netdev - Re: [net-next.git 2/7] stmmac: review barriers

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Date:	Wed, 03 Apr 2013 00:18:43 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
Cc:	netdev@...r.kernel.org, Deepak Sikri <deepak.sikri@...com>,
	Shiraz Hashim <shiraz.hashim@...com>
Subject: Re: [net-next.git 2/7] stmmac: review barriers

On Wed, 2013-04-03 at 07:41 +0200, Giuseppe CAVALLARO wrote:
> In all my tests performed on SH4 and ARM A9 platforms, I've never met problems
> that can be fixed by using memory barriers. In the past there was some issues
> on SMP ARM but fixed by reviewing xmit spinlock.
> 
> Further barriers have been added in the commits too: 8e83989106562326bf
> 
> This patch is to use the smp_wbm instead of wbm because the driver
> runs on UP systems. Then, IMO it could make sense to only maintain the barriers
> just in places where we touch the dma owner bits (that is the
> only real critical path as we had seen and fixed in the commit:
> eb0dc4bb2e22c04964d).
> 
> Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@...com>
> Cc: Deepak Sikri <deepak.sikri@...com>
> Cc: Shiraz Hashim <shiraz.hashim@...com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c |    9 +++------
>  1 files changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 8b69e3b..c92dcbc 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -1797,15 +1797,13 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
>  		priv->tx_skbuff[entry] = NULL;
>  		priv->hw->desc->prepare_tx_desc(desc, 0, len, csum_insertion,
>  						priv->mode);
> -		wmb();
> +		smp_wmb();
>  		priv->hw->desc->set_tx_owner(desc);

This looks pretty bogus to me.

If arch is UP, smp_wmb() is empty.

So why are you using it ?

Barrier here is not to protect the interaction with another cpu, but the
NIC itself.

If there is no need for barrier as you claim in changelog, don't add a
fake smp_wmb().



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