1// -*- C++ -*- header.
2
3// Copyright (C) 2015-2024 Free Software Foundation, Inc.
4//
5// This file is part of the GNU ISO C++ Library. This library is free
6// software; you can redistribute it and/or modify it under the
7// terms of the GNU General Public License as published by the
8// Free Software Foundation; either version 3, or (at your option)
9// any later version.
10
11// This library is distributed in the hope that it will be useful,
12// but WITHOUT ANY WARRANTY; without even the implied warranty of
13// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14// GNU General Public License for more details.
15
16// Under Section 7 of GPL version 3, you are granted additional
17// permissions described in the GCC Runtime Library Exception, version
18// 3.1, as published by the Free Software Foundation.
19
20// You should have received a copy of the GNU General Public License and
21// a copy of the GCC Runtime Library Exception along with this program;
22// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23// <http://www.gnu.org/licenses/>.
24
25/** @file bits/atomic_futex.h
26 * This is an internal header file, included by other library headers.
27 * Do not attempt to use it directly.
28 */
29
30#ifndef _GLIBCXX_ATOMIC_FUTEX_H
31#define _GLIBCXX_ATOMIC_FUTEX_H 1
32
33#pragma GCC system_header
34
35#include <atomic>
36#if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
37#include <mutex>
38#include <condition_variable>
39#endif
40#include <bits/chrono.h>
41
42#ifndef _GLIBCXX_ALWAYS_INLINE
43#define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
44#endif
45
46namespace std _GLIBCXX_VISIBILITY(default)
47{
48_GLIBCXX_BEGIN_NAMESPACE_VERSION
49
50#ifdef _GLIBCXX_HAS_GTHREADS
51#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
52 struct __atomic_futex_unsigned_base
53 {
54 // __s and __ns are measured against CLOCK_REALTIME. Returns false
55 // iff a timeout occurred.
56 bool
57 _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
58 chrono::seconds __s, chrono::nanoseconds __ns);
59
60 // __s and __ns are measured against CLOCK_MONOTONIC. Returns
61 // false iff a timeout occurred.
62 bool
63 _M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
64 bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns);
65
66 // This can be executed after the object has been destroyed.
67 static void _M_futex_notify_all(unsigned* __addr);
68 };
69
70 template <unsigned _Waiter_bit = 0x80000000>
71 class __atomic_futex_unsigned : __atomic_futex_unsigned_base
72 {
73 typedef chrono::steady_clock __clock_t;
74
75 // This must be lock-free and at offset 0.
76 atomic<unsigned> _M_data;
77
78 public:
79 explicit
80 __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
81 { }
82
83 _GLIBCXX_ALWAYS_INLINE unsigned
84 _M_load(memory_order __mo)
85 {
86 return _M_data.load(m: __mo) & ~_Waiter_bit;
87 }
88
89 private:
90 // If a timeout occurs, returns a current value after the timeout;
91 // otherwise, returns the operand's value if equal is true or a different
92 // value if equal is false.
93 // The assumed value is the caller's assumption about the current value
94 // when making the call.
95 // __s and __ns are measured against CLOCK_REALTIME.
96 unsigned
97 _M_load_and_test_until(unsigned __assumed, unsigned __operand,
98 bool __equal, memory_order __mo, bool __has_timeout,
99 chrono::seconds __s, chrono::nanoseconds __ns)
100 {
101 for (;;)
102 {
103 // Don't bother checking the value again because we expect the caller
104 // to have done it recently.
105 // memory_order_relaxed is sufficient because we can rely on just the
106 // modification order (store_notify uses an atomic RMW operation too),
107 // and the futex syscalls synchronize between themselves.
108 _M_data.fetch_or(i: _Waiter_bit, m: memory_order_relaxed);
109 bool __ret = _M_futex_wait_until(addr: (unsigned*)(void*)&_M_data,
110 val: __assumed | _Waiter_bit,
111 __has_timeout, __s, __ns);
112 // Fetch the current value after waiting (clears _Waiter_bit).
113 __assumed = _M_load(__mo);
114 if (!__ret || ((__operand == __assumed) == __equal))
115 return __assumed;
116 // TODO adapt wait time
117 }
118 }
119
120 // If a timeout occurs, returns a current value after the timeout;
121 // otherwise, returns the operand's value if equal is true or a different
122 // value if equal is false.
