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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_MEMORY_REF_COUNTED_H_
#define BASE_MEMORY_REF_COUNTED_H_
#include <stddef.h>
#include <utility>
#include "base/atomic_ref_count.h"
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/scoped_refptr.h"
#include "util/build_config.h"
namespace base {
namespace subtle {
class RefCountedBase {
public:
bool HasOneRef() const { return ref_count_ == 1; }
protected:
explicit RefCountedBase(StartRefCountFromZeroTag) {}
explicit RefCountedBase(StartRefCountFromOneTag) : ref_count_(1) {}
~RefCountedBase() {}
void AddRef() const { AddRefImpl(); }
// Returns true if the object should self-delete.
bool Release() const {
--ref_count_;
// TODO(maruel): Add back once it doesn't assert 500 times/sec.
// Current thread books the critical section "AddRelease"
// without release it.
// DFAKE_SCOPED_LOCK_THREAD_LOCKED(add_release_);
return ref_count_ == 0;
}
// Returns true if it is safe to read or write the object, from a thread
// safety standpoint. Should be DCHECK'd from the methods of RefCounted
// classes if there is a danger of objects being shared across threads.
//
// This produces fewer false positives than adding a separate SequenceChecker
// into the subclass, because it automatically detaches from the sequence when
// the reference count is 1 (and never fails if there is only one reference).
//
// This means unlike a separate SequenceChecker, it will permit a singly
// referenced object to be passed between threads (not holding a reference on
// the sending thread), but will trap if the sending thread holds onto a
// reference, or if the object is accessed from multiple threads
// simultaneously.
bool IsOnValidSequence() const { return true; }
private:
template <typename U>
friend scoped_refptr<U> base::AdoptRef(U*);
void Adopted() const {}
#if defined(ARCH_CPU_64_BIT)
void AddRefImpl() const;
#else
void AddRefImpl() const { ++ref_count_; }
#endif
mutable uint32_t ref_count_ = 0;
DISALLOW_COPY_AND_ASSIGN(RefCountedBase);
};
class RefCountedThreadSafeBase {
public:
bool HasOneRef() const;
protected:
explicit constexpr RefCountedThreadSafeBase(StartRefCountFromZeroTag) {}
explicit constexpr RefCountedThreadSafeBase(StartRefCountFromOneTag)
: ref_count_(1) {}
~RefCountedThreadSafeBase() = default;
// Release and AddRef are suitable for inlining on X86 because they generate
// very small code sequences. On other platforms (ARM), it causes a size
// regression and is probably not worth it.
#if defined(ARCH_CPU_X86_FAMILY)
// Returns true if the object should self-delete.
bool Release() const { return ReleaseImpl(); }
void AddRef() const { AddRefImpl(); }
#else
// Returns true if the object should self-delete.
bool Release() const;
void AddRef() const;
#endif
private:
template <typename U>
friend scoped_refptr<U> base::AdoptRef(U*);
void Adopted() const {}
ALWAYS_INLINE void AddRefImpl() const { ref_count_.Increment(); }
ALWAYS_INLINE bool ReleaseImpl() const {
if (!ref_count_.Decrement()) {
return true;
}
return false;
}
mutable AtomicRefCount ref_count_{0};
DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafeBase);
};
} // namespace subtle
// ScopedAllowCrossThreadRefCountAccess disables the check documented on
// RefCounted below for rare pre-existing use cases where thread-safety was
// guaranteed through other means (e.g. explicit sequencing of calls across
// execution sequences when bouncing between threads in order). New callers
// should refrain from using this (callsites handling thread-safety through
// locks should use RefCountedThreadSafe per the overhead of its atomics being
// negligible compared to locks anyways and callsites doing explicit sequencing
// should properly std::move() the ref to avoid hitting this check).
// TODO(tzik): Cleanup existing use cases and remove
// ScopedAllowCrossThreadRefCountAccess.
class ScopedAllowCrossThreadRefCountAccess final {
public:
ScopedAllowCrossThreadRefCountAccess() {}
~ScopedAllowCrossThreadRefCountAccess() {}
};
//
// A base class for reference counted classes. Otherwise, known as a cheap
// knock-off of WebKit's RefCounted<T> class. To use this, just extend your
// class from it like so:
//
// class MyFoo : public base::RefCounted<MyFoo> {
// ...
// private:
// friend class base::RefCounted<MyFoo>;
// ~MyFoo();
// };
//
// You should always make your destructor non-public, to avoid any code deleting
// the object accidentally while there are references to it.
//
//
// The ref count manipulation to RefCounted is NOT thread safe and has DCHECKs
// to trap unsafe cross thread usage. A subclass instance of RefCounted can be
// passed to another execution sequence only when its ref count is 1. If the ref
// count is more than 1, the RefCounted class verifies the ref updates are made
// on the same execution sequence as the previous ones. The subclass can also
// manually call IsOnValidSequence to trap other non-thread-safe accesses; see
// the documentation for that method.
