[PATCH net-next] net: use cmpxchg() in lock_sock_nested()

[Eric Dumazet <edumazet@google.com>]

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:

ffffffff81e96100 <lock_sock_nested>:
ffffffff81e96100:       f3 0f 1e fa             endbr64
ffffffff81e96104:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
ffffffff81e96109:       b9 01 00 00 00          mov    $0x1,%ecx
ffffffff81e9610e:       31 c0                   xor    %eax,%eax
ffffffff81e96110:       f0 48 0f b1 8f 48 01    lock cmpxchg %rcx,0x148(%rdi)
ffffffff81e96117:       00 00
ffffffff81e96119:       48 85 c0                test   %rax,%rax
ffffffff81e9611c:       74 33                   je     ffffffff81e96151 <lock_sock_nested+0x51>
...// slow path
ffffffff81e96151:       2e e9 19 c4 33 00       cs jmp ffffffff821d2570 <__pi___x86_return_thunk>

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/net/sock.h |  9 +++++++--
 net/core/sock.c    | 10 ++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 55b61e4b0d8318887d527e919fc1103d78ac6d14..84c21fb38a28406e387dc33e0fb5decd18893950 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 cfb2a6209946089669882cdbd5d1b36c53838989..2a03d1432111f2dcea9d700f80efb7b0bb4a47d8 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3780,6 +3780,16 @@ 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;
+
+		tmp.slock = __SPIN_LOCK_UNLOCKED(tmp.slock);
+		tmp.owned = 1;
+		if (likely(!cmpxchg(&sk->sk_lock.combined, 0, tmp.combined)))
+			return;
+	}
+#endif
 	spin_lock_bh(&sk->sk_lock.slock);
 	if (unlikely(sock_owned_by_user_nocheck(sk)))
 		__lock_sock(sk);
-- 
2.53.0.414.gf7e9f6c205-goog