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

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 18 mai 2010 à 15:40 +0200, Eric Dumazet a écrit :
> Hi David
> 
> I know you said 'only patches', but I found following patch small
> enough ?
> 

> Thanks
> 
> [PATCH] 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.
>    
> Add a WARN_ON_ONCE() for a while, to catch wrong assumptions.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> ---
>  net/core/skbuff.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c543dd2..4444f15 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -628,7 +628,8 @@ 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);
> +		WARN_ON_ONCE(atomic_read(fclone_ref) != 1);
> +		atomic_set(fclone_ref, 2);
>  	} else {
>  		n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
>  		if (!n)
> 
> 

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@gmail.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;
 		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)