[PATCH v1 net-next 3/4] af_unix: Move spin_lock() in manage_oob().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

When OOB skb has been already consumed, manage_oob() returns the next
skb if exists.  In such a case, we need to fall back to the else branch
below.

Then, we want to keep holding spin_lock(&sk->sk_receive_queue.lock).

Let's move it out of if-else branch and add lightweight check before
spin_lock() for major use cases without OOB skb.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 net/unix/af_unix.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 91d7877a1079..159d78fc3d14 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2657,9 +2657,12 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 	struct sk_buff *read_skb = NULL, *unread_skb = NULL;
 	struct unix_sock *u = unix_sk(sk);
 
-	if (!unix_skb_len(skb)) {
-		spin_lock(&sk->sk_receive_queue.lock);
+	if (likely(unix_skb_len(skb) && skb != READ_ONCE(u->oob_skb)))
+		return skb;
 
+	spin_lock(&sk->sk_receive_queue.lock);
+
+	if (!unix_skb_len(skb)) {
 		if (copied && (!u->oob_skb || skb == u->oob_skb)) {
 			skb = NULL;
 		} else if (flags & MSG_PEEK) {
@@ -2670,14 +2673,9 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 			__skb_unlink(read_skb, &sk->sk_receive_queue);
 		}
 
-		spin_unlock(&sk->sk_receive_queue.lock);
-
-		consume_skb(read_skb);
-		return skb;
+		goto unlock;
 	}
 
-	spin_lock(&sk->sk_receive_queue.lock);
-
 	if (skb != u->oob_skb)
 		goto unlock;
 
@@ -2698,6 +2696,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 unlock:
 	spin_unlock(&sk->sk_receive_queue.lock);
 
+	consume_skb(read_skb);
 	kfree_skb(unread_skb);
 
 	return skb;
-- 
2.30.2