From 26a029d407be480d791972afb5975cf62c9360a6 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Fri, 19 Apr 2024 02:47:55 +0200 Subject: Adding upstream version 124.0.1. Signed-off-by: Daniel Baumann --- mfbt/RefCounted.h | 327 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 327 insertions(+) create mode 100644 mfbt/RefCounted.h (limited to 'mfbt/RefCounted.h') diff --git a/mfbt/RefCounted.h b/mfbt/RefCounted.h new file mode 100644 index 0000000000..5c083f3524 --- /dev/null +++ b/mfbt/RefCounted.h @@ -0,0 +1,327 @@ +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ +/* vim: set ts=8 sts=2 et sw=2 tw=80: */ +/* This Source Code Form is subject to the terms of the Mozilla Public + * License, v. 2.0. If a copy of the MPL was not distributed with this + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ + +/* CRTP refcounting templates. Do not use unless you are an Expert. */ + +#ifndef mozilla_RefCounted_h +#define mozilla_RefCounted_h + +#include +#include + +#include "mozilla/AlreadyAddRefed.h" +#include "mozilla/Assertions.h" +#include "mozilla/Atomics.h" +#include "mozilla/Attributes.h" +#include "mozilla/RefCountType.h" + +#ifdef __wasi__ +# include "mozilla/WasiAtomic.h" +#else +# include +#endif // __wasi__ + +#if defined(MOZILLA_INTERNAL_API) +# include "nsXPCOM.h" +#endif + +#if defined(MOZILLA_INTERNAL_API) && defined(NS_BUILD_REFCNT_LOGGING) +# define MOZ_REFCOUNTED_LEAK_CHECKING +#endif + +namespace mozilla { + +/** + * RefCounted is a sort of a "mixin" for a class T. RefCounted + * manages, well, refcounting for T, and because RefCounted is + * parameterized on T, RefCounted can call T's destructor directly. + * This means T doesn't need to have a virtual dtor and so doesn't + * need a vtable. + * + * RefCounted is created with refcount == 0. Newly-allocated + * RefCounted must immediately be assigned to a RefPtr to make the + * refcount > 0. It's an error to allocate and free a bare + * RefCounted, i.e. outside of the RefPtr machinery. Attempts to + * do so will abort DEBUG builds. + * + * Live RefCounted have refcount > 0. The lifetime (refcounts) of + * live RefCounted are controlled by RefPtr and + * RefPtr. Upon a transition from refcounted==1 + * to 0, the RefCounted "dies" and is destroyed. The "destroyed" + * state is represented in DEBUG builds by refcount==0xffffdead. This + * state distinguishes use-before-ref (refcount==0) from + * use-after-destroy (refcount==0xffffdead). + * + * Note that when deriving from RefCounted or AtomicRefCounted, you + * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public + * section of your class, where ClassName is the name of your class. + * + * Note: SpiderMonkey should use js::RefCounted instead since that type + * will use appropriate js_delete and also not break ref-count logging. + */ +namespace detail { +const MozRefCountType DEAD = 0xffffdead; + +// When building code that gets compiled into Gecko, try to use the +// trace-refcount leak logging facilities. +class RefCountLogger { + public: + // Called by `RefCounted`-like classes to log a successful AddRef call in the + // Gecko leak-logging system. This call is a no-op outside of Gecko. Should be + // called afer incrementing the reference count. + template + static void logAddRef(const T* aPointer, MozRefCountType aRefCount) { +#ifdef MOZ_REFCOUNTED_LEAK_CHECKING + const void* pointer = aPointer; + const char* typeName = aPointer->typeName(); + uint32_t typeSize = aPointer->typeSize(); + NS_LogAddRef(const_cast(pointer), aRefCount, typeName, typeSize); +#endif + } + + // Created by `RefCounted`-like classes to log a successful Release call in + // the Gecko leak-logging system. The constructor should be invoked before the + // refcount is decremented to avoid invoking `typeName()` with a zero + // reference count. This call is a no-op outside of Gecko. + class MOZ_STACK_CLASS ReleaseLogger final { + public: + template + explicit ReleaseLogger(const T* aPointer) +#ifdef MOZ_REFCOUNTED_LEAK_CHECKING + : mPointer(aPointer), + mTypeName(aPointer->typeName()) +#endif + { + } + + void logRelease(MozRefCountType aRefCount) { +#ifdef MOZ_REFCOUNTED_LEAK_CHECKING + MOZ_ASSERT(aRefCount != DEAD); + NS_LogRelease(const_cast(mPointer), aRefCount, mTypeName); +#endif + } + +#ifdef MOZ_REFCOUNTED_LEAK_CHECKING + const void* mPointer; + const char* mTypeName; +#endif + }; +}; + +// This is used WeakPtr.h as well as this file. +enum RefCountAtomicity { AtomicRefCount, NonAtomicRefCount }; + +template +class RC { + public: + explicit RC(T aCount) : mValue(aCount) {} + + RC(const RC&) = delete; + RC& operator=(const RC&) = delete; + RC(RC&&) = delete; + RC& operator=(RC&&) = delete; + + T operator++() { return ++mValue; } + T operator--() { return --mValue; } + +#ifdef DEBUG + void operator=(const T& aValue) { mValue = aValue; } +#endif + + operator T() const { return mValue; } + + private: + T mValue; +}; + +template +class RC { + public: + explicit RC(T aCount) : mValue(aCount) {} + + RC(const RC&) = delete; + RC& operator=(const RC&) = delete; + RC(RC&&) = delete; + RC& operator=(RC&&) = delete; + + T operator++() { + // Memory synchronization is not required when incrementing a + // reference count. The first increment of a reference count on a + // thread is not important, since the first use of the object on a + // thread can happen before it. What is important