[PATCH net-next 2/2] pppoe: drop sock reference counting on fast path

[Qingfang Deng]

Now that PPPoE sockets are freed via RCU (SOCK_RCU_FREE), it is no longer
necessary to take a reference count when looking up sockets on the receive
path. Readers are protected by RCU, so the socket memory remains valid
until after a grace period.

Convert fast-path lookups to avoid refcounting:
 - Replace get_item() and sk_receive_skb() in pppoe_rcv() with
   __get_item() and __sk_receive_skb().
 - Rework get_item_by_addr() into __get_item_by_addr() (no refcount)
   and add check_item_by_addr() for existence checks in pppoe_ioctl().
 - Remove unnecessary sock_put() calls.

This avoids cacheline bouncing from atomic reference counting and improves
performance on the receive fast path.

Signed-off-by: Qingfang Deng <dqfext@xxxxxxxxx>
---
 drivers/net/ppp/pppoe.c | 54 +++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 23 deletions(-)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index f99533c80b66..70a7e1e88799 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -237,13 +237,30 @@ static inline struct pppox_sock *get_item(struct pppoe_net *pn, __be16 sid,
 	return po;
 }
 
-static inline struct pppox_sock *get_item_by_addr(struct net *net,
-						struct sockaddr_pppox *sp)
+static inline struct pppox_sock *__get_item_by_addr(struct net *net,
+						    struct sockaddr_pppox *sp)
 {
 	struct net_device *dev;
 	struct pppoe_net *pn;
 	struct pppox_sock *pppox_sock = NULL;
+	int ifindex;
+
+	dev = dev_get_by_name_rcu(net, sp->sa_addr.pppoe.dev);
+	if (dev) {
+		ifindex = dev->ifindex;
+		pn = pppoe_pernet(net);
+		pppox_sock = __get_item(pn, sp->sa_addr.pppoe.sid,
+					sp->sa_addr.pppoe.remote, ifindex);
+	}
+	return pppox_sock;
+}
 
+static inline bool check_item_by_addr(struct net *net,
+				      struct sockaddr_pppox *sp)
+{
+	struct net_device *dev;
+	struct pppoe_net *pn;
+	bool ret = false;
 	int ifindex;
 
 	rcu_read_lock();
@@ -251,11 +268,11 @@ static inline struct pppox_sock *get_item_by_addr(struct net *net,
 	if (dev) {
 		ifindex = dev->ifindex;
 		pn = pppoe_pernet(net);
-		pppox_sock = get_item(pn, sp->sa_addr.pppoe.sid,
-				sp->sa_addr.pppoe.remote, ifindex);
+		ret = !!__get_item(pn, sp->sa_addr.pppoe.sid,
+				   sp->sa_addr.pppoe.remote, ifindex);
 	}
 	rcu_read_unlock();
-	return pppox_sock;
+	return ret;
 }
 
 static inline void delete_item(struct pppoe_net *pn, __be16 sid,
@@ -381,18 +398,16 @@ static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
 	if (sk->sk_state & PPPOX_BOUND) {
 		ppp_input(&po->chan, skb);
 	} else if (sk->sk_state & PPPOX_RELAY) {
-		relay_po = get_item_by_addr(sock_net(sk),
-					    &po->pppoe_relay);
+		relay_po = __get_item_by_addr(sock_net(sk),
+					      &po->pppoe_relay);
 		if (relay_po == NULL)
 			goto abort_kfree;
 
 		if ((sk_pppox(relay_po)->sk_state & PPPOX_CONNECTED) == 0)
-			goto abort_put;
+			goto abort_kfree;
 
 		if (!__pppoe_xmit(sk_pppox(relay_po), skb))
-			goto abort_put;
-
-		sock_put(sk_pppox(relay_po));
+			goto abort_kfree;
 	} else {
 		if (sock_queue_rcv_skb(sk, skb))
 			goto abort_kfree;
@@ -400,9 +415,6 @@ static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
 
 	return NET_RX_SUCCESS;
 
-abort_put:
-	sock_put(sk_pppox(relay_po));
-
 abort_kfree:
 	kfree_skb(skb);
 	return NET_RX_DROP;
@@ -447,14 +459,11 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
 	ph = pppoe_hdr(skb);
 	pn = pppoe_pernet(dev_net(dev));
 
-	/* Note that get_item does a sock_hold(), so sk_pppox(po)
-	 * is known to be safe.
-	 */
-	po = get_item(pn, ph->sid, eth_hdr(skb)->h_source, dev->ifindex);
+	po = __get_item(pn, ph->sid, eth_hdr(skb)->h_source, dev->ifindex);
 	if (!po)
 		goto drop;
 
-	return sk_receive_skb(sk_pppox(po), skb, 0);
+	return __sk_receive_skb(sk_pppox(po), skb, 0, 1, false);
 
 drop:
 	kfree_skb(skb);
@@ -790,7 +799,7 @@ static int pppoe_ioctl(struct socket *sock, unsigned int cmd,
 
 	case PPPOEIOCSFWD:
 	{
-		struct pppox_sock *relay_po;
+		bool ret;
 
 		err = -EBUSY;
 		if (sk->sk_state & (PPPOX_BOUND | PPPOX_DEAD))
@@ -815,11 +824,10 @@ static int pppoe_ioctl(struct socket *sock, unsigned int cmd,
 
 		/* Check that the socket referenced by the address
 		   actually exists. */
-		relay_po = get_item_by_addr(sock_net(sk), &po->pppoe_relay);
-		if (!relay_po)
+		ret = check_item_by_addr(sock_net(sk), &po->pppoe_relay);
+		if (!ret)
 			break;
 
-		sock_put(sk_pppox(relay_po));
 		sk->sk_state |= PPPOX_RELAY;
 		err = 0;
 		break;
-- 
2.43.0