tcp_metrics: annotate data-races around tm->tcpm_stamp

tm->tcpm_stamp can be read or written locklessly.

Add needed READ_ONCE()/WRITE_ONCE() to document this.

Also constify tcpm_check_stamp() dst argument.

Fixes: 51c5d0c ("tcp: Maintain dynamic metrics in local cache.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230802131500.1478140-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/tcp_metrics.c

@@ -97,7 +97,7 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm,
 	u32 msval;
 	u32 val;
 
-	tm->tcpm_stamp = jiffies;
+	WRITE_ONCE(tm->tcpm_stamp, jiffies);
 
 	val = 0;
 	if (dst_metric_locked(dst, RTAX_RTT))
@@ -131,9 +131,15 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm,
 
 #define TCP_METRICS_TIMEOUT		(60 * 60 * HZ)
 
-static void tcpm_check_stamp(struct tcp_metrics_block *tm, struct dst_entry *dst)
+static void tcpm_check_stamp(struct tcp_metrics_block *tm,
+			     const struct dst_entry *dst)
 {
-	if (tm && unlikely(time_after(jiffies, tm->tcpm_stamp + TCP_METRICS_TIMEOUT)))
+	unsigned long limit;
+
+	if (!tm)
+		return;
+	limit = READ_ONCE(tm->tcpm_stamp) + TCP_METRICS_TIMEOUT;
+	if (unlikely(time_after(jiffies, limit)))
 		tcpm_suck_dst(tm, dst, false);
 }
 
@@ -174,7 +180,8 @@ static struct tcp_metrics_block *tcpm_new(struct dst_entry *dst,
 		oldest = deref_locked(tcp_metrics_hash[hash].chain);
 		for (tm = deref_locked(oldest->tcpm_next); tm;
 		     tm = deref_locked(tm->tcpm_next)) {
-			if (time_before(tm->tcpm_stamp, oldest->tcpm_stamp))
+			if (time_before(READ_ONCE(tm->tcpm_stamp),
+					READ_ONCE(oldest->tcpm_stamp)))
 				oldest = tm;
 		}
 		tm = oldest;
@@ -434,7 +441,7 @@ void tcp_update_metrics(struct sock *sk)
 					       tp->reordering);
 		}
 	}
-	tm->tcpm_stamp = jiffies;
+	WRITE_ONCE(tm->tcpm_stamp, jiffies);
 out_unlock:
 	rcu_read_unlock();
 }
@@ -647,7 +654,7 @@ static int tcp_metrics_fill_info(struct sk_buff *msg,
 	}
 
 	if (nla_put_msecs(msg, TCP_METRICS_ATTR_AGE,
-			  jiffies - tm->tcpm_stamp,
+			  jiffies - READ_ONCE(tm->tcpm_stamp),
 			  TCP_METRICS_ATTR_PAD) < 0)
 		goto nla_put_failure;