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