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net: use try_cmpxchg() in lock_sock_nested()

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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>
This commit is contained in:
Eric Dumazet
2026-02-26 02:12:15 +00:00
committed by Jakub Kicinski
parent b99ccb37ed
commit 5151ec54f5
2 changed files with 20 additions and 2 deletions
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9
include/net/sock.h
View File

@@ -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

13
net/core/sock.c
View File

@@ -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);