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/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include "Assertions.h"
#include "Atomic.h"
#include "RefCounted.h"
#include "RefPtr.h"
#include "StdLibExtras.h"
#ifdef KERNEL
# include <Kernel/Arch/x86/CPU.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>, typename EnableIf<IsBaseOf<RefCountedBase, T>>::Type* = nullptr>
RefPtr<T, PtrTraits> strong_ref() const
{
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(*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>();
}
void revoke()
{
auto current_consumers = m_consumers.fetch_or(1u, AK::MemoryOrder::memory_order_relaxed);
VERIFY(!(current_consumers & 1u));
// We flagged revokation, now wait until everyone trying to obtain
// a strong reference is done
while (current_consumers > 0) {
#ifdef KERNEL
Kernel::Processor::wait_check();
#else
// TODO: 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 revokation in progress
};
template<typename T>
class Weakable {
private:
class Link;
public:
template<typename U = T>
WeakPtr<U> make_weak_ptr() const;
protected:
Weakable() = default;
~Weakable()
{
m_being_destroyed.store(true, AK::MemoryOrder::memory_order_release);
revoke_weak_ptrs();
}
void revoke_weak_ptrs()
{
if (auto link = move(m_link))
link->revoke();
}
private:
mutable RefPtr<WeakLink> m_link;
Atomic<bool> m_being_destroyed { false };
};
}
using AK::Weakable;
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