libstdc++
atomic_wait.h
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1// -*- C++ -*- header.
2
3// Copyright (C) 2020-2021 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
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_wait.h
26 * This is an internal header file, included by other library headers.
27 * Do not attempt to use it directly. @headername{atomic}
28 */
29
30#ifndef _GLIBCXX_ATOMIC_WAIT_H
31#define _GLIBCXX_ATOMIC_WAIT_H 1
32
33#pragma GCC system_header
34
35#include <bits/c++config.h>
36#if defined _GLIBCXX_HAS_GTHREADS || defined _GLIBCXX_HAVE_LINUX_FUTEX
38#include <bits/gthr.h>
39#include <ext/numeric_traits.h>
40
41#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
42# include <cerrno>
43# include <climits>
44# include <unistd.h>
45# include <syscall.h>
46# include <bits/functexcept.h>
47#endif
48
49# include <bits/std_mutex.h> // std::mutex, std::__condvar
50
51#define __cpp_lib_atomic_wait 201907L
52
53namespace std _GLIBCXX_VISIBILITY(default)
54{
55_GLIBCXX_BEGIN_NAMESPACE_VERSION
56 namespace __detail
57 {
58#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
59#define _GLIBCXX_HAVE_PLATFORM_WAIT 1
60 using __platform_wait_t = int;
61 static constexpr size_t __platform_wait_alignment = 4;
62#else
63// define _GLIBCX_HAVE_PLATFORM_WAIT and implement __platform_wait()
64// and __platform_notify() if there is a more efficient primitive supported
65// by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better than
66// a mutex/condvar based wait.
67 using __platform_wait_t = uint64_t;
68 static constexpr size_t __platform_wait_alignment
69 = __alignof__(__platform_wait_t);
70#endif
71 } // namespace __detail
72
73 template<typename _Tp>
74 inline constexpr bool __platform_wait_uses_type
75#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
76 = is_scalar_v<_Tp>
77 && ((sizeof(_Tp) == sizeof(__detail::__platform_wait_t))
78 && (alignof(_Tp*) >= __detail::__platform_wait_alignment));
79#else
80 = false;
81#endif
82
83 namespace __detail
84 {
85#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
86 enum class __futex_wait_flags : int
87 {
88#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
89 __private_flag = 128,
90#else
91 __private_flag = 0,
92#endif
93 __wait = 0,
94 __wake = 1,
95 __wait_bitset = 9,
96 __wake_bitset = 10,
97 __wait_private = __wait | __private_flag,
98 __wake_private = __wake | __private_flag,
99 __wait_bitset_private = __wait_bitset | __private_flag,
100 __wake_bitset_private = __wake_bitset | __private_flag,
101 __bitset_match_any = -1
102 };
103
104 template<typename _Tp>
105 void
106 __platform_wait(const _Tp* __addr, __platform_wait_t __val) noexcept
107 {
108 auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
109 static_cast<int>(__futex_wait_flags::__wait_private),
110 __val, nullptr);
111 if (!__e || errno == EAGAIN)
112 return;
113 if (errno != EINTR)
114 __throw_system_error(errno);
115 }
116
117 template<typename _Tp>
118 void
119 __platform_notify(const _Tp* __addr, bool __all) noexcept
120 {
121 syscall (SYS_futex, static_cast<const void*>(__addr),
122 static_cast<int>(__futex_wait_flags::__wake_private),
123 __all ? INT_MAX : 1);
124 }
125#endif
126
127 inline void
128 __thread_yield() noexcept
129 {
130#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
131 __gthread_yield();
132#endif
133 }
134
135 inline void
136 __thread_relax() noexcept
137 {
138#if defined __i386__ || defined __x86_64__
139 __builtin_ia32_pause();
140#else
141 __thread_yield();
142#endif
143 }
144
145 constexpr auto __atomic_spin_count_relax = 12;
146 constexpr auto __atomic_spin_count = 16;
147
148 struct __default_spin_policy
149 {
150 bool
151 operator()() const noexcept
152 { return false; }
153 };
154
155 template<typename _Pred,
156 typename _Spin = __default_spin_policy>
157 bool
158 __atomic_spin(_Pred& __pred, _Spin __spin = _Spin{ }) noexcept
159 {
160 for (auto __i = 0; __i < __atomic_spin_count; ++__i)
161 {
162 if (__pred())
163 return true;
164
165 if (__i < __atomic_spin_count_relax)
166 __detail::__thread_relax();
