Subject: [PATCH 3/4] net: add support for socket no-lock

From: Jens Axboe <[email-masked]>
Date: Tue, 12 Apr 2022 14:26:12 -0600

If we have a guaranteed single user of a socket, then we can optimize
the lock/release of it.

Signed-off-by: Jens Axboe <[email-masked]>
---
 include/net/sock.h | 10 ++++++++--
 net/core/sock.c    | 31 +++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 99fcc4d7eed9..aefc94677c94 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1674,7 +1674,7 @@ do {									\
 
 static inline bool lockdep_sock_is_held(const struct sock *sk)
 {
-	return lockdep_is_held(&sk->sk_lock) ||
+	return sk->sk_no_lock || lockdep_is_held(&sk->sk_lock) ||
 	       lockdep_is_held(&sk->sk_lock.slock);
 }
 
@@ -1774,18 +1774,20 @@ static inline void unlock_sock_fast(struct sock *sk, bool slow)
 static inline void sock_owned_by_me(const struct sock *sk)
 {
 #ifdef CONFIG_LOCKDEP
-	WARN_ON_ONCE(!lockdep_sock_is_held(sk) && debug_locks);
+	WARN_ON_ONCE(!sk->sk_no_lock && !lockdep_sock_is_held(sk) && debug_locks);
 #endif
 }
 
 static inline bool sock_owned_by_user(const struct sock *sk)
 {
 	sock_owned_by_me(sk);
+	smp_rmb();
 	return sk->sk_lock.owned;
 }
 
 static inline bool sock_owned_by_user_nocheck(const struct sock *sk)
 {
+	smp_rmb();
 	return sk->sk_lock.owned;
 }
 
@@ -1794,6 +1796,10 @@ static inline void sock_release_ownership(struct sock *sk)
 	if (sock_owned_by_user_nocheck(sk)) {
 		sk->sk_lock.owned = 0;
 
+		if (sk->sk_no_lock) {
+			smp_wmb();
+			return;
+		}
 		/* The sk_lock has mutex_unlock() semantics: */
 		mutex_release(&sk->sk_lock.dep_map, _RET_IP_);
 	}
diff --git a/net/core/sock.c b/net/core/sock.c
index fec892b384a4..d7eea29c5699 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2764,6 +2764,9 @@ void __lock_sock(struct sock *sk)
 {
 	DEFINE_WAIT(wait);
 
+	if (WARN_ON_ONCE(sk->sk_no_lock))
+		return;
+
 	for (;;) {
 		prepare_to_wait_exclusive(&sk->sk_lock.wq, &wait,
 					TASK_UNINTERRUPTIBLE);
@@ -3307,8 +3310,21 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 }
 EXPORT_SYMBOL(sock_init_data);
 
+static inline bool lock_sock_nolock(struct sock *sk)
+{
+	if (sk->sk_no_lock) {
+		sk->sk_lock.owned = 1;
+		smp_wmb();
+		return true;
+	}
+	return false;
+}
+
 void lock_sock_nested(struct sock *sk, int subclass)
 {
+	if (lock_sock_nolock(sk))
+		return;
+
 	/* The sk_lock has mutex_lock() semantics here. */
 	mutex_acquire(&sk->sk_lock.dep_map, subclass, 0, _RET_IP_);
 
@@ -3321,8 +3337,23 @@ void lock_sock_nested(struct sock *sk, int subclass)
 }
 EXPORT_SYMBOL(lock_sock_nested);
 
+static inline bool release_sock_nolock(struct sock *sk)
+{
+	if (!sk->sk_no_lock)
+		return false;
+	if (READ_ONCE(sk->sk_backlog.tail))
+		return false;
+	if (sk->sk_prot->release_cb)
+		sk->sk_prot->release_cb(sk);
+	sock_release_ownership(sk);
+	return true;
+}
+
 void release_sock(struct sock *sk)
 {
+	if (release_sock_nolock(sk))
+		return;
+
 	spin_lock_bh(&sk->sk_lock.slock);
 	if (sk->sk_backlog.tail)
 		__release_sock(sk);
-- 
2.35.1