libstdc++
atomic_futex.h
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1// -*- C++ -*- header.
2
3// Copyright (C) 2015-2021 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
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9// any later version.
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11// This library is distributed in the hope that it will be useful,
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13// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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15
16// Under Section 7 of GPL version 3, you are granted additional
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18// 3.1, as published by the Free Software Foundation.
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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 <bits/c++config.h>
36#include <atomic>
37#include <chrono>
38#if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
39#include <mutex>
40#include <condition_variable>
41#endif
42
43#ifndef _GLIBCXX_ALWAYS_INLINE
44#define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
45#endif
46
47namespace std _GLIBCXX_VISIBILITY(default)
48{
49_GLIBCXX_BEGIN_NAMESPACE_VERSION
50
51#ifdef _GLIBCXX_HAS_GTHREADS
52#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
53 struct __atomic_futex_unsigned_base
54 {
55 // __s and __ns are measured against CLOCK_REALTIME. Returns false
56 // iff a timeout occurred.
57 bool
58 _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
59 chrono::seconds __s, chrono::nanoseconds __ns);
60
61 // __s and __ns are measured against CLOCK_MONOTONIC. Returns
62 // false iff a timeout occurred.
63 bool
64 _M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
65 bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns);
66
67 // This can be executed after the object has been destroyed.
68 static void _M_futex_notify_all(unsigned* __addr);
69 };
70
71 template <unsigned _Waiter_bit = 0x80000000>
72 class __atomic_futex_unsigned : __atomic_futex_unsigned_base
73 {
74 typedef chrono::steady_clock __clock_t;
75
76 // This must be lock-free and at offset 0.
77 atomic<unsigned> _M_data;
78
79 public:
80 explicit
81 __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
82 { }
83
84 _GLIBCXX_ALWAYS_INLINE unsigned
85 _M_load(memory_order __mo)
86 {
87 return _M_data.load(__mo) & ~_Waiter_bit;
88 }
89
90 private:
91 // If a timeout occurs, returns a current value after the timeout;
92 // otherwise, returns the operand's value if equal is true or a different
93 // value if equal is false.
94 // The assumed value is the caller's assumption about the current value
95 // when making the call.
96 // __s and __ns are measured against CLOCK_REALTIME.
97 unsigned
98 _M_load_and_test_until(unsigned __assumed, unsigned __operand,
99 bool __equal, memory_order __mo, bool __has_timeout,
101 {
102 for (;;)
103 {
104 // Don't bother checking the value again because we expect the caller
105 // to have done it recently.
106 // memory_order_relaxed is sufficient because we can rely on just the
107 // modification order (store_notify uses an atomic RMW operation too),
108 // and the futex syscalls synchronize between themselves.
109 _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
110 bool __ret = _M_futex_wait_until((unsigned*)(void*)&_M_data,
111 __assumed | _Waiter_bit,
112 __has_timeout, __s, __ns);
113 // Fetch the current value after waiting (clears _Waiter_bit).
114 __assumed = _M_load(__mo);
115 if (!__ret || ((__operand == __assumed) == __equal))
116 return __assumed;
117 // TODO adapt wait time
118 }
119 }
120
121 // If a timeout occurs, returns a current value after the timeout;
122 // otherwise, returns the operand's value if equal is true or a different
123 // value if equal is false.
124 // The assumed value is the caller's assumption about the current value
125 // when making the call.
126 // __s and __ns are measured against CLOCK_MONOTONIC.
127 unsigned
128 _M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
129 bool __equal, memory_order __mo, bool __has_timeout,
131 {
132 for (;;)
133 {
134 // Don't bother checking the value again because we expect the caller
135 // to have done it recently.
136 // memory_order_relaxed is sufficient because we can rely on just the
137 // modification order (store_notify uses an atomic RMW operation too),
138 // and the futex syscalls synchronize between themselves.
