net: dont hold rtnl mutex during netlink dump callbacks

Four years ago, Patrick made a change to hold rtnl mutex during netlink
dump callbacks.

I believe it was a wrong move. This slows down concurrent dumps, making
good old /proc/net/ files faster than rtnetlink in some situations.

This occurred to me because one "ip link show dev ..." was _very_ slow
on a workload adding/removing network devices in background.

All dump callbacks are able to use RCU locking now, so this patch does
roughly a revert of commits :

1c2d670f366 : [RTNETLINK]: Hold rtnl_mutex during netlink dump callbacks
6313c1e0992 : [RTNETLINK]: Remove unnecessary locking in dump callbacks

This let writers fight for rtnl mutex and readers going full speed.

It also takes care of phonet : phonet_route_get() is now called from rcu
read section. I renamed it to phonet_route_get_rcu()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Remi Denis-Courmont <remi.denis-courmont@nokia.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index 0dcaa903e00e..404fa1591027 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -752,7 +752,8 @@ static int dn_nl_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)
         skip_naddr = cb->args[1];
 
         idx = 0;
-        for_each_netdev(&init_net, dev) {
+        rcu_read_lock();
+        for_each_netdev_rcu(&init_net, dev) {
                 if (idx < skip_ndevs)
                         goto cont;
                 else if (idx > skip_ndevs) {
@@ -761,11 +762,11 @@ static int dn_nl_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)
                         skip_naddr = 0;
                 }
 
-                if ((dn_db = rtnl_dereference(dev->dn_ptr)) == NULL)
+                if ((dn_db = rcu_dereference(dev->dn_ptr)) == NULL)
                         goto cont;
 
-                for (ifa = rtnl_dereference(dn_db->ifa_list), dn_idx = 0; ifa;
-                     ifa = rtnl_dereference(ifa->ifa_next), dn_idx++) {
+                for (ifa = rcu_dereference(dn_db->ifa_list), dn_idx = 0; ifa;
+                     ifa = rcu_dereference(ifa->ifa_next), dn_idx++) {
                         if (dn_idx < skip_naddr)
                                 continue;
 
@@ -778,6 +779,7 @@ cont:
                 idx++;
         }
 done:
+        rcu_read_unlock();
         cb->args[0] = idx;
         cb->args[1] = dn_idx;