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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <atomic>
#include <thread>
namespace vespalib {
/**
* A spin-lock implementation that favors uncontended performance.
* Some measures are taken to reduce the impact of threads waiting to
* get the lock since this will not affect the fast-path of obtaining
* the lock immediately.
*
* Note that multiple threads trying to obtain the lock at the same
* time will reduce performance due to atomic writes against the same
* cache line.
*
* Note that being preempted while holding the lock will reduce
* performance, even more if the thread holding the lock is lower
* priority than the threads trying to obtain the lock. With a
* deterministic scheduler this could even lead to deadlock.
*
* This implementation satisfies the BasicLockable requirements,
* making it work with things like std::lock_guard.
**/
class SpinLock {
private:
std::atomic<bool> _lock;
public:
SpinLock() noexcept : _lock(false) {
static_assert(std::atomic<bool>::is_always_lock_free);
}
void lock() noexcept {
while (__builtin_expect(_lock.exchange(true, std::memory_order_acquire), false)) {
while (_lock.load(std::memory_order_relaxed)) {
std::this_thread::yield();
}
}
}
void unlock() noexcept { _lock.store(false, std::memory_order_release); }
};
}
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