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>

2 files changed, 41 insertions, 9 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index e5ece8dceaa..bcf1083857f 100644
--- 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 94f42a15fff..f54d8b8a434 100644
--- 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);