Concurrency

Reader-Writer Lock

Difficulty

Asked at Chicago Trading Company, Squarepoint Capital

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xxxxxxxxx xxxx, x xxxxxx, x xxxxxx xxxxx: xxxx xxxxxxxxxx, xxxxxxx xxxxxx. x xxxxx xxxxx xxxxxxxxxx xxx xxxxxxx xxxxxxx xxxx xxxxx xxx xx xxxxxx, xxx xxx xxxxxxx xxx xxxxxx xxx xx xxx xxxxxxx.

x xxxxxx/xxxxxxxxx xxxx xxxx xxxx xx xxxxxxxx xxx xxxxx xxxxxxx xxxxx. xxxxxxxxx xxxxxx xxxx xxx xxxxx xxx xxx xxxxxxxxx xxxxxxxx.

xxxx_xxxxxx() / xxxxxx_xxxxxx() — xxxx xxxxxxx xx xxxx
xxxx() / xxxxxx() — xxx xxxxxx, xxx xx xxxxxxx
xxxxxxx_xxx(), xxxxxx_xxx(), xxxxxxx_xxxxxxx() — xx xxx xxxxx xxx xxx xxxxxx

xx xxxx xx xxxxxx-xxxxxxxxxx: xxxx x xxxxxx xx xxxxxxx, xxx xxxxxxx xxxxx xxxxxx xx xxxxxx xxxx xxxxxxx xxx xxxxxxx xxxxxxx xxxxxx. xxxxxxx xxxxxxx xxxxxxx xxx xxxx xxx xxxxx xxxxxxx; xxx xxxxxxx xxxxxx xxxxx xxx xxxx.

xxxxxxxx

xxxx.xxxx_xxxxxx();
xxxx.xxxx_xxxxxx();
xxxx.xxxxxxx_xxx();   // x
xxxx.xxxxxx_xxx();    // xxxxx

xxxxxxxxxxx

xxx xxxxxxxxxx xxx xxxxx xxxxxx xxxx xxxxx xxxxx xxxxxxxxxxxx: xx xxxxxx xx xxxx xxxxxx xxxxx x xxxxxx xx, xx xxxxxx xx xxxx xxxxxx xxxxx x xxxxxx xx, xxx xxx xxxxxxxx xxxxxx xx xxxx.
x xxxxxx-xxxxxxxxxx xxxx xx xxxxxxx xxx xxxxxxx xxxxxxx. xxxxx x xxxxxx xxxx xxxx xxxxx xx xxxxxxxxxxx xxxxx x xxxxxx xxxx xxxx xxxxxxx, xx x xxxxxx xxxx xxxxx xxx xxx xxxxx xxxxxxx. xxx xxxx xxxxxxx xxx xxxx xxxx xxxx xxxxx-xxxxxx xxxxxxx xxx xxxxxxxx xxx xxxxxx xx xxxxxxxx.
xxx xxxx xx xxx xxxxxxxxxx xx xxx xxxxxx xxxxxxx: x xxxxxx xx xxxxxxx xxx xxxxxx xxxxxxx. xxxx xx x xxxx xxxxx, xxx x xxx, xxx xx xx xxx xxxxx xxx xxxxx xxxx xxxxxx xxx xxxx xxx xxxxxx xxxxxx.

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Language: C++RwLock

#include <atomic>
#include <condition_variable>
#include <cstddef>
#include <mutex>
#include <thread>
#include <vector>

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class RwLock {
public:
    void lock_shared() {
        // ...
    }

    void unlock_shared() {
        // ...
    }

    void lock() {
        // ...
    }

    void unlock() {
        // ...
    }

    long long readers_now() const { return readers_.load(); }
    bool writer_now() const { return writer_.load(); }
    long long waiting_writers() const { return waiting_.load(); }

private:
    std::mutex mutex_;
    std::condition_variable cv_;
    std::atomic<long long> readers_{0};
    std::atomic<bool> writer_{false};
    std::atomic<long long> waiting_{0};
};

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