//
//
// The reference count starts from zero by default, and we intended to migrate
// to start-from-one ref count. Put REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE() to
// the ref counted class to opt-in.
//
// If an object has start-from-one ref count, the first scoped_refptr need to be
// created by base::AdoptRef() or base::MakeRefCounted(). We can use
// base::MakeRefCounted() to create create both type of ref counted object.
//
// The motivations to use start-from-one ref count are:
// - Start-from-one ref count doesn't need the ref count increment for the
// first reference.
// - It can detect an invalid object acquisition for a being-deleted object
// that has zero ref count. That tends to happen on custom deleter that
// delays the deletion.
// TODO(tzik): Implement invalid acquisition detection.
// - Behavior parity to Blink's WTF::RefCounted, whose count starts from one.
// And start-from-one ref count is a step to merge WTF::RefCounted into
// base::RefCounted.
//
#define REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE() \
static constexpr ::base::subtle::StartRefCountFromOneTag \
kRefCountPreference = ::base::subtle::kStartRefCountFromOneTag
template <class T, typename Traits>
class RefCounted;
template <typename T>
struct DefaultRefCountedTraits {
static void Destruct(const T* x) {
RefCounted<T, DefaultRefCountedTraits>::DeleteInternal(x);
}
};
template <class T, typename Traits = DefaultRefCountedTraits<T>>
class RefCounted : public subtle::RefCountedBase {
public:
static constexpr subtle::StartRefCountFromZeroTag kRefCountPreference =
subtle::kStartRefCountFromZeroTag;
RefCounted() : subtle::RefCountedBase(T::kRefCountPreference) {}
void AddRef() const { subtle::RefCountedBase::AddRef(); }
void Release() const {
if (subtle::RefCountedBase::Release()) {
// Prune the code paths which the static analyzer may take to simulate
// object destruction. Use-after-free errors aren't possible given the
// lifetime guarantees of the refcounting system.
ANALYZER_SKIP_THIS_PATH();
Traits::Destruct(static_cast<const T*>(this));
}
}
protected:
~RefCounted() = default;
private:
friend struct DefaultRefCountedTraits<T>;
template <typename U>
static void DeleteInternal(const U* x) {
delete x;
}
DISALLOW_COPY_AND_ASSIGN(RefCounted);
};
// Forward declaration.
template <class T, typename Traits>
class RefCountedThreadSafe;
// Default traits for RefCountedThreadSafe<T>. Deletes the object when its ref
// count reaches 0. Overload to delete it on a different thread etc.
template <typename T>
struct DefaultRefCountedThreadSafeTraits {
static void Destruct(const T* x) {
// Delete through RefCountedThreadSafe to make child classes only need to be
// friend with RefCountedThreadSafe instead of this struct, which is an
// implementation detail.
RefCountedThreadSafe<T, DefaultRefCountedThreadSafeTraits>::DeleteInternal(
x);
}
};
//
// A thread-safe variant of RefCounted<T>
//
// class MyFoo : public base::RefCountedThreadSafe<MyFoo> {
// ...
// };
//
// If you're using the default trait, then you should add compile time
// asserts that no one else is deleting your object. i.e.
// private:
// friend class base::RefCountedThreadSafe<MyFoo>;
// ~MyFoo();
//
// We can use REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE() with RefCountedThreadSafe
// too. See the comment above the RefCounted definition for details.
template <class T, typename Traits = DefaultRefCountedThreadSafeTraits<T>>
class RefCountedThreadSafe : public subtle::RefCountedThreadSafeBase {
public:
static constexpr subtle::StartRefCountFromZeroTag kRefCountPreference =
subtle::kStartRefCountFromZeroTag;
explicit RefCountedThreadSafe()
: subtle::RefCountedThreadSafeBase(T::kRefCountPreference) {}
void AddRef() const { subtle::RefCountedThreadSafeBase::AddRef(); }
void Release() const {
if (subtle::RefCountedThreadSafeBase::Release()) {
ANALYZER_SKIP_THIS_PATH();
Traits::Destruct(static_cast<const T*>(this));
}
}
protected:
~RefCountedThreadSafe() = default;
private:
friend struct DefaultRefCountedThreadSafeTraits<T>;
template <typename U>
static void DeleteInternal(const U* x) {
delete x;
}
DISALLOW_COPY_AND_ASSIGN(RefCountedThreadSafe);
};
//
// A thread-safe wrapper for some piece of data so we can place other
// things in scoped_refptrs<>.
//
template <typename T>
class RefCountedData
: public base::RefCountedThreadSafe<base::RefCountedData<T>> {
public:
RefCountedData() : data() {}
RefCountedData(const T& in_value) : data(in_value) {}
RefCountedData(T&& in_value) : data(std::move(in_value)) {}
T data;
private:
friend class base::RefCountedThreadSafe<base::RefCountedData<T>>;
~RefCountedData() = default;
};
} // namespace base
#endif // BASE_MEMORY_REF_COUNTED_H_