Commit 5151ec54 authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

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: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarKuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: default avatarJason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20260226021215.1764237-1-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent b99ccb37

include/net/sock.h

+7 −2
Original line number Diff line number Diff line
@@ -81,8 +81,13 @@ 
 * mini-semaphore synchronizes multiple users amongst themselves.

 */

typedef struct {

	spinlock_t		slock;

	union {

		struct slock_owned {

			int		owned;

			spinlock_t	slock;

		};

		long	combined;

	};

	wait_queue_head_t	wq;

	/*

	 * We express the mutex-alike socket_lock semantics

net/core/sock.c

+13 −0
Original line number Diff line number Diff line
@@ -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);