netdev - Re: [PATCH 05/11] netpoll: dont need rx

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Message-Id: <20071119.193928.167909754.davem@davemloft.net>
Date:	Mon, 19 Nov 2007 19:39:28 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	satyam@...radead.org, netdev@...r.kernel.org
Subject: Re: [PATCH 05/11] netpoll: dont need rx_flags

From: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Sat, 03 Nov 2007 11:43:19 -0700

> The rx_flags variable is redundant. Turning rx on/off is done
> via setting the rx_np pointer.
> 
> Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>

Not all of these transformations are equivalent, and as a result
you're adding a bug.

> @@ -51,12 +50,11 @@ static inline int netpoll_rx(struct sk_b
>  	unsigned long flags;
>  	int ret = 0;
>  
> -	if (!npinfo || (!npinfo->rx_np && !npinfo->rx_flags))
> +	if (!npinfo || !npinfo->rx_np)
>  		return 0;
>  
>  	spin_lock_irqsave(&npinfo->rx_lock, flags);
> -	/* check rx_flags again with the lock held */
> -	if (npinfo->rx_flags && __netpoll_rx(skb))
> +	if (__netpoll_rx(skb))
>  		ret = 1;
>  	spin_unlock_irqrestore(&npinfo->rx_lock, flags);

If you're using ->rx_np == NULL as your new guard, you have
to duplicate that test inside of holding the lock.

->rx_np goes to NULL under the lock, but in the previous code if
the rx_flags is set we know that ->rx_np is also NULL, that's
why it was OK to only recheck ->rx_flags in the lock and not
->rx_np as well.

Therefore we have to replace the test on ->rx_flags with ->rx_np in
all spots to retain correct semantics.

I've made this correction to your patch and applied it to net-2.6.25

Thanks.
-
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