// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP___STOP_TOKEN_STOP_STATE_H
#define _LIBCPP___STOP_TOKEN_STOP_STATE_H
#include <__assert>
#include <__config>
#include <__stop_token/atomic_unique_lock.h>
#include <__stop_token/intrusive_list_view.h>
#include <__thread/id.h>
#include <atomic>
#include <cstdint>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_THREADS)
struct __stop_callback_base : __intrusive_node_base<__stop_callback_base> {
using __callback_fn_t = void(__stop_callback_base*) noexcept;
_LIBCPP_HIDE_FROM_ABI explicit __stop_callback_base(__callback_fn_t* __callback_fn) : __callback_fn_(__callback_fn) {}
_LIBCPP_HIDE_FROM_ABI void __invoke() noexcept { __callback_fn_(this); }
__callback_fn_t* __callback_fn_;
atomic<bool> __completed_ = false;
bool* __destroyed_ = nullptr;
};
class __stop_state {
static constexpr uint32_t __stop_requested_bit = 1;
static constexpr uint32_t __callback_list_locked_bit = 1 << 1;
static constexpr uint32_t __stop_source_counter_shift = 2;
// The "stop_source counter" is not used for lifetime reference counting.
// When the number of stop_source reaches 0, the remaining stop_tokens's
// stop_possible will return false. We need this counter to track this.
//
// The "callback list locked" bit implements the atomic_unique_lock to
// guard the operations on the callback list
//
// 31 - 2 | 1 | 0 |
// stop_source counter | callback list locked | stop_requested |
atomic<uint32_t> __state_ = 0;
// Reference count for stop_token + stop_callback + stop_source
// When the counter reaches zero, the state is destroyed
// It is used by __intrusive_shared_ptr, but it is stored here for better layout
atomic<uint32_t> __ref_count_ = 0;
using __state_t = uint32_t;
using __callback_list_lock = __atomic_unique_lock<__state_t, __callback_list_locked_bit>;
using __callback_list = __intrusive_list_view<__stop_callback_base>;
__callback_list __callback_list_;
__thread_id __requesting_thread_;
public:
_LIBCPP_HIDE_FROM_ABI __stop_state() noexcept = default;
_LIBCPP_HIDE_FROM_ABI void __increment_stop_source_counter() noexcept {
_LIBCPP_ASSERT_UNCATEGORIZED(
__state_.load(std::memory_order_relaxed) <= static_cast<__state_t>(~(1 << __stop_source_counter_shift)),
"stop_source's counter reaches the maximum. Incrementing the counter will overflow");
__state_.fetch_add(1 << __stop_source_counter_shift, std::memory_order_relaxed);
}
// We are not destroying the object after counter decrements to zero, nor do we have
// operations depend on the ordering of decrementing the counter. relaxed is enough.
_LIBCPP_HIDE_FROM_ABI void __decrement_stop_source_counter() noexcept {
_LIBCPP_ASSERT_UNCATEGORIZED(
__state_.load(std::memory_order_relaxed) >= static_cast<__state_t>(1 << __stop_source_counter_shift),
"stop_source's counter is 0. Decrementing the counter will underflow");
__state_.fetch_sub(1 << __stop_source_counter_shift, std::memory_order_relaxed);
}
_LIBCPP_HIDE_FROM_ABI bool __stop_requested() const noexcept {
// acquire because [thread.stoptoken.intro] A call to request_stop that returns true
// synchronizes with a call to stop_requested on an associated stop_token or stop_source
// object that returns true.
// request_stop's compare_exchange_weak has release which syncs with this acquire
return (__state_.load(std::memory_order_acquire) & __stop_requested_bit) != 0;
}
_LIBCPP_HIDE_FROM_ABI bool __stop_possible_for_stop_token() const noexcept {
// [stoptoken.mem] false if "a stop request was not made and there are no associated stop_source objects"
// Todo: Can this be std::memory_order_relaxed as the standard does not say anything except not to introduce data
// race?
