net: lockless sock_i_ino()

Followup of commit c51da3f7 ("net: remove sock_i_uid()")

A recent syzbot report was the trigger for this change.

Over the years, we had many problems caused by the
read_lock[_bh](&sk->sk_callback_lock) in sock_i_uid().

We could fix smc_diag_dump_proto() or make a more radical move:

Instead of waiting for new syzbot reports, cache the socket
inode number in sk->sk_ino, so that we no longer
need to acquire sk->sk_callback_lock in sock_i_ino().

This makes socket dumps faster (one less cache line miss,
and two atomic ops avoided).

Prior art:

commit 25a9c8a4 ("netlink: Add __sock_i_ino() for __netlink_diag_dump().")
commit 4f9bf2a2 ("tcp: Don't acquire inet_listen_hashbucket::lock with disabled BH.")
commit efc3dbc3 ("rds: Make rds_sock_lock BH rather than IRQ safe.")

Fixes: d2d6422f ("x86: Allow to enable PREEMPT_RT.")
Reported-by:  <syzbot+50603c05bbdf4dfdaffa@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/68b73804.050a0220.3db4df.01d8.GAE@google.com/T/#u


Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250902183603.740428-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>