ipv6: elide flowlabel check if no exclusive leases exist

Processes can request ipv6 flowlabels with cmsg IPV6_FLOWINFO.
If not set, by default an autogenerated flowlabel is selected.

Explicit flowlabels require a control operation per label plus a
datapath check on every connection (every datagram if unconnected).
This is particularly expensive on unconnected sockets multiplexing
many flows, such as QUIC.

In the common case, where no lease is exclusive, the check can be
safely elided, as both lease request and check trivially succeed.
Indeed, autoflowlabel does the same even with exclusive leases.

Elide the check if no process has requested an exclusive lease.

fl6_sock_lookup previously returns either a reference to a lease or
NULL to denote failure. Modify to return a real error and update
all callers. On return NULL, they can use the label and will elide
the atomic_dec in fl6_sock_release.

This is an optimization. Robust applications still have to revert to
requesting leases if the fast path fails due to an exclusive lease.

Changes RFC->v1:
  - use static_key_false_deferred to rate limit jump label operations
    - call static_key_deferred_flush to stop timers on exit
  - move decrement out of RCU context
  - defer optimization also if opt data is associated with a lease
  - updated all fp6_sock_lookup callers, not just udp

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 1 deletions

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index 8eca5fb30376..8dfc65639aa4 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -13,6 +13,7 @@
 #include <linux/hardirq.h>
 #include <linux/jhash.h>
 #include <linux/refcount.h>
+#include <linux/jump_label_ratelimit.h>
 #include <net/if_inet6.h>
 #include <net/ndisc.h>
 #include <net/flow.h>
@@ -389,7 +390,18 @@ static inline void txopt_put(struct ipv6_txoptions *opt)
 		kfree_rcu(opt, rcu);
 }
 
-struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk, __be32 label);
+struct ip6_flowlabel *__fl6_sock_lookup(struct sock *sk, __be32 label);
+
+extern struct static_key_false_deferred ipv6_flowlabel_exclusive;
+static inline struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk,
+						    __be32 label)
+{
+	if (static_branch_unlikely(&ipv6_flowlabel_exclusive.key))
+		return __fl6_sock_lookup(sk, label) ? : ERR_PTR(-ENOENT);
+
+	return NULL;
+}
+
 struct ipv6_txoptions *fl6_merge_options(struct ipv6_txoptions *opt_space,
 					 struct ip6_flowlabel *fl,
 					 struct ipv6_txoptions *fopt);