net: add SOCK_RCU_FREE socket flag

We want a generic way to insert an RCU grace period before socket
freeing for cases where RCU_SLAB_DESTROY_BY_RCU is adding too
much overhead.

SLAB_DESTROY_BY_RCU strict rules force us to take a reference
on the socket sk_refcnt, and it is a performance problem for UDP
encapsulation, or TCP synflood behavior, as many CPUs might
attempt the atomic operations on a shared sk_refcnt

UDP sockets and TCP listeners can set SOCK_RCU_FREE so that their
lookup can use traditional RCU rules, without refcount changes.
They can set the flag only once hashed and visible by other cpus.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Tested-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -438,6 +438,7 @@ struct sock {
 						  struct sk_buff *skb);
 	void                    (*sk_destruct)(struct sock *sk);
 	struct sock_reuseport __rcu	*sk_reuseport_cb;
+	struct rcu_head		sk_rcu;
 };
 
 #define __sk_user_data(sk) ((*((void __rcu **)&(sk)->sk_user_data)))
@@ -720,6 +721,7 @@ enum sock_flags {
 		     */
 	SOCK_FILTER_LOCKED, /* Filter cannot be changed anymore */
 	SOCK_SELECT_ERR_QUEUE, /* Wake select on error queue */
+	SOCK_RCU_FREE, /* wait rcu grace period in sk_destruct() */
 };
 
 #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))

net/core/sock.c

@@ -1419,8 +1419,12 @@ struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
 }
 EXPORT_SYMBOL(sk_alloc);
 
-void sk_destruct(struct sock *sk)
+/* Sockets having SOCK_RCU_FREE will call this function after one RCU
+ * grace period. This is the case for UDP sockets and TCP listeners.
+ */
+static void __sk_destruct(struct rcu_head *head)
 {
+	struct sock *sk = container_of(head, struct sock, sk_rcu);
 	struct sk_filter *filter;
 
 	if (sk->sk_destruct)
@@ -1449,6 +1453,14 @@ void sk_destruct(struct sock *sk)
 	sk_prot_free(sk->sk_prot_creator, sk);
 }
 
+void sk_destruct(struct sock *sk)
+{
+	if (sock_flag(sk, SOCK_RCU_FREE))
+		call_rcu(&sk->sk_rcu, __sk_destruct);
+	else
+		__sk_destruct(&sk->sk_rcu);
+}
+
 static void __sk_free(struct sock *sk)
 {
 	if (unlikely(sock_diag_has_destroy_listeners(sk) && sk->sk_net_refcnt))