nbsp; uint32_t m_mis[2]; // for misaligned locks m_tid/m_seq will span into here
} psynch;
struct _pthread_mutex_ulock_s ulock;
};
#if defined(__LP64__)
uint32_t _reserved[4];
#else
uint32_t _reserved[1];
#endif
};
代码来验证一下：线程优先级是否会被提升？
// printThreadPriority用来打印线程的优先级信息
void printThreadPriority() {
thread_t cur_thread = mach_thread_self();
mach_port_deallocate(mach_task_self(), cur_thread);
mach_msg_type_number_t thread_info_count = THREAD_INFO_MAX;
thread_info_data_t thinfo;
kern_return_t kr = thread_info(cur_thread, THREAD_EXTENDED_INFO, (thread_info_t)thinfo, &thread_info_count);
if (kr != KERN_SUCCESS) {
return;
}
thread_extended_info_t extend_info = (thread_extended_info_t)thinfo;
printf("pth_priority: %d, pth_curpri: %d, pth_maxpriority: %d\n", extend_info->pth_priority, extend_info->pth_curpri, extend_info->pth_maxpriority);
}
先在子线程上锁并休眠，然后主线程请求该锁
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
printf("begin : \n");
printThreadPriority();
printf("queue before lock \n");
pthread_mutex_lock(&_lock); //确保 backgroundQueue 先得到锁
printf("queue lock \n");
printThreadPriority();
dispatch_async(dispatch_get_main_queue(), ^{
printf("before main lock\n");
pthread_mutex_lock(&_lock);
printf("in main lock\n");
pthread_mutex_unlock(&_lock);
printf("after main unlock\n");
});
sleep(10);
printThreadPriority();
printf("queue unlock\n");
pthread_mutex_unlock(&_lock);
printf("queue after unlock\n");
});
begin :
pth_priority: 4, pth_curpri: 4, pth_maxpriority: 63
queue before lock
queue lock
pth_priority: 4, pth_curpri: 4, pth_maxpriority: 63
before main lock
pth_priority: 47, pth_curpri: 47, pth_maxpriority: 63
queue unlock
in main lock
after main unlock
queue after unlock
可以看到，低优先级子线程先持有锁，当时的优先级为4，而该锁被主线程请求的时候，子线程的优先级被提升为47
os_unfair_lock
os_unfair_lock用来替换OSSpinLock，解决优先级反转问题。等待os_unfair_lock锁的线程会处于休眠状态，从用户态切换到内核态，而并非忙等。os_unfair_lock将线程ID保存到了锁的内部，锁的等待者会把自己的优先级让出来，从而避免优先级反转。验证一下：
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PR