What is the role of spinlocks in C++ multi-threaded programming?

Spin lock is a lightweight lock used to protect shared resources. It obtains it by continuously polling the status of the lock to avoid context switching. The advantages include high efficiency, responsiveness, and scalability, but the disadvantages are that it may cause CPU waste and is not suitable for long-term locking situations.

Spin lock in C++ multi-threaded programming

Introduction

The spin lock is a lightweight lock. When a thread Used when trying to access a shared resource, it avoids context switches by always polling the status of the lock.

Principle

The working principle of a spin lock is: when a thread tries to acquire a lock, it will continuously check the status of the lock. If the lock is released, the thread acquires it immediately. If the lock has been acquired by another thread, the thread will continue to poll the lock's status until it is released.

Advantages

• High efficiency: Spin lock is more efficient than other locking mechanisms (such as mutex locks) because it avoids expensive context switches.
• Responsive: When a thread continuously polls the status of a lock, it can quickly react to the release of the lock.
• Strong scalability: Spin locks perform well in multi-processor systems because each thread can spin on its own cache line.

Limitations

• May cause CPU waste: If a lock is frequently contested, continuously polling the lock status may cause Wastes a lot of CPU resources.
• Not applicable to long-term lock situations: If a lock is held for a long time, the spin lock may cause thread starvation.

Practical case

The following code example demonstrates how to use std::atomic<bool></bool> in C++ to implement a spin lock:

#include <atomic>

class Spinlock {
private:
    std::atomic<bool> locked;

public:
    Spinlock() : locked(false) {}

    void lock() {
        while (locked.exchange(true)) { /* 旋转直到锁被释放 */ }
    }

    void unlock() {
        locked.store(false);
    }
};

int main() {
    Spinlock lock;

    // 创建多个线程来争用锁
    std::vector<std::thread> threads;
    for (int i = 0; i < 10; i++) {
        threads.push_back(std::thread([&lock] {
            lock.lock();
            // 访问共享资源
            lock.unlock();
        }));
    }

    // 等待所有线程完成
    for (std::thread& thread : threads) {
        thread.join();
    }

    return 0;
}

Conclusion

Spin lock is a powerful synchronization primitive that can be used to protect shared resources in multi-threaded C++ programs. However, they can cause CPU waste when locks are frequently contested, so caution is needed when using them.

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