net: Introduce net_rwsem to protect net_namespace_list

rtnl_lock() is used everywhere, and contention is very high.
When someone wants to iterate over alive net namespaces,
he/she has no a possibility to do that without exclusive lock.
But the exclusive rtnl_lock() in such places is overkill,
and it just increases the contention. Yes, there is already
for_each_net_rcu() in kernel, but it requires rcu_read_lock(),
and this can't be sleepable. Also, sometimes it may be need
really prevent net_namespace_list growth, so for_each_net_rcu()
is not fit there.

This patch introduces new rw_semaphore, which will be used
instead of rtnl_mutex to protect net_namespace_list. It is
sleepable and allows not-exclusive iterations over net
namespaces list. It allows to stop using rtnl_lock()
in several places (what is made in next patches) and makes
less the time, we keep rtnl_mutex. Here we just add new lock,
while the explanation of we can remove rtnl_lock() there are
in next patches.

Fine grained locks generally are better, then one big lock,
so let's do that with net_namespace_list, while the situation
allows that.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

7 files changed, 31 insertions, 5 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index e13807b5c84d..eca5458b2753 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1629,6 +1629,7 @@ int register_netdevice_notifier(struct notifier_block *nb)
                 goto unlock;
         if (dev_boot_phase)
                 goto unlock;
+        down_read(&net_rwsem);
         for_each_net(net) {
                 for_each_netdev(net, dev) {
                         err = call_netdevice_notifier(nb, NETDEV_REGISTER, dev);
@@ -1642,6 +1643,7 @@ int register_netdevice_notifier(struct notifier_block *nb)
                         call_netdevice_notifier(nb, NETDEV_UP, dev);
                 }
         }
+        up_read(&net_rwsem);
 
 unlock:
         rtnl_unlock();
@@ -1664,6 +1666,7 @@ rollback:
         }
 
 outroll:
+        up_read(&net_rwsem);
         raw_notifier_chain_unregister(&netdev_chain, nb);
         goto unlock;
 }
@@ -1694,6 +1697,7 @@ int unregister_netdevice_notifier(struct notifier_block *nb)
         if (err)
                 goto unlock;
 
+        down_read(&net_rwsem);
         for_each_net(net) {
                 for_each_netdev(net, dev) {
                         if (dev->flags & IFF_UP) {
@@ -1704,6 +1708,7 @@ int unregister_netdevice_notifier(struct notifier_block *nb)
                         call_netdevice_notifier(nb, NETDEV_UNREGISTER, dev);
                 }
         }
+        up_read(&net_rwsem);
 unlock:
         rtnl_unlock();
         return err;
diff --git a/net/core/fib_notifier.c b/net/core/fib_notifier.c
index 0c048bdeb016..614b985c92a4 100644
--- a/net/core/fib_notifier.c
+++ b/net/core/fib_notifier.c
@@ -33,6 +33,7 @@ static unsigned int fib_seq_sum(void)
         struct net *net;
 
         rtnl_lock();
+        down_read(&net_rwsem);
         for_each_net(net) {
                 rcu_read_lock();
                 list_for_each_entry_rcu(ops, &net->fib_notifier_ops, list) {
@@ -43,6 +44,7 @@ static unsigned int fib_seq_sum(void)
                 }
                 rcu_read_unlock();
         }
+        up_read(&net_rwsem);
         rtnl_unlock();
 
         return fib_seq;
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index b5796d17a302..7fdf321d4997 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -33,6 +33,10 @@ static struct list_head *first_device = &pernet_list;
 LIST_HEAD(net_namespace_list);
 EXPORT_SYMBOL_GPL(net_namespace_list);
 
+/* Protects net_namespace_list. Nests iside rtnl_lock() */
+DECLARE_RWSEM(net_rwsem);
+EXPORT_SYMBOL_GPL(net_rwsem);
+
 struct net init_net = {
         .count          = REFCOUNT_INIT(1),
         .dev_base_head  = LIST_HEAD_INIT(init_net.dev_base_head),
@@ -309,9 +313,9 @@ static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
                 if (error < 0)
                         goto out_undo;
         }
-        rtnl_lock();
+        down_write(&net_rwsem);
         list_add_tail_rcu(&net->list, &net_namespace_list);
-        rtnl_unlock();
+        up_write(&net_rwsem);
 out:
         return error;
 