139 _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
140 bool __ret = _M_futex_wait_until_steady((unsigned*)(void*)&_M_data,
141 __assumed | _Waiter_bit,
142 __has_timeout, __s, __ns);
143 // Fetch the current value after waiting (clears _Waiter_bit).
144 __assumed = _M_load(__mo);
145 if (!__ret || ((__operand == __assumed) == __equal))
146 return __assumed;
147 // TODO adapt wait time
148 }
149 }
150
151 // Returns the operand's value if equal is true or a different value if
152 // equal is false.
153 // The assumed value is the caller's assumption about the current value
154 // when making the call.
155 unsigned
156 _M_load_and_test(unsigned __assumed, unsigned __operand,
157 bool __equal, memory_order __mo)
158 {
159 return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
160 false, {}, {});
161 }
162
163 // If a timeout occurs, returns a current value after the timeout;
164 // otherwise, returns the operand's value if equal is true or a different
165 // value if equal is false.
166 // The assumed value is the caller's assumption about the current value
167 // when making the call.
168 template<typename _Dur>
169 unsigned
170 _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
171 bool __equal, memory_order __mo,
172 const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
173 {
174 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
175 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
176 // XXX correct?
177 return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
178 true, __s.time_since_epoch(), __ns);
179 }
180
181 template<typename _Dur>
182 unsigned
183 _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
184 bool __equal, memory_order __mo,
185 const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
186 {
187 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
188 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
189 // XXX correct?
190 return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
191 true, __s.time_since_epoch(), __ns);
192 }
193
194 public:
195
196 _GLIBCXX_ALWAYS_INLINE unsigned
197 _M_load_when_not_equal(unsigned __val, memory_order __mo)
198 {
199 unsigned __i = _M_load(__mo);
200 if ((__i & ~_Waiter_bit) != __val)
201 return (__i & ~_Waiter_bit);
202 // TODO Spin-wait first.
203 return _M_load_and_test(__i, __val, false, __mo);
204 }
205
206 _GLIBCXX_ALWAYS_INLINE void
207 _M_load_when_equal(unsigned __val, memory_order __mo)
208 {
209 unsigned __i = _M_load(__mo);
210 if ((__i & ~_Waiter_bit) == __val)
211 return;
212 // TODO Spin-wait first.
213 _M_load_and_test(__i, __val, true, __mo);
214 }
215
216 // Returns false iff a timeout occurred.
217 template<typename _Rep, typename _Period>
218 _GLIBCXX_ALWAYS_INLINE bool
219 _M_load_when_equal_for(unsigned __val, memory_order __mo,
220 const chrono::duration<_Rep, _Period>& __rtime)
221 {
222 using __dur = typename __clock_t::duration;
223 return _M_load_when_equal_until(__val, __mo,
224 __clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
225 }
226
227 // Returns false iff a timeout occurred.
228 template<typename _Clock, typename _Duration>
229 _GLIBCXX_ALWAYS_INLINE bool
230 _M_load_when_equal_until(unsigned __val, memory_order __mo,
231 const chrono::time_point<_Clock, _Duration>& __atime)
232 {
233 typename _Clock::time_point __c_entry = _Clock::now();
234 do {
235 const __clock_t::time_point __s_entry = __clock_t::now();
236 const auto __delta = __atime - __c_entry;
237 const auto __s_atime = __s_entry +
238 chrono::__detail::ceil<__clock_t::duration>(__delta);
239 if (_M_load_when_equal_until(__val, __mo, __s_atime))
240 return true;
241 __c_entry = _Clock::now();
242 } while (__c_entry < __atime);
243 return false;
244 }
245
246 // Returns false iff a timeout occurred.
247 template<typename _Duration>
248 _GLIBCXX_ALWAYS_INLINE bool
249 _M_load_when_equal_until(unsigned __val, memory_order __mo,
250 const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
251 {
252 unsigned __i = _M_load(__mo);
253 if ((__i & ~_Waiter_bit) == __val)
254 return true;
255 // TODO Spin-wait first. Ignore effect on timeout.