167 else
168 __detail::__thread_yield();
169 }
170
171 while (__spin())
172 {
173 if (__pred())
174 return true;
175 }
176
177 return false;
178 }
179
180 // return true if equal
181 template<typename _Tp>
182 bool __atomic_compare(const _Tp& __a, const _Tp& __b)
183 {
184 // TODO make this do the correct padding bit ignoring comparison
185 return __builtin_memcmp(&__a, &__b, sizeof(_Tp)) == 0;
186 }
187
188 struct __waiter_pool_base
189 {
190#ifdef __cpp_lib_hardware_interference_size
191 static constexpr auto _S_align = hardware_destructive_interference_size;
192#else
193 static constexpr auto _S_align = 64;
194#endif
195
196 alignas(_S_align) __platform_wait_t _M_wait = 0;
197
198#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
199 mutex _M_mtx;
200#endif
201
202 alignas(_S_align) __platform_wait_t _M_ver = 0;
203
204#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
205 __condvar _M_cv;
206#endif
207 __waiter_pool_base() = default;
208
209 void
210 _M_enter_wait() noexcept
211 { __atomic_fetch_add(&_M_wait, 1, __ATOMIC_SEQ_CST); }
212
213 void
214 _M_leave_wait() noexcept
215 { __atomic_fetch_sub(&_M_wait, 1, __ATOMIC_RELEASE); }
216
217 bool
218 _M_waiting() const noexcept
219 {
220 __platform_wait_t __res;
221 __atomic_load(&_M_wait, &__res, __ATOMIC_SEQ_CST);
222 return __res != 0;
223 }
224
225 void
226 _M_notify(__platform_wait_t* __addr, [[maybe_unused]] bool __all,
227 bool __bare) noexcept
228 {
229#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
230 if (__addr == &_M_ver)
231 {
232 __atomic_fetch_add(__addr, 1, __ATOMIC_SEQ_CST);
233 __all = true;
234 }
235
236 if (__bare || _M_waiting())
237 __platform_notify(__addr, __all);
238#else
239 {
240 lock_guard<mutex> __l(_M_mtx);
241 __atomic_fetch_add(__addr, 1, __ATOMIC_RELAXED);
242 }
243 if (__bare || _M_waiting())
244 _M_cv.notify_all();
245#endif
246 }
247
248 static __waiter_pool_base&
249 _S_for(const void* __addr) noexcept
250 {
251 constexpr uintptr_t __ct = 16;
252 static __waiter_pool_base __w[__ct];
253 auto __key = (uintptr_t(__addr) >> 2) % __ct;
254 return __w[__key];
255 }
256 };
257
258 struct __waiter_pool : __waiter_pool_base
259 {
260 void
261 _M_do_wait(const __platform_wait_t* __addr, __platform_wait_t __old) noexcept
262 {
263#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
264 __platform_wait(__addr, __old);
265#else
266 __platform_wait_t __val;
267 __atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
268 if (__val == __old)
269 {
270 lock_guard<mutex> __l(_M_mtx);
271 __atomic_load(__addr, &__val, __ATOMIC_RELAXED);
272 if (__val == __old)
273 _M_cv.wait(_M_mtx);
274 }
275#endif // __GLIBCXX_HAVE_PLATFORM_WAIT
276 }
277 };
278
279 template<typename _Tp>
280 struct __waiter_base
281 {
282 using __waiter_type = _Tp;
283
284 __waiter_type& _M_w;
285 __platform_wait_t* _M_addr;
286
287 template<typename _Up>
288 static __platform_wait_t*
289 _S_wait_addr(const _Up* __a, __platform_wait_t* __b)
290 {
291 if constexpr (__platform_wait_uses_type<_Up>)
292 return reinterpret_cast<__platform_wait_t*>(const_cast<_Up*>(__a));
293 else
294 return __b;
295 }
296
297 static __waiter_type&
298 _S_for(const void* __addr) noexcept
299 {
300 static_assert(sizeof(__waiter_type) == sizeof(__waiter_pool_base));
301 auto& res = __waiter_pool_base::_S_for(__addr);
302 return reinterpret_cast<__waiter_type&>(res);
303 }
304
305 template<typename _Up>
306 explicit __waiter_base(const _Up* __addr) noexcept
307 : _M_w(_S_for(__addr))
308 , _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
309 { }
310
311 void
312 _M_notify(bool __all, bool __bare = false) noexcept
313 { _M_w._M_notify(_M_addr, __all, __bare); }
314
315 template<typename _Up, typename _ValFn,
316 typename _Spin = __default_spin_policy>
317 static bool
318 _S_do_spin_v(__platform_wait_t* __addr,
319 const _Up& __old, _ValFn __vfn,
320 __platform_wait_t& __val,
321 _Spin __spin = _Spin{ })
322 {
323 auto const __pred = [=]
324 { return !__detail::__atomic_compare(__old, __vfn()); };
325
326 if constexpr (__platform_wait_uses_type<_Up>)
