libstdc++
atomic_wait.h
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1 // -*- C++ -*- header.
2 
3 // Copyright (C) 2020-2021 Free Software Foundation, Inc.
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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
37 #include <bits/functional_hash.h>
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 
53 namespace 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(const __platform_wait_t* __addr, bool __all, bool __bare) noexcept
227  {
228  if (!(__bare || _M_waiting()))
229  return;
230 
231 #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
232  __platform_notify(__addr, __all);
233 #else
234  if (__all)
235  _M_cv.notify_all();
236  else
237  _M_cv.notify_one();
238 #endif
239  }
240 
241  static __waiter_pool_base&
242  _S_for(const void* __addr) noexcept
243  {
244  constexpr uintptr_t __ct = 16;
245  static __waiter_pool_base __w[__ct];
246  auto __key = (uintptr_t(__addr) >> 2) % __ct;
247  return __w[__key];
248  }
249  };
250 
251  struct __waiter_pool : __waiter_pool_base
252  {
253  void
254  _M_do_wait(const __platform_wait_t* __addr, __platform_wait_t __old) noexcept
255  {
256 #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
257  __platform_wait(__addr, __old);
258 #else
259  __platform_wait_t __val;
260  __atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
261  if (__val == __old)
262  {
263  lock_guard<mutex> __l(_M_mtx);
264  _M_cv.wait(_M_mtx);
265  }
266 #endif // __GLIBCXX_HAVE_PLATFORM_WAIT
267  }
268  };
269 
270  template<typename _Tp>
271  struct __waiter_base
272  {
273  using __waiter_type = _Tp;
274 
275  __waiter_type& _M_w;
276  __platform_wait_t* _M_addr;
277 
278  template<typename _Up>
279  static __platform_wait_t*
280  _S_wait_addr(const _Up* __a, __platform_wait_t* __b)
281  {
282  if constexpr (__platform_wait_uses_type<_Up>)
283  return reinterpret_cast<__platform_wait_t*>(const_cast<_Up*>(__a));
284  else
285  return __b;
286  }
287 
288  static __waiter_type&
289  _S_for(const void* __addr) noexcept
290  {
291  static_assert(sizeof(__waiter_type) == sizeof(__waiter_pool_base));
292  auto& res = __waiter_pool_base::_S_for(__addr);
293  return reinterpret_cast<__waiter_type&>(res);
294  }
295 
296  template<typename _Up>
297  explicit __waiter_base(const _Up* __addr) noexcept
298  : _M_w(_S_for(__addr))
299  , _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
300  { }
301 
302  bool
303  _M_laundered() const
304  { return _M_addr == &_M_w._M_ver; }
305 
306  void
307  _M_notify(bool __all, bool __bare = false)
308  {
309  if (_M_laundered())
310  {
311  __atomic_fetch_add(_M_addr, 1, __ATOMIC_SEQ_CST);
312  __all = true;
313  }
314  _M_w._M_notify(_M_addr, __all, __bare);
315  }
316 
317  template<typename _Up, typename _ValFn,
318  typename _Spin = __default_spin_policy>
319  static bool
320  _S_do_spin_v(__platform_wait_t* __addr,
321  const _Up& __old, _ValFn __vfn,
322  __platform_wait_t& __val,
323  _Spin __spin = _Spin{ })
324  {
325  auto const __pred = [=]
326  { return !__detail::__atomic_compare(__old, __vfn()); };
327 
328  if constexpr (__platform_wait_uses_type<_Up>)
329  {
330  __builtin_memcpy(&__val, &__old, sizeof(__val));
331  }
332  else
333  {
334  __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
335  }
336  return __atomic_spin(__pred, __spin);
337  }
338 
339  template<typename _Up, typename _ValFn,
340  typename _Spin = __default_spin_policy>
341  bool
342  _M_do_spin_v(const _Up& __old, _ValFn __vfn,
343  __platform_wait_t& __val,
344  _Spin __spin = _Spin{ })
345  { return _S_do_spin_v(_M_addr, __old, __vfn, __val, __spin); }
346 
347  template<typename _Pred,