256 __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
257 return (__i & ~_Waiter_bit) == __val;
258 }
259
260 // Returns false iff a timeout occurred.
261 template<typename _Duration>
262 _GLIBCXX_ALWAYS_INLINE bool
263 _M_load_when_equal_until(unsigned __val, memory_order __mo,
264 const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
265 {
266 unsigned __i = _M_load(__mo);
267 if ((__i & ~_Waiter_bit) == __val)
268 return true;
269 // TODO Spin-wait first. Ignore effect on timeout.
270 __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
271 return (__i & ~_Waiter_bit) == __val;
272 }
273
274 _GLIBCXX_ALWAYS_INLINE void
275 _M_store_notify_all(unsigned __val, memory_order __mo)
276 {
277 unsigned* __futex = (unsigned *)(void *)&_M_data;
278 if (_M_data.exchange(__val, __mo) & _Waiter_bit)
279 _M_futex_notify_all(__futex);
280 }
281 };
282
283#else // ! (_GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1)
284
285 // If futexes are not available, use a mutex and a condvar to wait.
286 // Because we access the data only within critical sections, all accesses
287 // are sequentially consistent; thus, we satisfy any provided memory_order.
288 template <unsigned _Waiter_bit = 0x80000000>
289 class __atomic_futex_unsigned
290 {
291 typedef chrono::system_clock __clock_t;
292
293 unsigned _M_data;
294 mutex _M_mutex;
295 condition_variable _M_condvar;
296
297 public:
298 explicit
299 __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
300 { }
301
302 _GLIBCXX_ALWAYS_INLINE unsigned
303 _M_load(memory_order __mo)
304 {
305 unique_lock<mutex> __lock(_M_mutex);
306 return _M_data;
307 }
308
309 _GLIBCXX_ALWAYS_INLINE unsigned
310 _M_load_when_not_equal(unsigned __val, memory_order __mo)
311 {
312 unique_lock<mutex> __lock(_M_mutex);
313 while (_M_data == __val)
314 _M_condvar.wait(__lock);
315 return _M_data;
316 }
317
318 _GLIBCXX_ALWAYS_INLINE void
319 _M_load_when_equal(unsigned __val, memory_order __mo)
320 {
321 unique_lock<mutex> __lock(_M_mutex);
322 while (_M_data != __val)
323 _M_condvar.wait(__lock);
324 }
325
326 template<typename _Rep, typename _Period>
327 _GLIBCXX_ALWAYS_INLINE bool
328 _M_load_when_equal_for(unsigned __val, memory_order __mo,
329 const chrono::duration<_Rep, _Period>& __rtime)
330 {
331 unique_lock<mutex> __lock(_M_mutex);
332 return _M_condvar.wait_for(__lock, __rtime,
333 [&] { return _M_data == __val;});
334 }
335
336 template<typename _Clock, typename _Duration>
337 _GLIBCXX_ALWAYS_INLINE bool
338 _M_load_when_equal_until(unsigned __val, memory_order __mo,
339 const chrono::time_point<_Clock, _Duration>& __atime)
340 {
341 unique_lock<mutex> __lock(_M_mutex);
342 return _M_condvar.wait_until(__lock, __atime,
343 [&] { return _M_data == __val;});
344 }
345
346 _GLIBCXX_ALWAYS_INLINE void
347 _M_store_notify_all(unsigned __val, memory_order __mo)
348 {
349 unique_lock<mutex> __lock(_M_mutex);
350 _M_data = __val;
351 _M_condvar.notify_all();
352 }
353 };
354
355#endif // _GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1
356#endif // _GLIBCXX_HAS_GTHREADS
357
358_GLIBCXX_END_NAMESPACE_VERSION
359} // namespace std
360
361#endif
duration< int64_t > seconds
seconds
Definition: chrono:847
duration< int64_t, nano > nanoseconds
nanoseconds
Definition: chrono:838
memory_order
Enumeration for memory_order.
Definition: atomic_base.h:79
ISO C++ entities toplevel namespace is std.