unix: Improve locking scheme in unix_show_fdinfo()

After switching to TCP_ESTABLISHED or TCP_LISTEN sk_state, alive SOCK_STREAM
and SOCK_SEQPACKET sockets can't change it anymore (since commit 3ff8bff704f4
"unix: Fix race in SOCK_SEQPACKET's unix_dgram_sendmsg()").

Thus, we do not need to take lock here.

Signed-off-by: Kirill Tkhai <tkhai@ya.ru>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f0c2293f1d3b8bd409cff9902d4c2f48e4bb151b..009616fa02563dd5d8dbe925ca97e31347f3cbc4 100644 (file)
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -807,23 +807,23 @@ static int unix_count_nr_fds(struct sock *sk)
 static void unix_show_fdinfo(struct seq_file *m, struct socket *sock)
 {
        struct sock *sk = sock->sk;
+       unsigned char s_state;
        struct unix_sock *u;
-       int nr_fds;
+       int nr_fds = 0;
 
        if (sk) {
+               s_state = READ_ONCE(sk->sk_state);
                u = unix_sk(sk);
-               if (sock->type == SOCK_DGRAM) {
-                       nr_fds = atomic_read(&u->scm_stat.nr_fds);
-                       goto out_print;
-               }
 
-               unix_state_lock(sk);
-               if (sk->sk_state != TCP_LISTEN)
+               /* SOCK_STREAM and SOCK_SEQPACKET sockets never change their
+                * sk_state after switching to TCP_ESTABLISHED or TCP_LISTEN.
+                * SOCK_DGRAM is ordinary. So, no lock is needed.
+                */
+               if (sock->type == SOCK_DGRAM || s_state == TCP_ESTABLISHED)
                        nr_fds = atomic_read(&u->scm_stat.nr_fds);
-               else
+               else if (s_state == TCP_LISTEN)
                        nr_fds = unix_count_nr_fds(sk);
-               unix_state_unlock(sk);
-out_print:
+
                seq_printf(m, "scm_fds: %u\n", nr_fds);
        }
 }