net: Revert "net: avoid one atomic operation in skb_clone()"

Not sure what I was thinking, but doing anything after
releasing a refcount is suicidal or/and embarrassing.

By the time we set skb->fclone to SKB_FCLONE_FREE, another cpu
could have released last reference and freed whole skb.

We potentially corrupt memory or trap if CONFIG_DEBUG_PAGEALLOC is set.
Reported-by: NChris Mason <clm@fb.com>
Fixes: ce1a4ea3 ("net: avoid one atomic operation in skb_clone()")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -552,20 +552,13 @@ static void kfree_skbmem(struct sk_buff *skb)
 	case SKB_FCLONE_CLONE:
 		fclones = container_of(skb, struct sk_buff_fclones, skb2);
 
-		/* Warning : We must perform the atomic_dec_and_test() before
-		 * setting skb->fclone back to SKB_FCLONE_FREE, 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.
-		 */
-		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_FREE;
-		}
+
+		if (atomic_dec_and_test(&fclones->fclone_ref))
+			kmem_cache_free(skbuff_fclone_cache, fclones);
 		break;
 	}
 }
@@ -887,11 +880,7 @@ struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
 	if (skb->fclone == SKB_FCLONE_ORIG &&
 	    n->fclone == SKB_FCLONE_FREE) {
 		n->fclone = SKB_FCLONE_CLONE;
-		/* 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);
+		atomic_inc(&fclones->fclone_ref);
 	} else {
 		if (skb_pfmemalloc(skb))
 			gfp_mask |= __GFP_MEMALLOC;