net: add a noref bit on skb dst

Use low order bit of skb->_skb_dst to tell dst is not refcounted.

Change _skb_dst to _skb_refdst to make sure all uses are catched.

skb_dst() returns the dst, regardless of noref bit set or not, but
with a lockdep check to make sure a noref dst is not given if current
user is not rcu protected.

New skb_dst_set_noref() helper to set an notrefcounted dst on a skb.
(with lockdep check)

skb_dst_drop() drops a reference only if skb dst was refcounted.

skb_dst_force() helper is used to force a refcount on dst, when skb
is queued and not anymore RCU protected.

Use skb_dst_force() in __sk_add_backlog(), __dev_xmit_skb() if
!IFF_XMIT_DST_RELEASE or skb enqueued on qdisc queue, in
sock_queue_rcv_skb(), in __nf_queue().

Use skb_dst_force() in dev_requeue_skb().

Note: dst_use_noref() still dirties dst, we might transform it
later to do one dirtying per jiffies.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 5 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index aed16eb..5697caf 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -600,12 +600,15 @@ static inline int sk_stream_memory_free(struct sock *sk)
 /* OOB backlog add */
 static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
-	if (!sk->sk_backlog.tail) {
-		sk->sk_backlog.head = sk->sk_backlog.tail = skb;
-	} else {
+	/* dont let skb dst not refcounted, we are going to leave rcu lock */
+	skb_dst_force(skb);
+
+	if (!sk->sk_backlog.tail)
+		sk->sk_backlog.head = skb;
+	else
 		sk->sk_backlog.tail->next = skb;
-		sk->sk_backlog.tail = skb;
-	}
+
+	sk->sk_backlog.tail = skb;
 	skb->next = NULL;
 }