netdev - Re: [PATCH] net: avoid one atomic op per cloned skb

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Date:	Wed, 19 May 2010 07:18:58 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: avoid one atomic op per cloned skb

Le mardi 18 mai 2010 à 20:58 +0200, Eric Dumazet a écrit :

> Oops, it needs more thinking, definitely not a 2.6.35 thing...
> 
> There would be a race between skb_clone() and kfree_skbmem()
> 
> kfree_skbmem() must perform the atomic_dec_and_test() before setting
> skb->fclone to SKB_FCLONE_UNAVAILABLE.
> 
> Doing so avoids dirtying skb->fclone right before kmem_cache_free()...
> 
> 
> V2 would be :
> 
> [RFC v2] net: avoid one atomic op per cloned skb
> 
> skb_clone() can use atomic_set(clone_ref, 2) safely, because only
> current thread can possibly touch clone_ref at this point.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c543dd2..77d5a6b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -370,13 +370,13 @@ static void kfree_skbmem(struct sk_buff *skb)
>  		fclone_ref = (atomic_t *) (skb + 1);
>  		other = skb - 1;
>  
> -		/* The clone portion is available for
> -		 * fast-cloning again.
> -		 */
> -		skb->fclone = SKB_FCLONE_UNAVAILABLE;
> -
>  		if (atomic_dec_and_test(fclone_ref))
>  			kmem_cache_free(skbuff_fclone_cache, other);
> +		else
> +			/* The clone portion is available for fast-cloning. 
> +			 * Note this must be done after the fclone_ref change.
> +			 */
> +			skb->fclone = SKB_FCLONE_UNAVAILABLE;

This still is racy, because we are not allowed to access skb after the
atomic_dec_and_test() :

Other thread can now go past the final refcount decrement and free skb
under us.

Hmm...

>  		break;
>  	}
>  }
> @@ -628,7 +628,7 @@ struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
>  	    n->fclone == SKB_FCLONE_UNAVAILABLE) {
>  		atomic_t *fclone_ref = (atomic_t *) (n + 1);
>  		n->fclone = SKB_FCLONE_CLONE;
> -		atomic_inc(fclone_ref);
> +		atomic_set(fclone_ref, 2);
>  	} else {
>  		n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
>  		if (!n)
> 



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