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// Copyright 2025 Dolphin Emulator Project
// SPDX-License-Identifier: GPL-2.0-or-later
#pragma once
#include <atomic>
#include <cassert>
#include <cstdint>
namespace Common
{
// Behaves like `std::shared_mutex` but locks and unlocks may come from different threads.
class TransferableSharedMutex
{
public:
void lock()
{
while (true)
{
CounterType old_value{};
if (m_counter.compare_exchange_strong(old_value, EXCLUSIVE_LOCK_VALUE,
std::memory_order_acquire, std::memory_order_relaxed))
{
return;
}
// lock() or lock_shared() is already held.
// Wait for an unlock notification and try again.
m_counter.wait(old_value, std::memory_order_relaxed);
}
}
bool try_lock()
{
CounterType old_value{};
return m_counter.compare_exchange_weak(old_value, EXCLUSIVE_LOCK_VALUE,
std::memory_order_acquire, std::memory_order_relaxed);
}
void unlock()
{
m_counter.store(0, std::memory_order_release);
m_counter.notify_all(); // Notify potentially multiple wait()ers in lock_shared().
}
void lock_shared()
{
while (true)
{
auto old_value = m_counter.load(std::memory_order_relaxed);
while (old_value < LAST_SHARED_LOCK_VALUE)
{
if (m_counter.compare_exchange_strong(old_value, old_value + 1, std::memory_order_acquire,
std::memory_order_relaxed))
{
return;
}
}
// Something has gone very wrong if m_counter is nearly saturated with shared_lock().
assert(old_value != LAST_SHARED_LOCK_VALUE);
// lock() is already held.
// Wait for an unlock notification and try again.
m_counter.wait(old_value, std::memory_order_relaxed);
}
}
bool try_lock_shared()
{
auto old_value = m_counter.load(std::memory_order_relaxed);
return (old_value < LAST_SHARED_LOCK_VALUE) &&
m_counter.compare_exchange_weak(old_value, old_value + 1, std::memory_order_acquire,
std::memory_order_relaxed);
}
void unlock_shared()
{
if (m_counter.fetch_sub(1, std::memory_order_release) == 1)
m_counter.notify_one(); // Notify one of the wait()ers in lock().
}
private:
using CounterType = std::uintptr_t;
static constexpr auto EXCLUSIVE_LOCK_VALUE = CounterType(-1);
static constexpr auto LAST_SHARED_LOCK_VALUE = EXCLUSIVE_LOCK_VALUE - 1;
std::atomic<CounterType> m_counter{};
};
} // namespace Common
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