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/* 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/. */
// A class for values only accessible from a single designated thread.
#ifndef mozilla_ThreadBound_h
#define mozilla_ThreadBound_h
#include "mozilla/Atomics.h"
#include "prthread.h"
#include <type_traits>
namespace mozilla {
template <typename T>
class ThreadBound;
namespace detail {
template <bool Condition, typename T>
struct AddConstIf {
using type = T;
};
template <typename T>
struct AddConstIf<true, T> {
using type = typename std::add_const<T>::type;
};
} // namespace detail
// A ThreadBound<T> is a T that can only be accessed by a specific
// thread. To enforce this rule, the inner T is only accessible
// through a non-copyable, immovable accessor object.
// Given a ThreadBound<T> threadBoundData, it can be accessed like so:
//
// auto innerData = threadBoundData.Access();
// innerData->DoStuff();
//
// Trying to access a ThreadBound<T> from a different thread will
// trigger a MOZ_DIAGNOSTIC_ASSERT.
// The encapsulated T is constructed during the construction of the
// enclosing ThreadBound<T> by forwarding all of the latter's
// constructor parameters to the former. A newly constructed
// ThreadBound<T> is bound to the thread it's constructed in. It's
// possible to rebind the data to some otherThread by calling
//
// threadBoundData.Transfer(otherThread);
//
// on the thread that threadBoundData is currently bound to, as long
// as it's not currently being accessed. (Trying to rebind from
// another thread or while an accessor exists will trigger an
// assertion.)
//
// Note: A ThreadBound<T> may be destructed from any thread, not just
// its designated thread at the time the destructor is invoked.
template <typename T>
class ThreadBound final {
public:
template <typename... Args>
explicit ThreadBound(Args&&... aArgs)
: mData(std::forward<Args>(aArgs)...),
mThread(PR_GetCurrentThread()),
mAccessCount(0) {}
~ThreadBound() { AssertIsNotCurrentlyAccessed(); }
void Transfer(const PRThread* const aDest) {
AssertIsCorrectThread();
AssertIsNotCurrentlyAccessed();
mThread = aDest;
}
private:
T mData;
// This member is (potentially) accessed by multiple threads and is
// thus the first point of synchronization between them.
Atomic<const PRThread*, ReleaseAcquire> mThread;
// In order to support nested accesses (e.g. from different stack
// frames) it's necessary to maintain a counter of the existing
// accessor. Since it's possible to access a const ThreadBound, the
// counter is mutable. It's atomic because accessing it synchronizes
// access to mData (see comment in Accessor's constructor).
using AccessCountType = Atomic<int, ReleaseAcquire>;
mutable AccessCountType mAccessCount;
public:
template <bool IsConst>
class MOZ_STACK_CLASS Accessor final {
using DataType = typename detail::AddConstIf<IsConst, T>::type;
public:
explicit Accessor(
typename detail::AddConstIf<IsConst, ThreadBound>::type& aThreadBound)
: mData(aThreadBound.mData), mAccessCount(aThreadBound.mAccessCount) {
aThreadBound.AssertIsCorrectThread();
// This load/store serves as a memory fence that guards mData
// against accesses that would trip the thread assertion.
// (Otherwise one of the loads in the caller's instruction
// stream might be scheduled before the assertion.)
++mAccessCount;
}
Accessor(const Accessor&) = delete;
Accessor(Accessor&&) = delete;
Accessor& operator=(const Accessor&) = delete;
Accessor& operator=(Accessor&&) = delete;
~Accessor() { --mAccessCount; }
DataType* operator->() { return &mData; }
private:
DataType& mData;
AccessCountType& mAccessCount;
};
auto Access() { return Accessor<false>{*this}; }
auto Access() const { return Accessor<true>{*this}; }
private:
bool IsCorrectThread() const { return mThread == PR_GetCurrentThread(); }
bool IsNotCurrentlyAccessed() const { return mAccessCount == 0; }
#define MOZ_DEFINE_THREAD_BOUND_ASSERT(predicate) \
void Assert##predicate() const { MOZ_DIAGNOSTIC_ASSERT(predicate()); }
MOZ_DEFINE_THREAD_BOUND_ASSERT(IsCorrectThread)
MOZ_DEFINE_THREAD_BOUND_ASSERT(IsNotCurrentlyAccessed)
#undef MOZ_DEFINE_THREAD_BOUND_ASSERT
};
} // namespace mozilla
#endif // mozilla_ThreadBound_h
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