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net: avoid one atomic operation in skb_clone()

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Fast clone cloning can actually avoid an atomic_inc(), if we
guarantee prior clone_ref value is 1.

This requires a change kfree_skbmem(), to perform the
atomic_dec_and_test() on clone_ref before setting fclone to
SKB_FCLONE_UNAVAILABLE.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2014-10-01 15:27:15 -07:00 • committed by David S. Miller
commit ce1a4ea3f1
1 changed files with 17 additions and 6 deletions
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23
net/core/skbuff.c
View file

@ -541,13 +541,20 @@ static void kfree_skbmem(struct sk_buff *skb)
case SKB_FCLONE_CLONE:
fclones = container_of(skb, struct sk_buff_fclones, skb2);
/* The clone portion is available for
* fast-cloning again.
/* Warning : We must perform the atomic_dec_and_test() before
* setting skb->fclone back to SKB_FCLONE_UNAVAILABLE, otherwise
* skb_clone() could set clone_ref to 2 before our decrement.
* Anyway, if we are going to free the structure, no need to
* rewrite skb->fclone.
*/
skb->fclone = SKB_FCLONE_UNAVAILABLE;
if (atomic_dec_and_test(&fclones->fclone_ref))
if (atomic_dec_and_test(&fclones->fclone_ref)) {
kmem_cache_free(skbuff_fclone_cache, fclones);
} else {
/* The clone portion is available for
* fast-cloning again.
*/
skb->fclone = SKB_FCLONE_UNAVAILABLE;
}
break;
}
}
@ -869,7 +876,11 @@ struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
if (skb->fclone == SKB_FCLONE_ORIG &&
n->fclone == SKB_FCLONE_UNAVAILABLE) {
n->fclone = SKB_FCLONE_CLONE;
atomic_inc(&fclones->fclone_ref);
/* As our fastclone was free, clone_ref must be 1 at this point.
* We could use atomic_inc() here, but it is faster
* to set the final value.
*/
atomic_set(&fclones->fclone_ref, 2);
} else {
if (skb_pfmemalloc(skb))
gfp_mask |= __GFP_MEMALLOC;