tcp: remove volatile qualifier on tw_substate

Using a volatile qualifier for a specific struct field is unusual.

Use instead READ_ONCE()/WRITE_ONCE() where necessary.

tcp_timewait_state_process() can change tw_substate while other
threads are reading this field.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20240827015250.3509197-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/inet_timewait_sock.h

@@ -58,7 +58,7 @@ struct inet_timewait_sock {
 #define tw_dr			__tw_common.skc_tw_dr
 
 	__u32			tw_mark;
-	volatile unsigned char	tw_substate;
+	unsigned char		tw_substate;
 	unsigned char		tw_rcv_wscale;
 
 	/* Socket demultiplex comparisons on incoming packets. */

net/ipv4/inet_diag.c

@@ -442,7 +442,7 @@ static int inet_twsk_diag_fill(struct sock *sk,
 	inet_diag_msg_common_fill(r, sk);
 	r->idiag_retrans      = 0;
 
-	r->idiag_state	      = tw->tw_substate;
+	r->idiag_state	      = READ_ONCE(tw->tw_substate);
 	r->idiag_timer	      = 3;
 	tmo = tw->tw_timer.expires - jiffies;
 	r->idiag_expires      = jiffies_delta_to_msecs(tmo);
@@ -1209,7 +1209,7 @@ void inet_diag_dump_icsk(struct inet_hashinfo *hashinfo, struct sk_buff *skb,
 			if (num < s_num)
 				goto next_normal;
 			state = (sk->sk_state == TCP_TIME_WAIT) ?
-				inet_twsk(sk)->tw_substate : sk->sk_state;
+				READ_ONCE(inet_twsk(sk)->tw_substate) : sk->sk_state;
 			if (!(idiag_states & (1 << state)))
 				goto next_normal;
 			if (r->sdiag_family != AF_UNSPEC &&

net/ipv4/tcp_ipv4.c

@@ -120,7 +120,7 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
 	struct tcp_sock *tp = tcp_sk(sk);
 	int ts_recent_stamp;
 
-	if (tw->tw_substate == TCP_FIN_WAIT2)
+	if (READ_ONCE(tw->tw_substate) == TCP_FIN_WAIT2)
 		reuse = 0;
 
 	if (reuse == 2) {
@@ -2948,7 +2948,7 @@ static void get_timewait4_sock(const struct inet_timewait_sock *tw,
 
 	seq_printf(f, "%4d: %08X:%04X %08X:%04X"
 		" %02X %08X:%08X %02X:%08lX %08X %5d %8d %d %d %pK",
-		i, src, srcp, dest, destp, tw->tw_substate, 0, 0,
+		i, src, srcp, dest, destp, READ_ONCE(tw->tw_substate), 0, 0,
 		3, jiffies_delta_to_clock_t(delta), 0, 0, 0, 0,
 		refcount_read(&tw->tw_refcnt), tw);
 }

net/ipv4/tcp_minisocks.c

@@ -117,7 +117,7 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 		}
 	}
 
-	if (tw->tw_substate == TCP_FIN_WAIT2) {
+	if (READ_ONCE(tw->tw_substate) == TCP_FIN_WAIT2) {
 		/* Just repeat all the checks of tcp_rcv_state_process() */
 
 		/* Out of window, send ACK */
@@ -150,7 +150,7 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 			return TCP_TW_RST;
 
 		/* FIN arrived, enter true time-wait state. */
-		tw->tw_substate	  = TCP_TIME_WAIT;
+		WRITE_ONCE(tw->tw_substate, TCP_TIME_WAIT);
 		twsk_rcv_nxt_update(tcptw, TCP_SKB_CB(skb)->end_seq);
 
 		if (tmp_opt.saw_tstamp) {

net/ipv6/tcp_ipv6.c

@@ -2258,7 +2258,7 @@ static void get_timewait6_sock(struct seq_file *seq,
 		   src->s6_addr32[2], src->s6_addr32[3], srcp,
 		   dest->s6_addr32[0], dest->s6_addr32[1],
 		   dest->s6_addr32[2], dest->s6_addr32[3], destp,
-		   tw->tw_substate, 0, 0,
+		   READ_ONCE(tw->tw_substate), 0, 0,
 		   3, jiffies_delta_to_clock_t(delta), 0, 0, 0, 0,
 		   refcount_read(&tw->tw_refcnt), tw);
 }