libcruft-util/thread/semaphore_linux.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#include "semaphore.hpp"

#include "../cast.hpp"
#include "../posix/except.hpp"

#include <cerrno>
#include <linux/futex.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <limits>

using cruft::thread::semaphore;


///////////////////////////////////////////////////////////////////////////////
static long
sys_futex (void *addr1, int op, int val1, struct timespec *timeout, void *addr2, int val3)
{
    return syscall (SYS_futex, addr1, op | FUTEX_PRIVATE_FLAG, val1, timeout, addr2, val3);
}


///////////////////////////////////////////////////////////////////////////////
semaphore::semaphore ():
    semaphore (1)
{ ; }


//-----------------------------------------------------------------------------
semaphore::semaphore (int initial):
    m_value (initial)
{ ; }


///////////////////////////////////////////////////////////////////////////////
int
semaphore::acquire (int count)
{
    CHECK_GE (count, 0);

    do {
        int now = m_value;

        // if our value is positive then attempt to decrement it and return,
        // else retry because someone interfered with us.
        if (now - count >= 0) {
            if (m_value.compare_exchange_weak (now, now - count))
                return now - count;
            continue;
        }

        // the count doesn't appear to allow us to acquire. sleep until
        // there's been a modification and retry.
        if (-1 == sys_futex (&m_value, FUTEX_WAIT, now, nullptr, nullptr, 0)) {
            switch (errno) {
                case EAGAIN: break;
                case EINTR:  break;
                default:
                    posix::error::throw_code ();
            }
        }
    } while (1);
}


//-----------------------------------------------------------------------------
int
semaphore::acquire (void)
{
    return acquire (1);
}


//-----------------------------------------------------------------------------
int
semaphore::release (int count)
{
    auto res = m_value += count;
    if (sys_futex (&m_value, FUTEX_WAKE, count, nullptr, nullptr, 0) < 0)
        posix::error::throw_code ();
    return res;
}


//-----------------------------------------------------------------------------
int
semaphore::release (void)
{
    return release (1);
}


///////////////////////////////////////////////////////////////////////////////
int
semaphore::value (void) const
{
    return m_value;
}


//-----------------------------------------------------------------------------
int
semaphore::operator++ (void)
{
    return release ();
}


//-----------------------------------------------------------------------------
int
semaphore::operator-- (void)
{
    // we don't need to wake anyone because this will only serve to delay
    // their wakeup.
    return --m_value;
}