spinlock的一种实现

spinlock，中文译名为“自旋锁”。是专为防止多处理器并发而引入的一种锁。在Linux的内核中，spinlock用在多处理器环境中。当一个CPU访问一个临界资源 (critical section)的时候，需要预先取得spinlock，如果取不到的话，它就在空循环等待，直到另外的CPU释放spinlock。由于涉及到多个处理器，spinlock的效率非常重要。

skynet中的spinlock.h

#ifndef SKYNET_SPINLOCK_H
#define SKYNET_SPINLOCK_H

#define SPIN_INIT(q) spinlock_init(&(q)->lock);
#define SPIN_LOCK(q) spinlock_lock(&(q)->lock);
#define SPIN_UNLOCK(q) spinlock_unlock(&(q)->lock);
#define SPIN_DESTROY(q) spinlock_destroy(&(q)->lock);

#ifndef USE_PTHREAD_LOCK

struct spinlock {
	int lock;
};

static inline void
spinlock_init(struct spinlock *lock) {
	lock->lock = 0;
}

static inline void
spinlock_lock(struct spinlock *lock) {
	while (__sync_lock_test_and_set(&lock->lock,1)) {}
}

static inline int
spinlock_trylock(struct spinlock *lock) {
	return __sync_lock_test_and_set(&lock->lock,1) == 0;
}

static inline void
spinlock_unlock(struct spinlock *lock) {
	__sync_lock_release(&lock->lock);
}

static inline void
spinlock_destroy(struct spinlock *lock) {
	(void) lock;
}

#else

#include <pthread.h>

// we use mutex instead of spinlock for some reason
// you can also replace to pthread_spinlock

struct spinlock {
	pthread_mutex_t lock;
};

static inline void
spinlock_init(struct spinlock *lock) {
	pthread_mutex_init(&lock->lock, NULL);
}

static inline void
spinlock_lock(struct spinlock *lock) {
	pthread_mutex_lock(&lock->lock);
}

static inline int
spinlock_trylock(struct spinlock *lock) {
	return pthread_mutex_trylock(&lock->lock) == 0;
}

static inline void
spinlock_unlock(struct spinlock *lock) {
	pthread_mutex_unlock(&lock->lock);
}

static inline void
spinlock_destroy(struct spinlock *lock) {
	pthread_mutex_destroy(&lock->lock);
}

#endif

#endif

其中，使用了linux的原子操作：

type __sync_lock_test_and_set (type *ptr, type value, ...)
将*ptr设为value并返回*ptr操作之前的值。

void __sync_lock_release (type *ptr, ...)
将*ptr置0

其他的linux原子操作,参看：

https://blog.youkuaiyun.com/hzhsan/article/details/25124901

如何设计一个真正高性能的spin_lock? https://www.zhihu.com/question/55764216