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// futex -*- C++ -*-
// Copyright (C) 2015-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
#include <bits/atomic_futex.h>
#ifdef _GLIBCXX_HAS_GTHREADS
#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
#include <chrono>
#include <climits>
#include <syscall.h>
#include <unistd.h>
#include <sys/time.h>
#include <errno.h>
#include <ext/numeric_traits.h>
#include <debug/debug.h>
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
#include <unistd.h>
#include <sys/syscall.h>
#endif
// Constants for the wait/wake futex syscall operations
const unsigned futex_wait_op = 0;
const unsigned futex_wait_bitset_op = 9;
const unsigned futex_clock_monotonic_flag = 0;
const unsigned futex_clock_realtime_flag = 256;
const unsigned futex_bitset_match_any = ~0;
const unsigned futex_wake_op = 1;
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
namespace
{
std::atomic<bool> futex_clock_realtime_unavailable;
std::atomic<bool> futex_clock_monotonic_unavailable;
// Return the relative duration from (now_s + now_ns) to (abs_s + abs_ns)
// as a timespec.
struct timespec
relative_timespec(chrono::seconds abs_s, chrono::nanoseconds abs_ns,
time_t now_s, long now_ns)
{
struct timespec rt;
// Did we already time out?
if (now_s > abs_s.count())
{
rt.tv_sec = -1;
return rt;
}
auto rel_s = abs_s.count() - now_s;
// Avoid overflows
if (rel_s > __gnu_cxx::__int_traits<time_t>::__max)
rel_s = __gnu_cxx::__int_traits<time_t>::__max;
else if (rel_s < __gnu_cxx::__int_traits<time_t>::__min)
rel_s = __gnu_cxx::__int_traits<time_t>::__min;
// Convert the absolute timeout value to a relative timeout
rt.tv_sec = rel_s;
rt.tv_nsec = abs_ns.count() - now_ns;
if (rt.tv_nsec < 0)
{
rt.tv_nsec += 1000000000;
--rt.tv_sec;
}
return rt;
}
} // namespace
bool
__atomic_futex_unsigned_base::
_M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
chrono::seconds __s, chrono::nanoseconds __ns)
{
if (!__has_timeout)
{
// Ignore whether we actually succeeded to block because at worst,
// we will fall back to spin-waiting. The only thing we could do
// here on errors is abort.
int ret __attribute__((unused));
ret = syscall (SYS_futex, __addr, futex_wait_op, __val, nullptr);
__glibcxx_assert(ret == 0 || errno == EINTR || errno == EAGAIN);
return true;
}
else
{
if (!futex_clock_realtime_unavailable.load(std::memory_order_relaxed))
{
struct timespec rt;
rt.tv_sec = __s.count();
rt.tv_nsec = __ns.count();
// futex sets errno=EINVAL for absolute timeouts before the epoch.
if (__builtin_expect(rt.tv_sec < 0, false))
return false;
if (syscall (SYS_futex, __addr,
futex_wait_bitset_op | futex_clock_realtime_flag,
__val, &rt, nullptr, futex_bitset_match_any) == -1)
{
__glibcxx_assert(errno == EINTR || errno == EAGAIN
|| errno == ETIMEDOUT || errno == ENOSYS);
if (errno == ETIMEDOUT)
return false;
if (errno == ENOSYS)
{
futex_clock_realtime_unavailable.store(true,
std::memory_order_relaxed);
// Fall through to legacy implementation if the system
// call is unavailable.
}
else
return true;
}
else
return true;
}
// We only get to here if futex_clock_realtime_unavailable was
// true or has just been set to true.
struct timeval tv;
gettimeofday (&tv, NULL);
// Convert the absolute timeout value to a relative timeout
auto rt = relative_timespec(__s, __ns, tv.tv_sec, tv.tv_usec * 1000);
// Did we already time out?
if (rt.tv_sec < 0)
return false;
if (syscall (SYS_futex, __addr, futex_wait_op, __val, &rt) == -1)
{
__glibcxx_assert(errno == EINTR || errno == EAGAIN
|| errno == ETIMEDOUT);
if (errno == ETIMEDOUT)
return false;
}
return true;
}
}
bool
__atomic_futex_unsigned_base::
_M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
bool __has_timeout,
chrono::seconds __s, chrono::nanoseconds __ns)
{
if (!__has_timeout)
{
// Ignore whether we actually succeeded to block because at worst,
// we will fall back to spin-waiting. The only thing we could do
// here on errors is abort.
int ret __attribute__((unused));
ret = syscall (SYS_futex, __addr, futex_wait_op, __val, nullptr);
__glibcxx_assert(ret == 0 || errno == EINTR || errno == EAGAIN);
return true;
}
else
{
if (!futex_clock_monotonic_unavailable.load(std::memory_order_relaxed))
{
struct timespec rt;
rt.tv_sec = __s.count();
rt.tv_nsec = __ns.count();
// futex sets errno=EINVAL for absolute timeouts before the epoch.
if (__builtin_expect(rt.tv_sec < 0, false))
return false;
if (syscall (SYS_futex, __addr,
futex_wait_bitset_op | futex_clock_monotonic_flag,
__val, &rt, nullptr, futex_bitset_match_any) == -1)
{
__glibcxx_assert(errno == EINTR || errno == EAGAIN
|| errno == ETIMEDOUT || errno == ENOSYS);
if (errno == ETIMEDOUT)
return false;
else if (errno == ENOSYS)
{
futex_clock_monotonic_unavailable.store(true,
std::memory_order_relaxed);
// Fall through to legacy implementation if the system
// call is unavailable.
}
else
return true;
}
}
// We only get to here if futex_clock_monotonic_unavailable was
// true or has just been set to true.
struct timespec ts;
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
syscall(SYS_clock_gettime, CLOCK_MONOTONIC, &ts);
#else
clock_gettime(CLOCK_MONOTONIC, &ts);
#endif
// Convert the absolute timeout value to a relative timeout
auto rt = relative_timespec(__s, __ns, ts.tv_sec, ts.tv_nsec);
// Did we already time out?
if (rt.tv_sec < 0)
return false;
if (syscall (SYS_futex, __addr, futex_wait_op, __val, &rt) == -1)
{
__glibcxx_assert(errno == EINTR || errno == EAGAIN
|| errno == ETIMEDOUT);
if (errno == ETIMEDOUT)
return false;
}
return true;
}
}
void
__atomic_futex_unsigned_base::_M_futex_notify_all(unsigned* __addr)
{
// This syscall can fail for various reasons, including in situations
// in which there is no real error. Thus, we don't bother checking
// the error codes. See the futex documentation and glibc for background.
syscall (SYS_futex, __addr, futex_wake_op, INT_MAX);
}
_GLIBCXX_END_NAMESPACE_VERSION
}
#endif // defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
#endif // _GLIBCXX_HAS_GTHREADS
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