__state_t __curent_state = __state_.load(std::memory_order_acquire);
return ((__curent_state & __stop_requested_bit) != 0) || ((__curent_state >> __stop_source_counter_shift) != 0);
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __request_stop() noexcept {
auto __cb_list_lock = __try_lock_for_request_stop();
if (!__cb_list_lock.__owns_lock()) {
return false;
}
__requesting_thread_ = this_thread::get_id();
while (!__callback_list_.__empty()) {
auto __cb = __callback_list_.__pop_front();
// allow other callbacks to be removed while invoking the current callback
__cb_list_lock.__unlock();
bool __destroyed = false;
__cb->__destroyed_ = &__destroyed;
__cb->__invoke();
// __cb's invoke function could potentially delete itself. We need to check before accessing __cb's member
if (!__destroyed) {
// needs to set __destroyed_ pointer to nullptr, otherwise it points to a local variable
// which is to be destroyed at the end of the loop
__cb->__destroyed_ = nullptr;
// [stopcallback.cons] If callback is concurrently executing on another thread, then the return
// from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
// this release syncs with the acquire in the remove_callback
__cb->__completed_.store(true, std::memory_order_release);
__cb->__completed_.notify_all();
}
__cb_list_lock.__lock();
}
return true;
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __add_callback(__stop_callback_base* __cb) noexcept {
// If it is already stop_requested. Do not try to request it again.
const auto __give_up_trying_to_lock_condition = [__cb](__state_t __state) {
if ((__state & __stop_requested_bit) != 0) {
// already stop requested, synchronously run the callback and no need to lock the list again
__cb->__invoke();
return true;
}
// no stop source. no need to lock the list to add the callback as it can never be invoked
return (__state >> __stop_source_counter_shift) == 0;
};
__callback_list_lock __cb_list_lock(__state_, __give_up_trying_to_lock_condition);
if (!__cb_list_lock.__owns_lock()) {
return false;
}
__callback_list_.__push_front(__cb);
return true;
// unlock here: [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of
// that callback.
// Note: this release sync with the acquire in the request_stop' __try_lock_for_request_stop
}
// called by the destructor of stop_callback
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void __remove_callback(__stop_callback_base* __cb) noexcept {
__callback_list_lock __cb_list_lock(__state_);
// under below condition, the request_stop call just popped __cb from the list and could execute it now
bool __potentially_executing_now = __cb->__prev_ == nullptr && !__callback_list_.__is_head(__cb);
if (__potentially_executing_now) {
auto __requested_thread = __requesting_thread_;
__cb_list_lock.__unlock();
if (std::this_thread::get_id() != __requested_thread) {
// [stopcallback.cons] If callback is concurrently executing on another thread, then the return
// from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
__cb->__completed_.wait(false, std::memory_order_acquire);
} else {
// The destructor of stop_callback runs on the same thread of the thread that invokes the callback.
// The callback is potentially invoking its own destuctor. Set the flag to avoid accessing destroyed
// members on the invoking side
if (__cb->__destroyed_) {
*__cb->__destroyed_ = true;
}
}
} else {
__callback_list_.__remove(__cb);
}
}
private:
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI __callback_list_lock __try_lock_for_request_stop() noexcept {
// If it is already stop_requested, do not try to request stop or lock the list again.
const auto __lock_fail_condition = [](__state_t __state) { return (__state & __stop_requested_bit) != 0; };
// set locked and requested bit at the same time
const auto __after_lock_state = [](__state_t __state) {
return __state | __callback_list_locked_bit | __stop_requested_bit;
};
// acq because [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of that
// callback. We are going to invoke the callback after getting the lock, acquire so that we can see the
// registration of a callback (and other writes that happens-before the add_callback)
// Note: the rel (unlock) in the add_callback syncs with this acq
// rel because [thread.stoptoken.intro] A call to request_stop that returns true synchronizes with a call
// to stop_requested on an associated stop_token or stop_source object that returns true.
// We need to make sure that all writes (including user code) before request_stop will be made visible
// to the threads that waiting for `stop_requested == true`
// Note: this rel syncs with the acq in `stop_requested`
const auto __locked_ordering = std::memory_order_acq_rel;
return __callback_list_lock(__state_, __lock_fail_condition, __after_lock_state, __locked_ordering);
}
template <class _Tp>
friend struct __intrusive_shared_ptr_traits;
};
template <class _Tp>
struct __intrusive_shared_ptr_traits;
template <>
struct __intrusive_shared_ptr_traits<__stop_state> {
_LIBCPP_HIDE_FROM_ABI static atomic<uint32_t>& __get_atomic_ref_count(__stop_state& __state) {
return __state.__ref_count_;
}
};
#endif // _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_THREADS)
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___STOP_TOKEN_STOP_STATE_H
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