92.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 03 Feb 2017 09:56:25 -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 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.

>  		if (vnet_tx_dring_avail(dr) > VNET_TX_WAKEUP_THRESH(dr))
>  			netif_tx_wake_queue(txq);
>  	}