123 // The assumed value is the caller's assumption about the current value
124 // when making the call.
125 // __s and __ns are measured against CLOCK_MONOTONIC.
126 unsigned
127 _M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
128 bool __equal, memory_order __mo, bool __has_timeout,
129 chrono::seconds __s, chrono::nanoseconds __ns)
130 {
131 for (;;)
132 {
133 // Don't bother checking the value again because we expect the caller
134 // to have done it recently.
135 // memory_order_relaxed is sufficient because we can rely on just the
136 // modification order (store_notify uses an atomic RMW operation too),
137 // and the futex syscalls synchronize between themselves.
138 _M_data.fetch_or(i: _Waiter_bit, m: memory_order_relaxed);
139 bool __ret = _M_futex_wait_until_steady(addr: (unsigned*)(void*)&_M_data,
140 val: __assumed | _Waiter_bit,
141 __has_timeout, __s, __ns);
142 // Fetch the current value after waiting (clears _Waiter_bit).
143 __assumed = _M_load(__mo);
144 if (!__ret || ((__operand == __assumed) == __equal))
145 return __assumed;
146 // TODO adapt wait time
147 }
148 }
149
150 // Returns the operand's value if equal is true or a different value if
151 // equal is false.
152 // The assumed value is the caller's assumption about the current value
153 // when making the call.
154 unsigned
155 _M_load_and_test(unsigned __assumed, unsigned __operand,
156 bool __equal, memory_order __mo)
157 {
158 return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
159 has_timeout: false, s: {}, ns: {});
160 }
161
162 // If a timeout occurs, returns a current value after the timeout;
163 // otherwise, returns the operand's value if equal is true or a different
164 // value if equal is false.
165 // The assumed value is the caller's assumption about the current value
166 // when making the call.
167 template<typename _Dur>
168 unsigned
169 _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
170 bool __equal, memory_order __mo,
171 const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
172 {
173 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
174 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
175 // XXX correct?
176 return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
177 has_timeout: true, s: __s.time_since_epoch(), __ns);
178 }
179
180 template<typename _Dur>
181 unsigned
182 _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
183 bool __equal, memory_order __mo,
184 const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
185 {
186 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
187 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
188 // XXX correct?
189 return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
190 has_timeout: true, s: __s.time_since_epoch(), __ns);
191 }
192
193 public:
194
195 _GLIBCXX_ALWAYS_INLINE unsigned
196 _M_load_when_not_equal(unsigned __val, memory_order __mo)
197 {
198 unsigned __i = _M_load(__mo);
199 if ((__i & ~_Waiter_bit) != __val)
200 return (__i & ~_Waiter_bit);
201 // TODO Spin-wait first.
202 return _M_load_and_test(assumed: __i, operand: __val, equal: false, __mo);
203 }
204
205 _GLIBCXX_ALWAYS_INLINE void
206 _M_load_when_equal(unsigned __val, memory_order __mo)
207 {
208 unsigned __i = _M_load(__mo);
209 if ((__i & ~_Waiter_bit) == __val)
210 return;
211 // TODO Spin-wait first.
212 _M_load_and_test(assumed: __i, operand: __val, equal: true, __mo);
213 }
214
215 // Returns false iff a timeout occurred.
216 template<typename _Rep, typename _Period>
217 _GLIBCXX_ALWAYS_INLINE bool
218 _M_load_when_equal_for(unsigned __val, memory_order __mo,
219 const chrono::duration<_Rep, _Period>& __rtime)
220 {
221 using __dur = typename __clock_t::duration;
222 return _M_load_when_equal_until(__val, __mo,
223 __clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
224 }
225
226 // Returns false iff a timeout occurred.
227 template<typename _Clock, typename _Duration>
228 _GLIBCXX_ALWAYS_INLINE bool
229 _M_load_when_equal_until(unsigned __val, memory_order __mo,
230 const chrono::time_point<_Clock, _Duration>& __atime)
231 {
232 typename _Clock::time_point __c_entry = _Clock::now();
233 do {
234 const __clock_t::time_point __s_entry = __clock_t::now();
235 const auto __delta = __atime - __c_entry;
236 const auto __s_atime = __s_entry +
237 chrono::__detail::ceil<__clock_t::duration>(__delta);
238 if (_M_load_when_equal_until(__val, __mo, __s_atime))
239 return true;
240 __c_entry = _Clock::now();
241 } while (__c_entry < __atime);
242 return false;
243 }
244
245 // Returns false iff a timeout occurred.
