Revert "udp: avoid calling sock_def_readable() if possible"

This reverts commit 612b1c0. On a
scenario with multiple threads blocking on a recvfrom(), we need to call
sock_def_readable() on every __udp_enqueue_schedule_skb() otherwise the
threads won't be woken up as __skb_wait_for_more_packets() is using
prepare_to_wait_exclusive().

Link: https://bugzilla.redhat.com/2308477
Fixes: 612b1c0 ("udp: avoid calling sock_def_readable() if possible")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241202155620.1719-1-ffmancera@riseup.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: ffmancera

net/ipv4/udp.c

@@ -1674,7 +1674,6 @@ 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;
-	bool becomes_readable;
 	int size, rcvbuf;
 
 	/* Immediately drop when the receive queue is full.
@@ -1715,19 +1714,12 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	 */
 	sock_skb_set_dropcount(sk, skb);
 
-	becomes_readable = skb_queue_empty(list);
 	__skb_queue_tail(list, skb);
 	spin_unlock(&list->lock);
 
-	if (!sock_flag(sk, SOCK_DEAD)) {
-		if (becomes_readable ||
-		    sk->sk_data_ready != sock_def_readable ||
-		    READ_ONCE(sk->sk_peek_off) >= 0)
-			INDIRECT_CALL_1(sk->sk_data_ready,
-					sock_def_readable, sk);
-		else
-			sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
-	}
+	if (!sock_flag(sk, SOCK_DEAD))
+		INDIRECT_CALL_1(sk->sk_data_ready, sock_def_readable, sk);
+
 	busylock_release(busy);
 	return 0;