net: avoid one atomic operation in skb_clone()

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>

1 files changed, 17 insertions, 6 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a8cebb40699c..f77e64873caf 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -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;