1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___STOP_TOKEN_STOP_STATE_H
11#define _LIBCPP___STOP_TOKEN_STOP_STATE_H
12
13#include <__assert>
14#include <__config>
15#include <__stop_token/atomic_unique_lock.h>
16#include <__stop_token/intrusive_list_view.h>
17#include <__thread/id.h>
18#include <atomic>
19#include <cstdint>
20
21#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
22# pragma GCC system_header
23#endif
24
25_LIBCPP_BEGIN_NAMESPACE_STD
26
27#if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_THREADS
28
29struct __stop_callback_base : __intrusive_node_base<__stop_callback_base> {
30 using __callback_fn_t _LIBCPP_NODEBUG = void(__stop_callback_base*) noexcept;
31 _LIBCPP_HIDE_FROM_ABI explicit __stop_callback_base(__callback_fn_t* __callback_fn) : __callback_fn_(__callback_fn) {}
32
33 _LIBCPP_HIDE_FROM_ABI void __invoke() noexcept { __callback_fn_(this); }
34
35 __callback_fn_t* __callback_fn_;
36 atomic<bool> __completed_ = false;
37 bool* __destroyed_ = nullptr;
38};
39
40class __stop_state {
41 static constexpr uint32_t __stop_requested_bit = 1;
42 static constexpr uint32_t __callback_list_locked_bit = 1 << 1;
43 static constexpr uint32_t __stop_source_counter_shift = 2;
44
45 // The "stop_source counter" is not used for lifetime reference counting.
46 // When the number of stop_source reaches 0, the remaining stop_tokens's
47 // stop_possible will return false. We need this counter to track this.
48 //
49 // The "callback list locked" bit implements the atomic_unique_lock to
50 // guard the operations on the callback list
51 //
52 // 31 - 2 | 1 | 0 |
53 // stop_source counter | callback list locked | stop_requested |
54 atomic<uint32_t> __state_ = 0;
55
56 // Reference count for stop_token + stop_callback + stop_source
57 // When the counter reaches zero, the state is destroyed
58 // It is used by __intrusive_shared_ptr, but it is stored here for better layout
59 atomic<uint32_t> __ref_count_ = 0;
60
61 using __state_t _LIBCPP_NODEBUG = uint32_t;
62 using __callback_list_lock _LIBCPP_NODEBUG = __atomic_unique_lock<__state_t, __callback_list_locked_bit>;
63 using __callback_list _LIBCPP_NODEBUG = __intrusive_list_view<__stop_callback_base>;
64
65 __callback_list __callback_list_;
66 __thread_id __requesting_thread_;
67
68public:
69 _LIBCPP_HIDE_FROM_ABI __stop_state() noexcept = default;
70
71 _LIBCPP_HIDE_FROM_ABI void __increment_stop_source_counter() noexcept {
72 _LIBCPP_ASSERT_UNCATEGORIZED(
73 __state_.load(std::memory_order_relaxed) <= static_cast<__state_t>(~(1 << __stop_source_counter_shift)),
74 "stop_source's counter reaches the maximum. Incrementing the counter will overflow");
75 __state_.fetch_add(op: 1 << __stop_source_counter_shift, m: std::memory_order_relaxed);
76 }
77
78 // We are not destroying the object after counter decrements to zero, nor do we have
79 // operations depend on the ordering of decrementing the counter. relaxed is enough.
80 _LIBCPP_HIDE_FROM_ABI void __decrement_stop_source_counter() noexcept {
81 _LIBCPP_ASSERT_UNCATEGORIZED(
82 __state_.load(std::memory_order_relaxed) >= static_cast<__state_t>(1 << __stop_source_counter_shift),
83 "stop_source's counter is 0. Decrementing the counter will underflow");
84 __state_.fetch_sub(op: 1 << __stop_source_counter_shift, m: std::memory_order_relaxed);
85 }
86
87 _LIBCPP_HIDE_FROM_ABI bool __stop_requested() const noexcept {
88 // acquire because [thread.stoptoken.intro] A call to request_stop that returns true
89 // synchronizes with a call to stop_requested on an associated stop_token or stop_source
90 // object that returns true.
91 // request_stop's compare_exchange_weak has release which syncs with this acquire
92 return (__state_.load(m: std::memory_order_acquire) & __stop_requested_bit) != 0;
93 }
94
95 _LIBCPP_HIDE_FROM_ABI bool __stop_possible_for_stop_token() const noexcept {
96 // [stoptoken.mem] false if "a stop request was not made and there are no associated stop_source objects"
97 // Todo: Can this be std::memory_order_relaxed as the standard does not say anything except not to introduce data
98 // race?
