死锁检测组件-设想

现在有三个临界资源和三把锁绑定了，三把锁又分别被三个线程占用。（不用关注临界资源，因为锁和临界资源是绑定的）

但现在出现这种情况：线程1去申请获取锁2，线程2申请获取锁3，线程3申请获取锁1，这样就会造成死锁：

死锁问题，可转换为有向图的环路检测

死锁的构建

有四个线程，4把锁，以下代码一定会产生死锁


pthread_mutex_t mtx1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mtx2 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mtx3 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mtx4 = PTHREAD_MUTEX_INITIALIZER;

void *thread_routine_a(void *arg) {

	printf("thread_routine a \n");
	pthread_mutex_lock(&mtx1);
	sleep(1);	
	
	pthread_mutex_lock(&mtx2);

	pthread_mutex_unlock(&mtx2);

	pthread_mutex_unlock(&mtx1);

	printf("thread_routine a exit\n");

}

void *thread_routine_b(void *arg) {

	printf("thread_routine b \n");
	pthread_mutex_lock(&mtx2);
	sleep(1);

	pthread_mutex_lock(&mtx3);

	pthread_mutex_unlock(&mtx3);

	pthread_mutex_unlock(&mtx2);

	printf("thread_routine b exit \n");
// -----
	pthread_mutex_lock(&mtx1);
}


void *thread_routine_c(void *arg) {

	printf("thread_routine c \n");
	pthread_mutex_lock(&mtx3);
	sleep(1);

	pthread_mutex_lock(&mtx4);

	pthread_mutex_unlock(&mtx4);

	pthread_mutex_unlock(&mtx3);

	printf("thread_routine c exit \n");
}


void *thread_routine_d(void *arg) {

	printf("thread_routine d \n");
	pthread_mutex_lock(&mtx4);
	sleep(1);

	pthread_mutex_lock(&mtx1);

	pthread_mutex_unlock(&mtx1);

	pthread_mutex_unlock(&mtx4);

	printf("thread_routine d exit \n");
}


int main() {
#if 1
	init_hook();
	
	pthread_t tid1, tid2, tid3, tid4;

	pthread_create(&tid1, NULL, thread_routine_a, NULL);
	pthread_create(&tid2, NULL, thread_routine_b, NULL);
	pthread_create(&tid3, NULL, thread_routine_c, NULL);
	pthread_create(&tid4, NULL, thread_routine_d, NULL);

	pthread_join(tid1, NULL);
	pthread_join(tid2, NULL);
	pthread_join(tid3, NULL);
	pthread_join(tid4, NULL);
}

这里产生了：线程a => 线程b => 线程c => 线程d =》线程a的这样一个环路

但是我们不知道哪把锁被哪个线程占用了，没法构建有向图，也就无法得知是否产生了这样一个环路

这时，可以用hook，调自己写的 pthread_mutex_lock，将线程和锁的映射关系保存起来

pthread的hook

有点像装饰器模式

// 函数指针
typedef int (*pthread_mutex_lock_t)(pthread_mutex_t *mutex);
pthread_mutex_lock_t pthread_mutex_lock_f;

typedef int (*pthread_mutex_unlock_t)(pthread_mutex_t *mutex);
pthread_mutex_unlock_t pthread_mutex_unlock_f;

static int init_hook() {
	//  RTLD_NEXT可以理解为代码段，在这里面找pthread_mutex_lock函数名，把地址返回
    // 所以pthread_mutex_lock_f就是静态或动态库里的pthread_mutex_lock锁函数  
	pthread_mutex_lock_f = dlsym(RTLD_NEXT, "pthread_mutex_lock"); 
	
	pthread_mutex_unlock_f = dlsym(RTLD_NEXT, "pthread_mutex_unlock");
}

int pthread_mutex_lock(pthread_mutex_t *mutex) {

	printf("pthread_mutex_lock selfid %ld, mutex: %p\n", pthread_self(), mutex);
	beforelock(pthread_self(), mutex);
	
	pthread_mutex_lock_f(mutex);    // 用钩子的好处，给系统函数命一个别名

	afterlock(pthread_self(), mutex);

}

int pthread_mutex_unlock(pthread_mutex_t *mutex) {

	printf("pthread_mutex_unlock\n");

	pthread_mutex_unlock_f(mutex);
	afterunlock(pthread_self(), mutex);
	
}

这样，在加锁的时候，就能知道锁id(mutex)和线程id的对应关系