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>

1 files changed, 5 insertions, 0 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;