net: skb_free_datagram_locked() fix

Commit 4b0b72f7 ( net: speedup udp receive path )
introduced a bug in skb_free_datagram_locked().

We should not skb_orphan() skb if we dont have the guarantee we are the
last skb user, this might happen with MSG_PEEK concurrent users.

To keep socket locked for the smallest period of time, we split
consume_skb() logic, inlined in skb_free_datagram_locked()
Reported-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/datagram.c

@@ -229,13 +229,18 @@ EXPORT_SYMBOL(skb_free_datagram);
 
 void skb_free_datagram_locked(struct sock *sk, struct sk_buff *skb)
 {
+	if (likely(atomic_read(&skb->users) == 1))
+		smp_rmb();
+	else if (likely(!atomic_dec_and_test(&skb->users)))
+		return;
+
 	lock_sock_bh(sk);
 	skb_orphan(skb);
 	sk_mem_reclaim_partial(sk);
 	unlock_sock_bh(sk);
 
-	/* skb is now orphaned, might be freed outside of locked section */
-	consume_skb(skb);
+	/* skb is now orphaned, can be freed outside of locked section */
+	__kfree_skb(skb);
 }
 EXPORT_SYMBOL(skb_free_datagram_locked);