tcp: reclaim 8 bytes in struct request_sock_queue

synflood_warned had to be u32 for xchg(), but ensuring
atomicity is not really needed.

Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

include/net/request_sock.h

@@ -185,8 +185,8 @@ struct fastopen_queue {
 struct request_sock_queue {
 	spinlock_t		rskq_lock;
 	u8			rskq_defer_accept;
+	u8			synflood_warned;
 
-	u32			synflood_warned;
 	atomic_t		qlen;
 	atomic_t		young;
 

net/ipv4/tcp_input.c

@@ -7282,8 +7282,8 @@ static bool tcp_syn_flood_action(struct sock *sk, const char *proto)
 #endif
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
-	if (!READ_ONCE(queue->synflood_warned) && syncookies != 2 &&
-	    xchg(&queue->synflood_warned, 1) == 0) {
+	if (syncookies != 2 && !READ_ONCE(queue->synflood_warned)) {
+		WRITE_ONCE(queue->synflood_warned, 1);
 		if (IS_ENABLED(CONFIG_IPV6) && sk->sk_family == AF_INET6) {
 			net_info_ratelimited("%s: Possible SYN flooding on port [%pI6c]:%u. %s.\n",
 					proto, inet6_rcv_saddr(sk),