348  typename _Spin = __default_spin_policy>
349  static bool
350  _S_do_spin(const __platform_wait_t* __addr,
351  _Pred __pred,
352  __platform_wait_t& __val,
353  _Spin __spin = _Spin{ })
354  {
355  __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
356  return __atomic_spin(__pred, __spin);
357  }
358 
359  template<typename _Pred,
360  typename _Spin = __default_spin_policy>
361  bool
362  _M_do_spin(_Pred __pred, __platform_wait_t& __val,
363  _Spin __spin = _Spin{ })
364  { return _S_do_spin(_M_addr, __pred, __val, __spin); }
365  };
366 
367  template<typename _EntersWait>
368  struct __waiter : __waiter_base<__waiter_pool>
369  {
370  using __base_type = __waiter_base<__waiter_pool>;
371 
372  template<typename _Tp>
373  explicit __waiter(const _Tp* __addr) noexcept
374  : __base_type(__addr)
375  {
376  if constexpr (_EntersWait::value)
377  _M_w._M_enter_wait();
378  }
379 
380  ~__waiter()
381  {
382  if constexpr (_EntersWait::value)
383  _M_w._M_leave_wait();
384  }
385 
386  template<typename _Tp, typename _ValFn>
387  void
388  _M_do_wait_v(_Tp __old, _ValFn __vfn)
389  {
390  do
391  {
392  __platform_wait_t __val;
393  if (__base_type::_M_do_spin_v(__old, __vfn, __val))
394  return;
395  __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
396  }
397  while (__detail::__atomic_compare(__old, __vfn()));
398  }
399 
400  template<typename _Pred>
401  void
402  _M_do_wait(_Pred __pred) noexcept
403  {
404  do
405  {
406  __platform_wait_t __val;
407  if (__base_type::_M_do_spin(__pred, __val))
408  return;
409  __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
410  }
411  while (!__pred());
412  }
413  };
414 
415  using __enters_wait = __waiter<std::true_type>;
416  using __bare_wait = __waiter<std::false_type>;
417  } // namespace __detail
418 
419  template<typename _Tp, typename _ValFn>
420  void
421  __atomic_wait_address_v(const _Tp* __addr, _Tp __old,
422  _ValFn __vfn) noexcept
423  {
424  __detail::__enters_wait __w(__addr);
425  __w._M_do_wait_v(__old, __vfn);
426  }
427 
428  template<typename _Tp, typename _Pred>
429  void
430  __atomic_wait_address(const _Tp* __addr, _Pred __pred) noexcept
431  {
432  __detail::__enters_wait __w(__addr);
433  __w._M_do_wait(__pred);
434  }
435 
436  // This call is to be used by atomic types which track contention externally
437  template<typename _Pred>
438  void
439  __atomic_wait_address_bare(const __detail::__platform_wait_t* __addr,
440  _Pred __pred) noexcept
441  {
442 #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
443  do
444  {
445  __detail::__platform_wait_t __val;
446  if (__detail::__bare_wait::_S_do_spin(__addr, __pred, __val))
447  return;
448  __detail::__platform_wait(__addr, __val);
449  }
450  while (!__pred());
451 #else // !_GLIBCXX_HAVE_PLATFORM_WAIT
452  __detail::__bare_wait __w(__addr);
453  __w._M_do_wait(__pred);
454 #endif
455  }
456 
457  template<typename _Tp>
458  void
459  __atomic_notify_address(const _Tp* __addr, bool __all) noexcept
460  {
461  __detail::__bare_wait __w(__addr);
462  __w._M_notify(__all);
463  }
464 
465  // This call is to be used by atomic types which track contention externally
466  inline void
467  __atomic_notify_address_bare(const __detail::__platform_wait_t* __addr,
468  bool __all) noexcept
469  {
470 #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
471  __detail::__platform_notify(__addr, __all);
472 #else
473  __detail::__bare_wait __w(__addr);
474  __w._M_notify(__all, true);
475 #endif
476  }
477 _GLIBCXX_END_NAMESPACE_VERSION
478 } // namespace std
479 #endif // GTHREADS || LINUX_FUTEX
480 #endif // _GLIBCXX_ATOMIC_WAIT_H
ISO C++ entities toplevel namespace is std.