unix: uses an atomic type for scm files accounting

So the scm_stat_{add,del} helper can be invoked with no
additional lock held.

This clean-up the code a bit and will make the next
patch easier.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/af_unix.h

@@ -42,7 +42,7 @@ struct unix_skb_parms {
 } __randomize_layout;
 
 struct scm_stat {
-	u32 nr_fds;
+	atomic_t nr_fds;
 };
 
 #define UNIXCB(skb)	(*(struct unix_skb_parms *)&((skb)->cb))

net/unix/af_unix.c

@@ -690,7 +690,8 @@ static void unix_show_fdinfo(struct seq_file *m, struct socket *sock)
 
 	if (sk) {
 		u = unix_sk(sock->sk);
-		seq_printf(m, "scm_fds: %u\n", READ_ONCE(u->scm_stat.nr_fds));
+		seq_printf(m, "scm_fds: %u\n",
+			   atomic_read(&u->scm_stat.nr_fds));
 	}
 }
 #else
@@ -1602,10 +1603,8 @@ static void scm_stat_add(struct sock *sk, struct sk_buff *skb)
 	struct scm_fp_list *fp = UNIXCB(skb).fp;
 	struct unix_sock *u = unix_sk(sk);
 
-	lockdep_assert_held(&sk->sk_receive_queue.lock);
-
 	if (unlikely(fp && fp->count))
-		u->scm_stat.nr_fds += fp->count;
+		atomic_add(fp->count, &u->scm_stat.nr_fds);
 }
 
 static void scm_stat_del(struct sock *sk, struct sk_buff *skb)
@@ -1613,10 +1612,8 @@ static void scm_stat_del(struct sock *sk, struct sk_buff *skb)
 	struct scm_fp_list *fp = UNIXCB(skb).fp;
 	struct unix_sock *u = unix_sk(sk);
 
-	lockdep_assert_held(&sk->sk_receive_queue.lock);
-
 	if (unlikely(fp && fp->count))
-		u->scm_stat.nr_fds -= fp->count;
+		atomic_sub(fp->count, &u->scm_stat.nr_fds);
 }
 
 /*
@@ -1805,10 +1802,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	if (sock_flag(other, SOCK_RCVTSTAMP))
 		__net_timestamp(skb);
 	maybe_add_creds(skb, sock, other);
-	spin_lock(&other->sk_receive_queue.lock);
 	scm_stat_add(other, skb);
-	__skb_queue_tail(&other->sk_receive_queue, skb);
-	spin_unlock(&other->sk_receive_queue.lock);
+	skb_queue_tail(&other->sk_receive_queue, skb);
 	unix_state_unlock(other);
 	other->sk_data_ready(other);
 	sock_put(other);
@@ -1910,10 +1905,8 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 			goto pipe_err_free;
 
 		maybe_add_creds(skb, sock, other);
-		spin_lock(&other->sk_receive_queue.lock);
 		scm_stat_add(other, skb);
-		__skb_queue_tail(&other->sk_receive_queue, skb);
-		spin_unlock(&other->sk_receive_queue.lock);
+		skb_queue_tail(&other->sk_receive_queue, skb);
 		unix_state_unlock(other);
 		other->sk_data_ready(other);
 		sent += size;
@@ -2409,9 +2402,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 			sk_peek_offset_bwd(sk, chunk);
 
 			if (UNIXCB(skb).fp) {
-				spin_lock(&sk->sk_receive_queue.lock);
 				scm_stat_del(sk, skb);
-				spin_unlock(&sk->sk_receive_queue.lock);
 				unix_detach_fds(&scm, skb);
 			}