Merge branch 'udp-small-changes-on-receive-path'

Eric Dumazet says:

====================
udp: small changes on receive path

This series is based on an observation I made in UDP receive path.

The sock_def_readable() costs are pretty high, especially when
epoll is used to generate EPOLLIN events.

First patch annotates races on sk->sk_rcvbuf reads.

Second patch replaces an atomic_add_return()
 with a less expensive atomic_add()

Third patch avoids calling sock_def_readable() when possible.

Fourth patch adds sk_wake_async_rcu() to get better inlining
and code generation.
====================

Link: https://lore.kernel.org/r/20240328144032.1864988-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>