iOS-os_unfair_lock

os_unfair_lock是一种底层锁，允许waiters在竞争时有效阻塞。

通常，应优先选择使用高级同步工具。如 pthread、dispatch 等提供的同步方案。

存储在锁中的值应被认为是不透明的，并且在实现中定义。其包含线程信息，系统可用于解决优先级反转问题。

加锁和解锁必须在同一线程，负责会引起程序异常终止。

用于取代OSSpinLock，尝试获取已加锁的线程无需忙等(是互斥锁)，解锁时由内核唤醒。和OSSpinLock一样，os_unfair_lock也没有加强公平性和顺序。例如，释放锁的线程可能立即再次加锁，而之前等待锁的线程唤醒后没有机会尝试加锁。这样有利于提高性能，但也造成了饥饿（starvation）。Starvation 指贪婪线程占用共享资源太长时间，其他线程无法访问共享资源、无法取得进展。例如，某对象的同步方法占用时间过长，并且频繁调用，其他线程尝试调用该方法时会被堵塞，处于 starvation。

#include <os/lock.h>
 * Low-level lock that allows waiters to block efficiently(有效) on contention(竞争).
 *
 * In general, higher level synchronization primitives such as those provided by
 * the pthread or dispatch subsystems should be preferred.
 *
 * The values stored in the lock should be considered opaque and implementation
 * defined, they contain thread ownership information that the system may use
 * to attempt to resolve priority inversions.
 *
 * This lock must be unlocked from the same thread that locked it, attempts to
 * unlock from a different thread will cause an assertion aborting the process.
 *
 * This lock must not be accessed from multiple processes or threads via shared
 * or multiply-mapped memory, the lock implementation relies on the address of
 * the lock value and owning process.
 *
 * Must be initialized with OS_UNFAIR_LOCK_INIT
 *
 * @discussion
 * Replacement for the deprecated OSSpinLock. Does not spin on contention but
 * waits in the kernel to be woken up by an unlock.
 *
 * As with OSSpinLock there is no attempt at fairness or lock ordering, e.g. an
 * unlocker can potentially immediately reacquire the lock before a woken up
 * waiter gets an opportunity to attempt to acquire the lock. This may be
 * advantageous for performance reasons, but also makes starvation of waiters a
 * possibility.

参考：
os_unfair_lock_lock
Starvation and Livelock
解说os_unfair_lock