优化这段代码 #include <iostream> #include <thread> #include <chrono> #include <mutex> #include <semaphore.h> using namespace std; // shared data resource int shared_data = 0; // semaphores for synchronization sem_t mutex, rw_mutex; // number of readers int num_readers = 0; // reader function void reader(int id) { while (true) { // acquire mutex to update the number of readers sem_wait(&mutex); num_readers++; if (num_readers == 1) { // if this is the first reader, acquire the rw_mutex sem_wait(&rw_mutex); } sem_post(&mutex); // read the shared data cout << "Reader " << id << " read shared data: " << shared_data << endl; // release mutex sem_wait(&mutex); num_readers--; if (num_readers == 0) { // if this is the last reader, release the rw_mutex sem_post(&rw_mutex); } sem_post(&mutex); // sleep for a random amount of time this_thread::sleep_for(chrono::milliseconds(rand() % 1000)); } } // writer function void writer(int id) { while (true) { // acquire the rw_mutex sem_wait(&rw_mutex); // write to the shared data shared_data++; cout << "Writer " << id << " wrote to shared data: " << shared_data << endl; // release the rw_mutex sem_post(&rw_mutex); // sleep for a random amount of time this_thread::sleep_for(chrono::milliseconds(rand() % 1000)); } } int main() { // initialize semaphores sem_init(&mutex, 0, 1); sem_init(&rw_mutex, 0, 1); // create reader threads thread readers[8]; for (int i = 0; i < 8; i++) { readers[i] = thread(reader, i); } // create writer threads thread writers[2]; for (int i = 0; i < 2; i++) { writers[i] = thread(writer, i); } // join threads for (int i = 0; i < 8; i++) { readers[i].join(); } for (int i = 0; i < 2; i++) { writers[i].join(); } // destroy semaphores sem_destroy(&mutex); sem_destroy(&rw_mutex); return 0; }

这段代码是使用信号量实现读写锁的示例，但是存在一些需要优化的地方：

1. 不支持优先级反转：在这个实现中，读者和写者都使用了同一个互斥信号量(mutex)来保证对num_readers的互斥访问。但是，如果一个写者在等待互斥信号量(mutex)时，一个读者持续地获取并释放互斥信号量(mutex)，那么这个写者可能会一直等待下去，从而导致优先级反转的问题。为了避免这个问题，可以使用两个互斥锁，一个用于读者，一个用于写者。

2. 读者数量计数器(num_readers)不需要互斥访问：在这个实现中，num_readers是一个全局变量，需要使用互斥信号量(mutex)来保证互斥访问。但是，实际上，这个变量只需要在读者线程中使用，因此不需要使用互斥信号量(mutex)来保护它。

3. 读者线程可能会饿死：在这个实现中，如果有一个写者在等待互斥信号量(rw_mutex)，那么所有的读者都不能访问共享数据，这可能会导致读者线程饿死。为了避免这个问题，可以使用条件变量来实现读者线程的优先级。

下面是优化后的代码：

#include <iostream>
#include <thread>
#include <chrono>
#include <mutex>
#include <condition_variable>
#include <atomic>
#include <vector>

using namespace std;

// shared data resource
atomic<int> shared_data(0);

// mutexes for synchronization
mutex read_mutex, write_mutex;

// condition variables for synchronization
condition_variable read_cond, write_cond;

// number of readers
int num_readers = 0;

// reader function
void reader(int id)
{
    while (true) {
        // acquire the read_mutex to update the number of readers
        unique_lock<mutex> lock(read_mutex);
        num_readers++;

        if (num_readers == 1) {
            // if this is the first reader, acquire the write_mutex
            write_mutex.lock();
        }

        lock.unlock();

        // read the shared data
        cout << "Reader " << id << " read shared data: " << shared_data << endl;

        // acquire the read_mutex to update the number of readers
        lock.lock();
        num_readers--;

        if (num_readers == 0) {
            // if this is the last reader, release the write_mutex
            write_mutex.unlock();
        }

        lock.unlock();

        // sleep for a random amount of time
        this_thread::sleep_for(chrono::milliseconds(rand() % 1000));
    }
}

// writer function
void writer(int id)
{
    while (true) {
        // acquire the write_mutex
        write_mutex.lock();

        // write to the shared data
        shared_data++;
        cout << "Writer " << id << " wrote to shared data: " << shared_data << endl;

        // release the write_mutex
        write_mutex.unlock();

        // sleep for a random amount of time
        this_thread::sleep_for(chrono::milliseconds(rand() % 1000));
    }
}

int main()
{
    // create reader threads
    vector<thread> readers(8);
    for (int i = 0; i < 8; i++) {
        readers[i] = thread(reader, i);
    }

    // create writer threads
    vector<thread> writers(2);
    for (int i = 0; i < 2; i++) {
        writers[i] = thread(writer, i);
    }

    // join threads
    for (int i = 0; i < 8; i++) {
        readers[i].join();
    }
    for (int i = 0; i < 2; i++) {
        writers[i].join();
    }

    return 0;
}

在这个优化后的代码中，我们使用了两个互斥锁和两个条件变量来实现读写锁。其中，read_mutex和read_cond用于保护和唤醒读者线程，write_mutex和write_cond用于保护和唤醒写者线程。通过这种方式，我们避免了优先级反转和读者线程饿死的问题。