udp: annotate data-race in __udp_enqueue_schedule_skb()

sk->sk_rcvbuf is read locklessly twice, while other threads
could change its value.

Use a READ_ONCE() to annotate the race.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240328144032.1864988-2-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/udp.c

@@ -1492,13 +1492,14 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	struct sk_buff_head *list = &sk->sk_receive_queue;
 	int rmem, err = -ENOMEM;
 	spinlock_t *busy = NULL;
-	int size;
+	int size, rcvbuf;
 
-	/* try to avoid the costly atomic add/sub pair when the receive
-	 * queue is full; always allow at least a packet
+	/* Immediately drop when the receive queue is full.
+	 * Always allow at least one packet.
 	 */
 	rmem = atomic_read(&sk->sk_rmem_alloc);
-	if (rmem > sk->sk_rcvbuf)
+	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
+	if (rmem > rcvbuf)
 		goto drop;
 
 	/* Under mem pressure, it might be helpful to help udp_recvmsg()
@@ -1507,7 +1508,7 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	 * - Less cache line misses at copyout() time
 	 * - Less work at consume_skb() (less alien page frag freeing)
 	 */
-	if (rmem > (sk->sk_rcvbuf >> 1)) {
+	if (rmem > (rcvbuf >> 1)) {
 		skb_condense(skb);
 
 		busy = busylock_acquire(sk);