futex.cc revision 1.1.1.9 
1// futex -*- C++ -*-
2
3// Copyright (C) 2015-2020 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#include <bits/atomic_futex.h>
26#ifdef _GLIBCXX_HAS_GTHREADS
27#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
28#include <chrono>
29#include <climits>
30#include <syscall.h>
31#include <unistd.h>
32#include <sys/time.h>
33#include <errno.h>
34#include <ext/numeric_traits.h>
35#include <debug/debug.h>
36
37// Constants for the wait/wake futex syscall operations
38const unsigned futex_wait_op = 0;
39const unsigned futex_wake_op = 1;
40
41namespace std _GLIBCXX_VISIBILITY(default)
42{
43_GLIBCXX_BEGIN_NAMESPACE_VERSION
44
45  using __gnu_cxx::__int_traits;
46
47namespace
48{
49  // Return the relative duration from (now_s + now_ns) to (abs_s + abs_ns)
50  // as a timespec.
51  struct timespec
52  relative_timespec(chrono::seconds abs_s, chrono::nanoseconds abs_ns,
53		    time_t now_s, long now_ns)
54  {
55    struct timespec rt;
56
57    // Did we already time out?
58    if (now_s > abs_s.count())
59      {
60	rt.tv_sec = -1;
61	return rt;
62      }
63
64    const auto rel_s = abs_s.count() - now_s;
65
66    // Convert the absolute timeout to a relative timeout, without overflow.
67    if (rel_s > __int_traits<time_t>::__max) [[unlikely]]
68      {
69	rt.tv_sec = __int_traits<time_t>::__max;
70	rt.tv_nsec = 999999999;
71      }
72    else
73      {
74	rt.tv_sec = rel_s;
75	rt.tv_nsec = abs_ns.count() - now_ns;
76	if (rt.tv_nsec < 0)
77	  {
78	    rt.tv_nsec += 1000000000;
79	    --rt.tv_sec;
80	  }
81      }
82
83    return rt;
84  }
85} // namespace
86
87  bool
88  __atomic_futex_unsigned_base::
89  _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
90		      chrono::seconds __s, chrono::nanoseconds __ns)
91  {
92    if (!__has_timeout)
93      {
94	// Ignore whether we actually succeeded to block because at worst,
95	// we will fall back to spin-waiting.  The only thing we could do
96	// here on errors is abort.
97	int ret __attribute__((unused));
98	ret = syscall (SYS_futex, __addr, futex_wait_op, __val, nullptr);
99	__glibcxx_assert(ret == 0 || errno == EINTR || errno == EAGAIN);
100	return true;
101      }
102    else
103      {
104	struct timeval tv;
105	gettimeofday (&tv, NULL);
106
107	// Convert the absolute timeout value to a relative timeout
108	auto rt = relative_timespec(__s, __ns, tv.tv_sec, tv.tv_usec * 1000);
109
110	// Did we already time out?
111	if (rt.tv_sec < 0)
112	  return false;
113
114	if (syscall (SYS_futex, __addr, futex_wait_op, __val, &rt) == -1)
115	  {
116	    __glibcxx_assert(errno == EINTR || errno == EAGAIN
117			     || errno == ETIMEDOUT);
118	    if (errno == ETIMEDOUT)
119	      return false;
120	  }
121	return true;
122      }
123  }
124
125  void
126  __atomic_futex_unsigned_base::_M_futex_notify_all(unsigned* __addr)
127  {
128    // This syscall can fail for various reasons, including in situations
129    // in which there is no real error.  Thus, we don't bother checking
130    // the error codes.  See the futex documentation and glibc for background.
131    syscall (SYS_futex, __addr, futex_wake_op, INT_MAX);
132  }
133
134_GLIBCXX_END_NAMESPACE_VERSION
135}
136#endif // defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
137#endif // _GLIBCXX_HAS_GTHREADS
138