246 template<typename _Duration>
247 _GLIBCXX_ALWAYS_INLINE bool
248 _M_load_when_equal_until(unsigned __val, memory_order __mo,
249 const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
250 {
251 unsigned __i = _M_load(__mo);
252 if ((__i & ~_Waiter_bit) == __val)
253 return true;
254 // TODO Spin-wait first. Ignore effect on timeout.
255 __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
256 return (__i & ~_Waiter_bit) == __val;
257 }
258
259 // Returns false iff a timeout occurred.
260 template<typename _Duration>
261 _GLIBCXX_ALWAYS_INLINE bool
262 _M_load_when_equal_until(unsigned __val, memory_order __mo,
263 const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
264 {
265 unsigned __i = _M_load(__mo);
266 if ((__i & ~_Waiter_bit) == __val)
267 return true;
268 // TODO Spin-wait first. Ignore effect on timeout.
269 __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
270 return (__i & ~_Waiter_bit) == __val;
271 }
272
273 _GLIBCXX_ALWAYS_INLINE void
274 _M_store_notify_all(unsigned __val, memory_order __mo)
275 {
276 unsigned* __futex = (unsigned *)(void *)&_M_data;
277 if (_M_data.exchange(i: __val, m: __mo) & _Waiter_bit)
278 _M_futex_notify_all(addr: __futex);
279 }
280 };
281
282#else // ! (_GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1)
283
284 // If futexes are not available, use a mutex and a condvar to wait.
285 // Because we access the data only within critical sections, all accesses
286 // are sequentially consistent; thus, we satisfy any provided memory_order.
287 template <unsigned _Waiter_bit = 0x80000000>
288 class __atomic_futex_unsigned
289 {
290 typedef chrono::system_clock __clock_t;
291
292 unsigned _M_data;
293 mutex _M_mutex;
294 condition_variable _M_condvar;
295
296 public:
297 explicit
298 __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
299 { }
300
301 _GLIBCXX_ALWAYS_INLINE unsigned
302 _M_load(memory_order __mo)
303 {
304 unique_lock<mutex> __lock(_M_mutex);
305 return _M_data;
306 }
307
308 _GLIBCXX_ALWAYS_INLINE unsigned
309 _M_load_when_not_equal(unsigned __val, memory_order __mo)
310 {
311 unique_lock<mutex> __lock(_M_mutex);
312 while (_M_data == __val)
313 _M_condvar.wait(__lock);
314 return _M_data;
315 }
316
317 _GLIBCXX_ALWAYS_INLINE void
318 _M_load_when_equal(unsigned __val, memory_order __mo)
319 {
320 unique_lock<mutex> __lock(_M_mutex);
321 while (_M_data != __val)
322 _M_condvar.wait(__lock);
323 }
324
325 template<typename _Rep, typename _Period>
326 _GLIBCXX_ALWAYS_INLINE bool
327 _M_load_when_equal_for(unsigned __val, memory_order __mo,
328 const chrono::duration<_Rep, _Period>& __rtime)
329 {
330 unique_lock<mutex> __lock(_M_mutex);
331 return _M_condvar.wait_for(__lock, __rtime,
332 [&] { return _M_data == __val;});
333 }
334
335 template<typename _Clock, typename _Duration>
336 _GLIBCXX_ALWAYS_INLINE bool
337 _M_load_when_equal_until(unsigned __val, memory_order __mo,
338 const chrono::time_point<_Clock, _Duration>& __atime)
339 {
340 unique_lock<mutex> __lock(_M_mutex);
341 return _M_condvar.wait_until(__lock, __atime,
342 [&] { return _M_data == __val;});
343 }
344
345 _GLIBCXX_ALWAYS_INLINE void
346 _M_store_notify_all(unsigned __val, memory_order __mo)
347 {
348 unique_lock<mutex> __lock(_M_mutex);
349 _M_data = __val;
350 _M_condvar.notify_all();
351 }
352 };
353
354#endif // _GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1
355#endif // _GLIBCXX_HAS_GTHREADS
356
357_GLIBCXX_END_NAMESPACE_VERSION
358} // namespace std
359
360#endif
361