netdev - [PATCH net-next 1/2] unix: uses an atomic type for scm files accounting

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Date:   Thu, 27 Feb 2020 12:30:37 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: [PATCH net-next 1/2] 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@...hat.com>
---
 include/net/af_unix.h |  2 +-
 net/unix/af_unix.c    | 21 ++++++---------------
 2 files changed, 7 insertions(+), 16 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 17e10fba2152..5cb65227b7a9 100644
--- a/include/net/af_unix.h
+++ b/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))
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index cbd7dc01e147..145a3965341e 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -689,7 +689,8 @@ 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));
 	}
 }
 
@@ -1598,10 +1599,8 @@ 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)
@@ -1609,10 +1608,8 @@ 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);
 }
 
 /*
@@ -1801,10 +1798,8 @@ 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);
@@ -1906,10 +1901,8 @@ 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;
@@ -2405,9 +2398,7 @@ 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);
 			}
 
-- 
2.21.1