@@ -450,7 +454,7 @@ static void unhash_nsid(struct net *net, struct net *last)
          * and this work is the only process, that may delete
          * a net from net_namespace_list. So, when the below
          * is executing, the list may only grow. Thus, we do not
-         * use for_each_net_rcu() or rtnl_lock().
+         * use for_each_net_rcu() or net_rwsem.
          */
         for_each_net(tmp) {
                 int id;
@@ -485,7 +489,7 @@ static void cleanup_net(struct work_struct *work)
         down_read(&pernet_ops_rwsem);
 
         /* Don't let anyone else find us. */
-        rtnl_lock();
+        down_write(&net_rwsem);
         llist_for_each_entry(net, net_kill_list, cleanup_list)
                 list_del_rcu(&net->list);
         /* Cache last net. After we unlock rtnl, no one new net
@@ -499,7 +503,7 @@ static void cleanup_net(struct work_struct *work)
          * useless anyway, as netns_ids are destroyed there.
          */
         last = list_last_entry(&net_namespace_list, struct net, list);
-        rtnl_unlock();
+        up_write(&net_rwsem);
 
         llist_for_each_entry(net, net_kill_list, cleanup_list) {
                 unhash_nsid(net, last);
@@ -900,6 +904,9 @@ static int __register_pernet_operations(struct list_head *list,
 
         list_add_tail(&ops->list, list);
         if (ops->init || (ops->id && ops->size)) {
+                /* We held write locked pernet_ops_rwsem, and parallel
+                 * setup_net() and cleanup_net() are not possible.
+                 */
                 for_each_net(net) {
                         error = ops_init(ops, net);
                         if (error)
@@ -923,6 +930,7 @@ static void __unregister_pernet_operations(struct pernet_operations *ops)
         LIST_HEAD(net_exit_list);
 
         list_del(&ops->list);
+        /* See comment in __register_pernet_operations() */
         for_each_net(net)
                 list_add_tail(&net->exit_list, &net_exit_list);
         ops_exit_list(ops, &net_exit_list);
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 2d3949789cef..e86b28482ca7 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -418,9 +418,11 @@ void __rtnl_link_unregister(struct rtnl_link_ops *ops)
 {
         struct net *net;
 
+        down_read(&net_rwsem);
         for_each_net(net) {
                 __rtnl_kill_links(net, ops);
         }
+        up_read(&net_rwsem);
         list_del(&ops->list);
 }
 EXPORT_SYMBOL_GPL(__rtnl_link_unregister);
@@ -438,6 +440,9 @@ static void rtnl_lock_unregistering_all(void)
         for (;;) {
                 unregistering = false;
                 rtnl_lock();
+                /* We held write locked pernet_ops_rwsem, and parallel
+                 * setup_net() and cleanup_net() are not possible.
+                 */
                 for_each_net(net) {
                         if (net->dev_unreg_count > 0) {
                                 unregistering = true;
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 705198de671d..370f9b7f051b 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -1764,12 +1764,14 @@ nf_ct_iterate_destroy(int (*iter)(struct nf_conn *i, void *data), void *data)
         struct net *net;
 
         rtnl_lock();
+        down_read(&net_rwsem);
         for_each_net(net) {
                 if (atomic_read(&net->ct.count) == 0)
                         continue;
                 __nf_ct_unconfirmed_destroy(net);
                 nf_queue_nf_hook_drop(net);
         }
+        up_read(&net_rwsem);
         rtnl_unlock();
 
         /* Need to wait for netns cleanup worker to finish, if its
diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index ef38e5aecd28..9746ee30a99b 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -2364,8 +2364,10 @@ static void __net_exit ovs_exit_net(struct net *dnet)
                 __dp_destroy(dp);
 
         rtnl_lock();
+        down_read(&net_rwsem);
         for_each_net(net)
                 list_vports_from_net(net, dnet, &head);
+        up_read(&net_rwsem);
         rtnl_unlock();
 
         /* Detach all vports from given namespace. */
diff --git a/net/wireless/wext-core.c b/net/wireless/wext-core.c
index 9efbfc753347..544d7b62d7ca 100644
--- a/net/wireless/wext-core.c
+++ b/net/wireless/wext-core.c
@@ -349,11 +349,13 @@ void wireless_nlevent_flush(void)
 
         ASSERT_RTNL();
 
+        down_read(&net_rwsem);
         for_each_net(net) {
                 while ((skb = skb_dequeue(&net->wext_nlevents)))
                         rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL,
                                     GFP_KERNEL);
         }
+        up_read(&net_rwsem);
 }
 EXPORT_SYMBOL_GPL(wireless_nlevent_flush);