can: convert protocol handling to RCU

This patch removes spin_locks at CAN socket creation time by using RCU.

Inspired by the discussion with Kurt van Dijck and Eric Dumazet the RCU code
was partly derived from af_phonet.c

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/can/af_can.c b/net/can/af_can.c
index 733d66f..a8dcaa4 100644 (file)
--- a/net/can/af_can.c
+++ b/net/can/af_can.c
@@ -85,7 +85,7 @@ static struct kmem_cache *rcv_cache __read_mostly;
 
 /* table of registered CAN protocols */
 static struct can_proto *proto_tab[CAN_NPROTO] __read_mostly;
-static DEFINE_SPINLOCK(proto_tab_lock);
+static DEFINE_MUTEX(proto_tab_lock);
 
 struct timer_list can_stattimer;   /* timer for statistics update */
 struct s_stats    can_stats;       /* packet statistics */
@@ -115,6 +115,19 @@ static void can_sock_destruct(struct sock *sk)
        skb_queue_purge(&sk->sk_receive_queue);
 }
 
+static struct can_proto *can_try_module_get(int protocol)
+{
+       struct can_proto *cp;
+
+       rcu_read_lock();
+       cp = rcu_dereference(proto_tab[protocol]);
+       if (cp && !try_module_get(cp->prot->owner))
+               cp = NULL;
+       rcu_read_unlock();
+
+       return cp;
+}
+
 static int can_create(struct net *net, struct socket *sock, int protocol,
                      int kern)
 {
@@ -130,9 +143,12 @@ static int can_create(struct net *net, struct socket *sock, int protocol,
        if (!net_eq(net, &init_net))
                return -EAFNOSUPPORT;
 
+       cp = can_try_module_get(protocol);
+
 #ifdef CONFIG_MODULES
-       /* try to load protocol module kernel is modular */
-       if (!proto_tab[protocol]) {
+       if (!cp) {
+               /* try to load protocol module if kernel is modular */
+
                err = request_module("can-proto-%d", protocol);
 
                /*
@@ -143,22 +159,18 @@ static int can_create(struct net *net, struct socket *sock, int protocol,
                if (err && printk_ratelimit())
                        printk(KERN_ERR "can: request_module "
                               "(can-proto-%d) failed.\n", protocol);
+
+               cp = can_try_module_get(protocol);
        }
 #endif
 
-       spin_lock(&proto_tab_lock);
-       cp = proto_tab[protocol];
-       if (cp && !try_module_get(cp->prot->owner))
-               cp = NULL;
-       spin_unlock(&proto_tab_lock);
-
        /* check for available protocol and correct usage */
 
        if (!cp)
                return -EPROTONOSUPPORT;
 
        if (cp->type != sock->type) {
-               err = -EPROTONOSUPPORT;
+               err = -EPROTOTYPE;
                goto errout;
        }
 
@@ -694,15 +706,16 @@ int can_proto_register(struct can_proto *cp)
        if (err < 0)
                return err;
 
-       spin_lock(&proto_tab_lock);
+       mutex_lock(&proto_tab_lock);
+
        if (proto_tab[proto]) {
                printk(KERN_ERR "can: protocol %d already registered\n",
                       proto);
                err = -EBUSY;
        } else
-               proto_tab[proto] = cp;
+               rcu_assign_pointer(proto_tab[proto], cp);
 
-       spin_unlock(&proto_tab_lock);
+       mutex_unlock(&proto_tab_lock);
 
        if (err < 0)
                proto_unregister(cp->prot);
@@ -719,13 +732,12 @@ void can_proto_unregister(struct can_proto *cp)
 {
        int proto = cp->protocol;
 
-       spin_lock(&proto_tab_lock);
-       if (!proto_tab[proto]) {
-               printk(KERN_ERR "BUG: can: protocol %d is not registered\n",
-                      proto);
-       }
-       proto_tab[proto] = NULL;
-       spin_unlock(&proto_tab_lock);
+       mutex_lock(&proto_tab_lock);
+       BUG_ON(proto_tab[proto] != cp);
+       rcu_assign_pointer(proto_tab[proto], NULL);
+       mutex_unlock(&proto_tab_lock);
+
+       synchronize_rcu();
 
        proto_unregister(cp->prot);
 }