tcp: Fix fastreuse cache initialization on first bind

inet_csk_update_fastreuse() initializes the reuse caches only when
tb->bhash2 is empty, but both inet_csk_get_port() and
__inet_inherit_port() link tb2 before calling it, so the first-bind
branch is never taken.

As a result, SO_REUSEADDR binds never get fastreuse=1 and keep
scanning existing owners, and SO_REUSEPORT starts in STRICT instead
of ANY.

Initialize the caches when tb2 is the only entry in tb->bhash2 and has
no owners. Timewait sockets are also linked on tb2->owners, so this is
equivalent to the old hlist_empty(&tb->owners) test.

Found by manual code inspection and confirmed with GDB. Binding 16000
SO_REUSEADDR sockets to one address/port in a KVM guest takes 4.18 s
before and 19.4 ms after this patch. Existing bind/reuse selftests
pass. Analysis and testing were assisted by an AI tool (Codex).

Fixes: 8002d44fe84d ("tcp: Check hlist_empty(&tb->bhash2) instead of hlist_empty(&tb->owners).")
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Signed-off-by: Zhewei Yuan <yzwlhm@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20261026095208.58504-1-yzwlhm@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions