rtnetlink: Add per-netns RTNL.

The goal is to break RTNL down into per-netns mutex.

This patch adds per-netns mutex and its helper functions, rtnl_net_lock()
and rtnl_net_unlock().

rtnl_net_lock() acquires the global RTNL and per-netns RTNL mutex, and
rtnl_net_unlock() releases them.

We will replace 800+ rtnl_lock() with rtnl_net_lock() and finally removes
rtnl_lock() in rtnl_net_lock().

When we need to nest per-netns RTNL mutex, we will use __rtnl_net_lock(),
and its locking order is defined by rtnl_net_lock_cmp_fn() as follows:

1. init_net is first
2. netns address ascending order

Note that the conversion will be done under CONFIG_DEBUG_NET_SMALL_RTNL
with LOCKDEP so that we can carefully add the extra mutex without slowing
down RTNL operations during conversion.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

include/linux/rtnetlink.h

···
 #define rcu_replace_pointer_rtnl(rp, p)			\
 	rcu_replace_pointer(rp, p, lockdep_rtnl_is_held())
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+void __rtnl_net_lock(struct net *net);
+void __rtnl_net_unlock(struct net *net);
+void rtnl_net_lock(struct net *net);
+void rtnl_net_unlock(struct net *net);
+int rtnl_net_lock_cmp_fn(const struct lockdep_map *a, const struct lockdep_map *b);
+#else
+static inline void __rtnl_net_lock(struct net *net) {}
+static inline void __rtnl_net_unlock(struct net *net) {}
+
+static inline void rtnl_net_lock(struct net *net)
+{
+	rtnl_lock();
+}
+
+static inline void rtnl_net_unlock(struct net *net)
+{
+	rtnl_unlock();
+}
+#endif
+
 static inline struct netdev_queue *dev_ingress_queue(struct net_device *dev)
 {
 	return rtnl_dereference(dev->ingress_queue);

include/net/net_namespace.h

···
 #if IS_ENABLED(CONFIG_SMC)
 	struct netns_smc	smc;
 #endif
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	/* Move to a better place when the config guard is removed. */
+	struct mutex		rtnl_mutex;
+#endif
 } __randomize_layout;
 
 #include <linux/seq_file_net.h>

net/Kconfig.debug

···
 	help
 	  Enable extra sanity checks in networking.
 	  This is mostly used by fuzzers, but is safe to select.
+
+config DEBUG_NET_SMALL_RTNL
+	bool "Add extra per-netns mutex inside RTNL"
+	depends on DEBUG_KERNEL && NET && LOCK_DEBUGGING_SUPPORT
+	select PROVE_LOCKING
+	default n
+	help
+	  rtnl_lock() is being replaced with rtnl_net_lock() that
+	  acquires the global RTNL and a small per-netns RTNL mutex.
+
+	  During the conversion, rtnl_net_lock() just adds an extra
+	  mutex in every RTNL scope and slows down the operations.
+
+	  Once the conversion completes, rtnl_lock() will be removed
+	  and rtnetlink will gain per-netns scalability.

net/core/net_namespace.c

···
 	idr_init(&net->netns_ids);
 	spin_lock_init(&net->nsid_lock);
 	mutex_init(&net->ipv4.ra_mutex);
+
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+	mutex_init(&net->rtnl_mutex);
+	lock_set_cmp_fn(&net->rtnl_mutex, rtnl_net_lock_cmp_fn, NULL);
+#endif
+
 	preinit_net_sysctl(net);
 }
 

net/core/rtnetlink.c

···
 EXPORT_SYMBOL(lockdep_rtnl_is_held);
 #endif /* #ifdef CONFIG_PROVE_LOCKING */
 
+#ifdef CONFIG_DEBUG_NET_SMALL_RTNL
+void __rtnl_net_lock(struct net *net)
+{
+	ASSERT_RTNL();
+
+	mutex_lock(&net->rtnl_mutex);
+}
+EXPORT_SYMBOL(__rtnl_net_lock);
+
+void __rtnl_net_unlock(struct net *net)
+{
+	ASSERT_RTNL();
+
+	mutex_unlock(&net->rtnl_mutex);
+}
+EXPORT_SYMBOL(__rtnl_net_unlock);
+
+void rtnl_net_lock(struct net *net)
+{
+	rtnl_lock();
+	__rtnl_net_lock(net);
+}
+EXPORT_SYMBOL(rtnl_net_lock);
+
+void rtnl_net_unlock(struct net *net)
+{
+	__rtnl_net_unlock(net);
+	rtnl_unlock();
+}
+EXPORT_SYMBOL(rtnl_net_unlock);
+
+static int rtnl_net_cmp_locks(const struct net *net_a, const struct net *net_b)
+{
+	if (net_eq(net_a, net_b))
+		return 0;
+
+	/* always init_net first */
+	if (net_eq(net_a, &init_net))
+		return -1;
+
+	if (net_eq(net_b, &init_net))
+		return 1;
+
+	/* otherwise lock in ascending order */
+	return net_a < net_b ? -1 : 1;
+}
+
+int rtnl_net_lock_cmp_fn(const struct lockdep_map *a, const struct lockdep_map *b)
+{
+	const struct net *net_a, *net_b;
+
+	net_a = container_of(a, struct net, rtnl_mutex.dep_map);
+	net_b = container_of(b, struct net, rtnl_mutex.dep_map);
+
+	return rtnl_net_cmp_locks(net_a, net_b);
+}
+#endif
+
 static struct rtnl_link __rcu *__rcu *rtnl_msg_handlers[RTNL_FAMILY_MAX + 1];
 
 static inline int rtm_msgindex(int msgtype)