is the transfer + // of the pointer to that thread, which may happen prior to the + // first increment on that thread. The necessary memory + // synchronization is done by the mechanism that transfers the + // pointer between threads. + return mValue.fetch_add(1, std::memory_order_relaxed) + 1; + } + + T operator--() { + // Since this may be the last release on this thread, we need + // release semantics so that prior writes on this thread are visible + // to the thread that destroys the object when it reads mValue with + // acquire semantics. + T result = mValue.fetch_sub(1, std::memory_order_release) - 1; + if (result == 0) { + // We're going to destroy the object on this thread, so we need + // acquire semantics to synchronize with the memory released by + // the last release on other threads, that is, to ensure that + // writes prior to that release are now visible on this thread. +#if defined(MOZ_TSAN) || defined(__wasi__) + // TSan doesn't understand std::atomic_thread_fence, so in order + // to avoid a false positive for every time a refcounted object + // is deleted, we replace the fence with an atomic operation. + mValue.load(std::memory_order_acquire); +#else + std::atomic_thread_fence(std::memory_order_acquire); +#endif + } + return result; + } + +#ifdef DEBUG + // This method is only called in debug builds, so we're not too concerned + // about its performance. + void operator=(const T& aValue) { + mValue.store(aValue, std::memory_order_seq_cst); + } +#endif + + operator T() const { + // Use acquire semantics since we're not sure what the caller is + // doing. + return mValue.load(std::memory_order_acquire); + } + + T IncrementIfNonzero() { + // This can be a relaxed load as any write of 0 that we observe will leave + // the field in a permanently zero (or `DEAD`) state (so a "stale" read of 0 + // is fine), and any other value is confirmed by the CAS below. + // + // This roughly matches rust's Arc::upgrade implementation as of rust 1.49.0 + T prev = mValue.load(std::memory_order_relaxed); + while (prev != 0) { + MOZ_ASSERT(prev != detail::DEAD, + "Cannot IncrementIfNonzero if marked as dead!"); + // TODO: It may be possible to use relaxed success ordering here? + if (mValue.compare_exchange_weak(prev, prev + 1, + std::memory_order_acquire, + std::memory_order_relaxed)) { + return prev + 1; + } + } + return 0; + } + + private: + std::atomic mValue; +}; + +template +class RefCounted { + protected: + RefCounted() : mRefCnt(0) {} +#ifdef DEBUG + ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); } +#endif + + public: + // Compatibility with RefPtr. + void AddRef() const { + // Note: this method must be thread safe for AtomicRefCounted. + MOZ_ASSERT(int32_t(mRefCnt) >= 0); + MozRefCountType cnt = ++mRefCnt; + detail::RefCountLogger::logAddRef(static_cast(this), cnt); + } + + void Release() const { + // Note: this method must be thread safe for AtomicRefCounted. + MOZ_ASSERT(int32_t(mRefCnt) > 0); + detail::RefCountLogger::ReleaseLogger logger(static_cast(this)); + MozRefCountType cnt = --mRefCnt; + // Note: it's not safe to touch |this| after decrementing the refcount, + // except for below. + logger.logRelease(cnt); + if (0 == cnt) { + // Because we have atomically decremented the refcount above, only + // one thread can get a 0 count here, so as long as we can assume that + // everything else in the system is accessing this object through + // RefPtrs, it's safe to access |this| here. +#ifdef DEBUG + mRefCnt = detail::DEAD; +#endif + delete static_cast(this); + } + } + + using HasThreadSafeRefCnt = + std::integral_constant; + + // Compatibility with wtf::RefPtr. + void ref() { AddRef(); } + void deref() { Release(); } + MozRefCountType refCount() const { return mRefCnt; } + bool hasOneRef() const { + MOZ_ASSERT(mRefCnt > 0); + return mRefCnt == 1; + } + + private: + mutable RC mRefCnt; +}; + +#ifdef MOZ_REFCOUNTED_LEAK_CHECKING +// Passing override for the optional argument marks the typeName and +// typeSize functions defined by this macro as overrides. +# define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...) \ + virtual const char* typeName() const __VA_ARGS__ { return #T; } \ + virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); } +#else +# define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...) +#endif + +// Note that this macro is expanded unconditionally because it declares only +// two small inline functions which will hopefully get eliminated by the linker +// in non-leak-checking builds. +#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T) \ + const char* typeName() const { return #T; } \ + size_t typeSize() const { return sizeof(*this); } + +} // namespace detail + +template +class RefCounted : public detail::RefCounted { + public: + ~RefCounted() { + static_assert(std::is_base_of::value, + "T must derive from RefCounted"); + } +}; + +namespace external { + +/** + * AtomicRefCounted is like RefCounted, with an atomically updated + * reference counter. + * + * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING + * instead. + */ +template +class AtomicRefCounted + : public mozilla::detail::RefCounted { + public: + ~AtomicRefCounted() { + static_assert(std::is_base_of::value, + "T must derive from AtomicRefCounted"); + } +}; + +} // namespace external + +} // namespace mozilla + +#endif // mozilla_RefCounted_h -- cgit v1.2.3