From: Eric Dumazet <edumazet@google.com>
Date: Thu, 26 Feb 2026 02:12:15 +0000 (+0000)
Subject: net: use try_cmpxchg() in lock_sock_nested()
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=5151ec54f5861ad71e08190a7ce2173df788d36a;p=thirdparty%2Fkernel%2Flinux.git

net: use try_cmpxchg() in lock_sock_nested()

Add a fast path in lock_sock_nested(), to avoid acquiring
the socket spinlock only to set @owned to one:

        spin_lock_bh(&sk->sk_lock.slock);
        if (unlikely(sock_owned_by_user_nocheck(sk)))
                __lock_sock(sk);
        sk->sk_lock.owned = 1;
        spin_unlock_bh(&sk->sk_lock.slock);

On x86_64 compiler generates something quite efficient:

00000000000077c0 <lock_sock_nested>:
    77c0:       f3 0f 1e fa                 endbr64
    77c4:       e8 00 00 00 00              call   __fentry__
    77c9:       b9 01 00 00 00              mov    $0x1,%ecx
    77ce:       31 c0                       xor    %eax,%eax
    77d0:       f0 48 0f b1 8f 48 01 00 00  lock cmpxchg %rcx,0x148(%rdi)
    77d9:       75 06                       jne    slow_path
    77db:       2e e9 00 00 00 00           cs jmp __x86_return_thunk-0x4
slow_path:      ...

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20260226021215.1764237-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---

diff --git a/include/net/sock.h b/include/net/sock.h
index e7099d3d0416e..a7a8b31e98771 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -81,8 +81,13 @@
  * mini-semaphore synchronizes multiple users amongst themselves.
  */
 typedef struct {
-	spinlock_t		slock;
-	int			owned;
+	union {
+		struct slock_owned {
+			int		owned;
+			spinlock_t	slock;
+		};
+		long	combined;
+	};
 	wait_queue_head_t	wq;
 	/*
 	 * We express the mutex-alike socket_lock semantics
diff --git a/net/core/sock.c b/net/core/sock.c
index 2032be9a03b52..9d841975a7a14 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3780,6 +3780,19 @@ void noinline lock_sock_nested(struct sock *sk, int subclass)
 	mutex_acquire(&sk->sk_lock.dep_map, subclass, 0, _RET_IP_);
 
 	might_sleep();
+#ifdef CONFIG_64BIT
+	if (sizeof(struct slock_owned) == sizeof(long)) {
+		socket_lock_t tmp, old;
+
+		tmp.slock = __SPIN_LOCK_UNLOCKED(tmp.slock);
+		tmp.owned = 1;
+		old.slock = __SPIN_LOCK_UNLOCKED(old.slock);
+		old.owned = 0;
+		if (likely(try_cmpxchg(&sk->sk_lock.combined,
+				       &old.combined, tmp.combined)))
+			return;
+	}
+#endif
 	spin_lock_bh(&sk->sk_lock.slock);
 	if (unlikely(sock_owned_by_user_nocheck(sk)))
 		__lock_sock(sk);