327 {
328 __builtin_memcpy(&__val, &__old, sizeof(__val));
329 }
330 else
331 {
332 __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
333 }
334 return __atomic_spin(__pred, __spin);
335 }
336
337 template<typename _Up, typename _ValFn,
338 typename _Spin = __default_spin_policy>
339 bool
340 _M_do_spin_v(const _Up& __old, _ValFn __vfn,
341 __platform_wait_t& __val,
342 _Spin __spin = _Spin{ })
343 { return _S_do_spin_v(_M_addr, __old, __vfn, __val, __spin); }
344
345 template<typename _Pred,
346 typename _Spin = __default_spin_policy>
347 static bool
348 _S_do_spin(const __platform_wait_t* __addr,
349 _Pred __pred,
350 __platform_wait_t& __val,
351 _Spin __spin = _Spin{ })
352 {
353 __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
354 return __atomic_spin(__pred, __spin);
355 }
356
357 template<typename _Pred,
358 typename _Spin = __default_spin_policy>
359 bool
360 _M_do_spin(_Pred __pred, __platform_wait_t& __val,
361 _Spin __spin = _Spin{ })
362 { return _S_do_spin(_M_addr, __pred, __val, __spin); }
363 };
364
365 template<typename _EntersWait>
366 struct __waiter : __waiter_base<__waiter_pool>
367 {
368 using __base_type = __waiter_base<__waiter_pool>;
369
370 template<typename _Tp>
371 explicit __waiter(const _Tp* __addr) noexcept
372 : __base_type(__addr)
373 {
374 if constexpr (_EntersWait::value)
375 _M_w._M_enter_wait();
376 }
377
378 ~__waiter()
379 {
380 if constexpr (_EntersWait::value)
381 _M_w._M_leave_wait();
382 }
383
384 template<typename _Tp, typename _ValFn>
385 void
386 _M_do_wait_v(_Tp __old, _ValFn __vfn)
387 {
388 do
389 {
390 __platform_wait_t __val;
391 if (__base_type::_M_do_spin_v(__old, __vfn, __val))
392 return;
393 __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
394 }
395 while (__detail::__atomic_compare(__old, __vfn()));
396 }
397
398 template<typename _Pred>
399 void
400 _M_do_wait(_Pred __pred) noexcept
401 {
402 do
403 {
404 __platform_wait_t __val;
405 if (__base_type::_M_do_spin(__pred, __val))
406 return;
407 __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
408 }
409 while (!__pred());
410 }
411 };
412
413 using __enters_wait = __waiter<std::true_type>;
414 using __bare_wait = __waiter<std::false_type>;
415 } // namespace __detail
416
417 template<typename _Tp, typename _ValFn>
418 void
419 __atomic_wait_address_v(const _Tp* __addr, _Tp __old,
420 _ValFn __vfn) noexcept
421 {
422 __detail::__enters_wait __w(__addr);
423 __w._M_do_wait_v(__old, __vfn);
424 }
425
426 template<typename _Tp, typename _Pred>
427 void
428 __atomic_wait_address(const _Tp* __addr, _Pred __pred) noexcept
429 {
430 __detail::__enters_wait __w(__addr);
431 __w._M_do_wait(__pred);
432 }
433
434 // This call is to be used by atomic types which track contention externally
435 template<typename _Pred>
436 void
437 __atomic_wait_address_bare(const __detail::__platform_wait_t* __addr,
438 _Pred __pred) noexcept
439 {
440#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
441 do
442 {
443 __detail::__platform_wait_t __val;
444 if (__detail::__bare_wait::_S_do_spin(__addr, __pred, __val))
445 return;
446 __detail::__platform_wait(__addr, __val);
447 }
448 while (!__pred());
449#else // !_GLIBCXX_HAVE_PLATFORM_WAIT
450 __detail::__bare_wait __w(__addr);
451 __w._M_do_wait(__pred);
452#endif
453 }
454
455 template<typename _Tp>
456 void
457 __atomic_notify_address(const _Tp* __addr, bool __all) noexcept
458 {
459 __detail::__bare_wait __w(__addr);
460 __w._M_notify(__all);
461 }
462
463 // This call is to be used by atomic types which track contention externally
464 inline void
465 __atomic_notify_address_bare(const __detail::__platform_wait_t* __addr,
466 bool __all) noexcept
467 {
468#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
469 __detail::__platform_notify(__addr, __all);
470#else
471 __detail::__bare_wait __w(__addr);
472 __w._M_notify(__all, true);
473#endif
474 }
475_GLIBCXX_END_NAMESPACE_VERSION
476} // namespace std
477#endif // GTHREADS || LINUX_FUTEX
478#endif // _GLIBCXX_ATOMIC_WAIT_H
ISO C++ entities toplevel namespace is std.