net: Introduce dev_get_by_name_rcu()

Some workloads hit dev_base_lock rwlock pretty hard.
We can use RCU lookups to avoid touching this rwlock
(and avoid touching netdevice refcount)

netdevices are already freed after a RCU grace period, so this patch
adds no penalty at device dismantle time.

However, it adds a synchronize_rcu() call in dev_change_name()

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

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index e5ece8d..bcf1083 100644 (file)
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1115,6 +1115,7 @@ extern void               __dev_remove_pack(struct packet_type *pt);
 extern struct net_device       *dev_get_by_flags(struct net *net, unsigned short flags,
                                                  unsigned short mask);
 extern struct net_device       *dev_get_by_name(struct net *net, const char *name);
+extern struct net_device       *dev_get_by_name_rcu(struct net *net, const char *name);
 extern struct net_device       *__dev_get_by_name(struct net *net, const char *name);
 extern int             dev_alloc_name(struct net_device *dev, const char *name);
 extern int             dev_open(struct net_device *dev);

diff --git a/net/core/dev.c b/net/core/dev.c
index 94f42a1..f54d8b8 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -213,7 +213,7 @@ static int list_netdevice(struct net_device *dev)
 
        write_lock_bh(&dev_base_lock);
        list_add_tail(&dev->dev_list, &net->dev_base_head);
-       hlist_add_head(&dev->name_hlist, dev_name_hash(net, dev->name));
+       hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, dev->name));
        hlist_add_head_rcu(&dev->index_hlist,
                           dev_index_hash(net, dev->ifindex));
        write_unlock_bh(&dev_base_lock);
@@ -230,7 +230,7 @@ static void unlist_netdevice(struct net_device *dev)
        /* Unlink dev from the device chain */
        write_lock_bh(&dev_base_lock);
        list_del(&dev->dev_list);
-       hlist_del(&dev->name_hlist);
+       hlist_del_rcu(&dev->name_hlist);
        hlist_del_rcu(&dev->index_hlist);
        write_unlock_bh(&dev_base_lock);
 }
@@ -599,6 +599,32 @@ struct net_device *__dev_get_by_name(struct net *net, const char *name)
 EXPORT_SYMBOL(__dev_get_by_name);
 
 /**
+ *     dev_get_by_name_rcu     - find a device by its name
+ *     @net: the applicable net namespace
+ *     @name: name to find
+ *
+ *     Find an interface by name.
+ *     If the name is found a pointer to the device is returned.
+ *     If the name is not found then %NULL is returned.
+ *     The reference counters are not incremented so the caller must be
+ *     careful with locks. The caller must hold RCU lock.
+ */
+
+struct net_device *dev_get_by_name_rcu(struct net *net, const char *name)
+{
+       struct hlist_node *p;
+       struct net_device *dev;
+       struct hlist_head *head = dev_name_hash(net, name);
+
+       hlist_for_each_entry_rcu(dev, p, head, name_hlist)
+               if (!strncmp(dev->name, name, IFNAMSIZ))
+                       return dev;
+
+       return NULL;
+}
+EXPORT_SYMBOL(dev_get_by_name_rcu);
+
+/**
  *     dev_get_by_name         - find a device by its name
  *     @net: the applicable net namespace
  *     @name: name to find
@@ -614,11 +640,11 @@ struct net_device *dev_get_by_name(struct net *net, const char *name)
 {
        struct net_device *dev;
 
-       read_lock(&dev_base_lock);
-       dev = __dev_get_by_name(net, name);
+       rcu_read_lock();
+       dev = dev_get_by_name_rcu(net, name);
        if (dev)
                dev_hold(dev);
-       read_unlock(&dev_base_lock);
+       rcu_read_unlock();
        return dev;
 }
 EXPORT_SYMBOL(dev_get_by_name);
@@ -960,7 +986,12 @@ rollback:
 
        write_lock_bh(&dev_base_lock);
        hlist_del(&dev->name_hlist);
-       hlist_add_head(&dev->name_hlist, dev_name_hash(net, dev->name));
+       write_unlock_bh(&dev_base_lock);
+
+       synchronize_rcu();
+
+       write_lock_bh(&dev_base_lock);
+       hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, dev->name));
        write_unlock_bh(&dev_base_lock);
 
        ret = call_netdevice_notifiers(NETDEV_CHANGENAME, dev);
@@ -1062,9 +1093,9 @@ void dev_load(struct net *net, const char *name)
 {
        struct net_device *dev;
 
-       read_lock(&dev_base_lock);
-       dev = __dev_get_by_name(net, name);
-       read_unlock(&dev_base_lock);
+       rcu_read_lock();
+       dev = dev_get_by_name_rcu(net, name);
+       rcu_read_unlock();
 
        if (!dev && capable(CAP_NET_ADMIN))
                request_module("%s", name);