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/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include "Assertions.h"
#include "Atomic.h"
#include "RefCounted.h"
#include "RefPtr.h"
#include "StdLibExtras.h"
#ifdef KERNEL
# include <Kernel/Arch/Processor.h>
# include <Kernel/Arch/ScopedCritical.h>
#else
# include <sched.h>
#endif
namespace AK {
template<typename T>
class Weakable;
template<typename T>
class WeakPtr;
class WeakLink : public RefCounted<WeakLink> {
template<typename T>
friend class Weakable;
template<typename T>
friend class WeakPtr;
public:
template<typename T, typename PtrTraits = RefPtrTraits<T>>
RefPtr<T, PtrTraits> strong_ref() const
requires(IsBaseOf<RefCountedBase, T>)
{
RefPtr<T, PtrTraits> ref;
{
#ifdef KERNEL
// We don't want to be pre-empted while we are trying to obtain
// a strong reference
Kernel::ScopedCritical critical;
#endif
if (!(m_consumers.fetch_add(1u << 1, AK::MemoryOrder::memory_order_acquire) & 1u)) {
T* ptr = (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
if (ptr && ptr->try_ref())
ref = adopt_ref(*ptr);
}
m_consumers.fetch_sub(1u << 1, AK::MemoryOrder::memory_order_release);
}
return ref;
}
template<typename T>
T* unsafe_ptr() const
{
if (m_consumers.load(AK::MemoryOrder::memory_order_relaxed) & 1u)
return nullptr;
// NOTE: This may return a non-null pointer even if revocation
// has been triggered as there is a possible race! But it's "unsafe"
// anyway because we return a raw pointer without ensuring a
// reference...
return (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
}
bool is_null() const
{
return unsafe_ptr<void>() == nullptr;
}
void revoke()
{
auto current_consumers = m_consumers.fetch_or(1u, AK::MemoryOrder::memory_order_relaxed);
VERIFY(!(current_consumers & 1u));
// We flagged revocation, now wait until everyone trying to obtain
// a strong reference is done
while (current_consumers > 0) {
#ifdef KERNEL
Kernel::Processor::wait_check();
#else
sched_yield();
#endif
current_consumers = m_consumers.load(AK::MemoryOrder::memory_order_acquire) & ~1u;
}
// No one is trying to use it (anymore)
m_ptr.store(nullptr, AK::MemoryOrder::memory_order_release);
}
private:
template<typename T>
explicit WeakLink(T& weakable)
: m_ptr(&weakable)
{
}
mutable Atomic<void*> m_ptr;
mutable Atomic<unsigned> m_consumers; // LSB indicates revocation in progress
};
template<typename T>
class Weakable {
private:
class Link;
public:
#ifndef KERNEL
template<typename U = T>
WeakPtr<U> make_weak_ptr() const
{
return MUST(try_make_weak_ptr<U>());
}
#endif
template<typename U = T>
ErrorOr<WeakPtr<U>> try_make_weak_ptr() const;
protected:
Weakable() = default;
~Weakable()
{
#ifdef KERNEL
m_being_destroyed.store(true, AK::MemoryOrder::memory_order_release);
#endif
revoke_weak_ptrs();
}
void revoke_weak_ptrs()
{
if (auto link = move(m_link))
link->revoke();
}
private:
mutable RefPtr<WeakLink> m_link;
#ifdef KERNEL
Atomic<bool> m_being_destroyed { false };
#endif
};
}
using AK::Weakable;
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