rtnl: add helper to check if rtnl group has listeners

[ Upstream commit c5e2a973448d958feb7881e4d875eac59fdeff3d ]

As of today, rtnl code creates a new skb and unconditionally fills and
broadcasts it to the relevant group. For most operations this is okay
and doesn't waste resources in general.

When operations are done without the rtnl_lock, as in tc-flower, such
skb allocation, message fill and no-op broadcasting can happen in all
cores of the system, which contributes to system pressure and wastes
precious cpu cycles when no one will receive the built message.

Introduce this helper so rtnetlink operations can simply check if someone
is listening and then proceed if necessary.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20231208192847.714940-2-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 369609fc6272 ("tc: Ensure we have enough buffer space when sending filter netlink notifications")
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
index ae2c6a3cec5dba5e0b378e9354aee8869bcdf620..ad5f15d3692350fc84bf934bd847d133d1af9b9e 100644 (file)
--- a/include/linux/rtnetlink.h
+++ b/include/linux/rtnetlink.h
@@ -138,4 +138,11 @@ extern int ndo_dflt_bridge_getlink(struct sk_buff *skb, u32 pid, u32 seq,
 
 extern void rtnl_offload_xstats_notify(struct net_device *dev);
 
+static inline int rtnl_has_listeners(const struct net *net, u32 group)
+{
+       struct sock *rtnl = net->rtnl;
+
+       return netlink_has_listeners(rtnl, group);
+}
+
 #endif /* __LINUX_RTNETLINK_H */