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1111 jermar 1 
/*
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 * Copyright (c) 2008 Jakub Jermar
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 * All rights reserved.
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 *
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 * Redistribution and use in source and binary forms, with or without
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 * modification, are permitted provided that the following conditions
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 * are met:
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 *
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 * - Redistributions of source code must retain the above copyright
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 *   notice, this list of conditions and the following disclaimer.
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 * - Redistributions in binary form must reproduce the above copyright
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 *   notice, this list of conditions and the following disclaimer in the
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 *   documentation and/or other materials provided with the distribution.
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 * - The name of the author may not be used to endorse or promote products
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 *   derived from this software without specific prior written permission.
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 *
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 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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 */
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/** @addtogroup libc
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 * @{
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 */
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/** @file
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 */
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#include <futex.h>
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#include <atomic.h>
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#include <libc.h>
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#include <stdio.h>
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#include <sys/types.h>
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#include <kernel/synch/synch.h>
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/*
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 * Note about race conditions.
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 * Because of non-atomic nature of operations performed sequentially on the
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 * futex counter and the futex wait queue, there is a race condition:
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 *
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 * (wq->missed_wakeups == 1) && (futex->count = 1)
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 *
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 * Scenario 1 (wait queue timeout vs. futex_up()):
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 * 1. assume wq->missed_wakeups == 0 && futex->count == -1
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 *    (ie. thread A sleeping, thread B in the critical section)
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 * 2. A receives timeout and gets removed from the wait queue
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 * 3. B wants to leave the critical section and calls futex_up()
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 * 4. B thus changes futex->count from -1 to 0
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 * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
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 * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
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 * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it
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 *    from 0 to 1
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 *
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 * Scenario 2 (conditional down operation vs. futex_up)
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 * 1. assume wq->missed_wakeups == 0 && futex->count == 0
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 *    (i.e. thread A is in the critical section)
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 * 2. thread B performs futex_trydown() operation and changes futex->count from
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 *    0 to -1
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 *    B is now obliged to call SYS_FUTEX_SLEEP syscall
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 * 3. A wants to leave the critical section and does futex_up()
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 * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP
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 *    syscall
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 * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
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 * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from
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 *    0 to 1
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 * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it
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 *    from 0 to 1
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 *
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 * Both scenarios allow two threads to be in the critical section
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 * simultaneously. One without kernel intervention and the other through
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 * wq->missed_wakeups being 1.
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 *
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 * To mitigate this problem, futex_down_timeout() detects that the syscall
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 * didn't sleep in the wait queue, fixes the futex counter and RETRIES the
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 * whole operation again.
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 */
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/** Initialize futex counter.
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 *
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 * @param futex     Futex.
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 * @param val       Initialization value.
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 */
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void futex_initialize(futex_t *futex, int val)
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{
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    atomic_set(futex, val);
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}
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int futex_down(futex_t *futex)
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{
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    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
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}
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int futex_trydown(futex_t *futex)
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{
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    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
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        SYNCH_FLAGS_NON_BLOCKING);
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}
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/** Try to down the futex.
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 *
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 * @param futex     Futex.
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 * @param usec      Microseconds to wait. Zero value means sleep without
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 *          timeout.
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 * @param flags     Select mode of operation. See comment for
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 *          waitq_sleep_timeout().
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 *
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 * @return      ENOENT if there is no such virtual address. One of
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 *          ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or
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 *          ESYNCH_TIMEOUT if the lock was not acquired because of
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 *          a timeout or ESYNCH_WOULD_BLOCK if the operation could
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 *          not be carried out atomically (if requested so).
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 */
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int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
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{
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    int rc;
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    while (atomic_predec(futex) < 0) {
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        rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count,
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            (sysarg_t) usec, (sysarg_t) flags);
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        switch (rc) {
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        case ESYNCH_OK_ATOMIC:
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            /*
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             * Because of a race condition between timeout and
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             * futex_up() and between conditional
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             * futex_down_timeout() and futex_up(), we have to give
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             * up and try again in this special case.
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             */
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            atomic_inc(futex);
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            break;
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        case ESYNCH_TIMEOUT:
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            atomic_inc(futex);
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            return ESYNCH_TIMEOUT;
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            break;
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        case ESYNCH_WOULD_BLOCK:
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            /*
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             * The conditional down operation should be implemented
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             * this way. The userspace-only variant tends to
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             * accumulate missed wakeups in the kernel futex wait
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             * queue.
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             */
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            atomic_inc(futex);
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            return ESYNCH_WOULD_BLOCK;
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            break;
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        case ESYNCH_OK_BLOCKED:
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            /*
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             * Enter the critical section.
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             * The futex counter has already been incremented for
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             * us.
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             */
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            return ESYNCH_OK_BLOCKED;
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            break;
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        default:
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            return rc;
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        }
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    }
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    /*
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     * Enter the critical section.
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     */
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    return ESYNCH_OK_ATOMIC;
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}
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/** Up the futex.
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 *
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 * @param futex     Futex.
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 *
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 * @return      ENOENT if there is no such virtual address. Otherwise
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 *          zero.
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 */
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int futex_up(futex_t *futex)
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{
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    long val;
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184 
    val = atomic_postinc(futex);
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    if (val < 0)
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        return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
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    return 0;
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}
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/** @}
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 */