netdev - Re: [PATCH net-next 09/14] tg3: Improve small packet performance

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Date:	Thu, 5 Aug 2010 08:27:41 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Matt Carlson <mcarlson@...adcom.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, andy@...yhouse.net,
	Michael Chan <mchan@...adcom.com>
Subject: Re: [PATCH net-next 09/14] tg3: Improve small packet performance


Hi,

Just saw this go in:
  
>  static inline u32 tg3_tx_avail(struct tg3_napi *tnapi)
>  {
> -	smp_mb();
> +	/* Tell compiler to fetch tx indices from memory. */
> +	barrier();
>  	return tnapi->tx_pending -
>  	       ((tnapi->tx_prod - tnapi->tx_cons) & (TG3_TX_RING_SIZE - 1));
>  }

Which worries me. Are we sure we don't need any ordering (eg smp_rmb)?
A compiler barrier does nothing to ensure two loads are ordered.

Anton
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