125 lines
3.0 KiB
C++
125 lines
3.0 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2018 Danny Robson <danny@nerdcruft.net>
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*/
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#include "semaphore.hpp"
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#include "../cast.hpp"
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#include "../posix/except.hpp"
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#include <cerrno>
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#include <linux/futex.h>
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#include <unistd.h>
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#include <sys/syscall.h>
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#include <limits>
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using cruft::thread::semaphore;
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///////////////////////////////////////////////////////////////////////////////
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static long
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sys_futex (void *addr1, int op, int val1, struct timespec *timeout, void *addr2, int val3)
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{
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return syscall (SYS_futex, addr1, op | FUTEX_PRIVATE_FLAG, val1, timeout, addr2, val3);
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}
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///////////////////////////////////////////////////////////////////////////////
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semaphore::semaphore ():
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semaphore (1)
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{ ; }
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//-----------------------------------------------------------------------------
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semaphore::semaphore (int initial):
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m_value (initial)
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{ ; }
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///////////////////////////////////////////////////////////////////////////////
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int
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semaphore::acquire (int count)
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{
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CHECK_GE (count, 0);
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do {
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int now = m_value;
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// if our value is positive then attempt to decrement it and return,
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// else retry because someone interfered with us.
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if (now - count >= 0) {
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if (m_value.compare_exchange_weak (now, now - count))
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return now - count;
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continue;
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}
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// the count doesn't appear to allow us to acquire. sleep until
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// there's been a modification and retry.
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if (-1 == sys_futex (&m_value, FUTEX_WAIT, now, nullptr, nullptr, 0)) {
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switch (errno) {
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case EAGAIN: break;
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case EINTR: break;
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default:
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posix::error::throw_code ();
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}
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}
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} while (1);
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}
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//-----------------------------------------------------------------------------
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int
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semaphore::acquire (void)
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{
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return acquire (1);
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}
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//-----------------------------------------------------------------------------
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int
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semaphore::release (int count)
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{
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auto res = m_value += count;
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if (sys_futex (&m_value, FUTEX_WAKE, count, nullptr, nullptr, 0) < 0)
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posix::error::throw_code ();
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return res;
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}
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//-----------------------------------------------------------------------------
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int
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semaphore::release (void)
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{
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return release (1);
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}
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///////////////////////////////////////////////////////////////////////////////
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int
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semaphore::value (void) const
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{
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return m_value;
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}
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//-----------------------------------------------------------------------------
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int
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semaphore::operator++ (void)
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{
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return release ();
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}
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//-----------------------------------------------------------------------------
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int
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semaphore::operator-- (void)
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{
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// we don't need to wake anyone because this will only serve to delay
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// their wakeup.
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return --m_value;
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}
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