netdev - Re: [PATCH v2] gianfar: Fix invalid TX frames returned on error queue when time stamping.

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Date:	Mon, 09 Jan 2012 16:12:00 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Manfred Rudigier <manfred.rudigier@...cron.at>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	afleming@...escale.com, avorontsov@...sta.com,
	richardcochran@...il.com
Subject: Re: [PATCH v2] gianfar: Fix invalid TX frames returned on error
 queue when time stamping.

Le lundi 09 janvier 2012 à 15:52 +0100, Manfred Rudigier a écrit :
> When TX time stamping for PTP messages is enabled on a socket, a time
> stamp is returned on the socket error queue to the user space application
> after the frame was transmitted. The transmitted frame is also returned on
> the error queue so that an application knows to which frame the time stamp
> belongs.

...

>  	/* make space for additional header when fcb is needed */
>  	if (((skb->ip_summed == CHECKSUM_PARTIAL) ||
>  			vlan_tx_tag_present(skb) ||
>  			unlikely(do_tstamp)) &&
> -			(skb_headroom(skb) < GMAC_FCB_LEN)) {
> +			(skb_headroom(skb) < fcb_length)) {
>  		struct sk_buff *skb_new;
>  
> -		skb_new = skb_realloc_headroom(skb, GMAC_FCB_LEN);
> +		skb_new = skb_realloc_headroom(skb, fcb_length);
>  		if (!skb_new) {
>  			dev->stats.tx_errors++;
>  			kfree_skb(skb);
>  			return NETDEV_TX_OK;
>  		}
> +
> +		/* Hold sock reference for processing TX time stamps */
> +		if (unlikely(do_tstamp)) {
> +			struct sock *sk = skb->sk;

Not clear to me why sk_refcnt could be 0 here

> +			if (!atomic_inc_not_zero(&sk->sk_refcnt)) {
> +				dev->stats.tx_errors++;
> +				kfree_skb(skb_new);
> +				kfree_skb(skb);
> +				return NETDEV_TX_OK;
> +			}
> +			skb_new->sk = skb->sk;
> +		}
> +
>  		kfree_skb(skb);
>  		skb = skb_new;

Not clear to me why ->destructor access is not needed here.

This change should be isolated from your patch IMHO

I would rather do something like :

diff --git a/drivers/net/ethernet/freescale/gianfar.c b/drivers/net/ethernet/freescale/gianfar.c
index e01cdaa..8e00461 100644
--- a/drivers/net/ethernet/freescale/gianfar.c
+++ b/drivers/net/ethernet/freescale/gianfar.c
@@ -2086,6 +2086,8 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
 			kfree_skb(skb);
 			return NETDEV_TX_OK;
 		}
+		swap(skb_new->sk, skb->sk);
+		swap(skb_new->destructor, skb->destructor);
 		kfree_skb(skb);
 		skb = skb_new;
 	}


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