99 __state_t __curent_state = __state_.load(m: std::memory_order_acquire);
100 return ((__curent_state & __stop_requested_bit) != 0) || ((__curent_state >> __stop_source_counter_shift) != 0);
101 }
102
103 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __request_stop() noexcept {
104 auto __cb_list_lock = __try_lock_for_request_stop();
105 if (!__cb_list_lock.__owns_lock()) {
106 return false;
107 }
108 __requesting_thread_ = this_thread::get_id();
109
110 while (!__callback_list_.__empty()) {
111 auto __cb = __callback_list_.__pop_front();
112
113 // allow other callbacks to be removed while invoking the current callback
114 __cb_list_lock.__unlock();
115
116 bool __destroyed = false;
117 __cb->__destroyed_ = &__destroyed;
118
119 __cb->__invoke();
120
121 // __cb's invoke function could potentially delete itself. We need to check before accessing __cb's member
122 if (!__destroyed) {
123 // needs to set __destroyed_ pointer to nullptr, otherwise it points to a local variable
124 // which is to be destroyed at the end of the loop
125 __cb->__destroyed_ = nullptr;
126
127 // [stopcallback.cons] If callback is concurrently executing on another thread, then the return
128 // from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
129 // this release syncs with the acquire in the remove_callback
130 __cb->__completed_.store(d: true, m: std::memory_order_release);
131 __cb->__completed_.notify_all();
132 }
133
134 __cb_list_lock.__lock();
135 }
136
137 return true;
138 }
139
140 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __add_callback(__stop_callback_base* __cb) noexcept {
141 // If it is already stop_requested. Do not try to request it again.
142 const auto __give_up_trying_to_lock_condition = [__cb](__state_t __state) {
143 if ((__state & __stop_requested_bit) != 0) {
144 // already stop requested, synchronously run the callback and no need to lock the list again
145 __cb->__invoke();
146 return true;
147 }
148 // no stop source. no need to lock the list to add the callback as it can never be invoked
149 return (__state >> __stop_source_counter_shift) == 0;
150 };
151
152 __callback_list_lock __cb_list_lock(__state_, __give_up_trying_to_lock_condition);
153
154 if (!__cb_list_lock.__owns_lock()) {
155 return false;
156 }
157
158 __callback_list_.__push_front(node: __cb);
159
160 return true;
161 // unlock here: [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of
162 // that callback.
163 // Note: this release sync with the acquire in the request_stop' __try_lock_for_request_stop
164 }
165
166 // called by the destructor of stop_callback
167 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void __remove_callback(__stop_callback_base* __cb) noexcept {
168 __callback_list_lock __cb_list_lock(__state_);
169
170 // under below condition, the request_stop call just popped __cb from the list and could execute it now
171 bool __potentially_executing_now = __cb->__prev_ == nullptr && !__callback_list_.__is_head(node: __cb);
172
173 if (__potentially_executing_now) {
174 auto __requested_thread = __requesting_thread_;
175 __cb_list_lock.__unlock();
176
177 if (std::this_thread::get_id() != __requested_thread) {
178 // [stopcallback.cons] If callback is concurrently executing on another thread, then the return
179 // from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
180 __cb->__completed_.wait(v: false, m: std::memory_order_acquire);
181 } else {
182 // The destructor of stop_callback runs on the same thread of the thread that invokes the callback.
183 // The callback is potentially invoking its own destuctor. Set the flag to avoid accessing destroyed
184 // members on the invoking side
185 if (__cb->__destroyed_) {
186 *__cb->__destroyed_ = true;
187 }
188 }
189 } else {
190 __callback_list_.__remove(node: __cb);
191 }
192 }
193
194private:
195 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI __callback_list_lock __try_lock_for_request_stop() noexcept {
196 // If it is already stop_requested, do not try to request stop or lock the list again.
197 const auto __lock_fail_condition = [](__state_t __state) { return (__state & __stop_requested_bit) != 0; };
198
199 // set locked and requested bit at the same time
200 const auto __after_lock_state = [](__state_t __state) {
201 return __state | __callback_list_locked_bit | __stop_requested_bit;
202 };
203
204 // acq because [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of that
205 // callback. We are going to invoke the callback after getting the lock, acquire so that we can see the
206 // registration of a callback (and other writes that happens-before the add_callback)
207 // Note: the rel (unlock) in the add_callback syncs with this acq
208 // rel because [thread.stoptoken.intro] A call to request_stop that returns true synchronizes with a call
209 // to stop_requested on an associated stop_token or stop_source object that returns true.
210 // We need to make sure that all writes (including user code) before request_stop will be made visible
211 // to the threads that waiting for `stop_requested == true`
212 // Note: this rel syncs with the acq in `stop_requested`
213 const auto __locked_ordering = std::memory_order_acq_rel;
214
215 return __callback_list_lock(__state_, __lock_fail_condition, __after_lock_state, __locked_ordering);
216 }
217
218 template <class _Tp>
219 friend struct __intrusive_shared_ptr_traits;
220};
221
222template <class _Tp>
223struct __intrusive_shared_ptr_traits;
224
225template <>
226struct __intrusive_shared_ptr_traits<__stop_state> {
227 _LIBCPP_HIDE_FROM_ABI static atomic<uint32_t>& __get_atomic_ref_count(__stop_state& __state) {
228 return __state.__ref_count_;
229 }
230};
231
232#endif // _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_THREADS
233
234_LIBCPP_END_NAMESPACE_STD
235
236#endif // _LIBCPP___STOP_TOKEN_STOP_STATE_H
237