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/*
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/*
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 * Copyright (c) 2006 Jakub Jermar
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 * Copyright (c) 2008 Jakub Jermar
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 * All rights reserved.
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 * All rights reserved.
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 *
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 *
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 * Redistribution and use in source and binary forms, with or without
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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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 * modification, are permitted provided that the following conditions
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 * are met:
 7 
 * are met:
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#include <sys/types.h>
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#include <sys/types.h>
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#include <kernel/synch/synch.h>
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#include <kernel/synch/synch.h>
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/*
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/*
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 * Note about race conditions.
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 * Note about race conditions.
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 * Because of non-atomic nature of operations performed sequentially on the futex
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 * Because of non-atomic nature of operations performed sequentially on the
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 * counter and the futex wait queue, there is a race condition:
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 * futex counter and the futex wait queue, there is a race condition:
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 *
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 *
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 * (wq->missed_wakeups == 1) && (futex->count = 1)
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 * (wq->missed_wakeups == 1) && (futex->count = 1)
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 *
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 *
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 * Scenario 1 (wait queue timeout vs. futex_up()):
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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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 * 1. assume wq->missed_wakeups == 0 && futex->count == -1
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 * 2. A receives timeout and gets removed from the wait queue
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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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 * 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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 * 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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 * 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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 * 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 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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 *
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 * Scenario 2 (conditional down operation vs. futex_up)
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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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 * 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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 *    (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 0 to -1
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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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 *    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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 * 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 syscall
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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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 * 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 0 to 1
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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 from 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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 *
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 * Both scenarios allow two threads to be in the critical section simultaneously.
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 * Both scenarios allow two threads to be in the critical section
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 * One without kernel intervention and the other through wq->missed_wakeups being 1.
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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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 *
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 * To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
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 * To mitigate this problem, futex_down_timeout() detects that the syscall
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 * in the wait queue, fixes the futex counter and RETRIES the whole operation again.
 80 
 * didn't sleep in the wait queue, fixes the futex counter and RETRIES the
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 *
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 * whole operation again.
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 */
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 */
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 83 
 
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/** Initialize futex counter.
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/** Initialize futex counter.
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 *
 85 
 *
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 * @param futex Futex.
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 * @param futex     Futex.
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 * @param val Initialization value.
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 * @param val       Initialization value.
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 */
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 */
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void futex_initialize(atomic_t *futex, int val)
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void futex_initialize(futex_t *futex, int val)
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{
 90 
{
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    atomic_set(futex, val);
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    atomic_set(futex, val);
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}
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}
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 93 
 
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int futex_down(atomic_t *futex)
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int futex_down(futex_t *futex)
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{
 95 
{
90 
    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
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    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
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}
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}
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 98 
 
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int futex_trydown(atomic_t *futex)
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int futex_trydown(futex_t *futex)
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{
 100 
{
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    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
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    return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
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        SYNCH_FLAGS_NON_BLOCKING);
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}
 103 
}
97 
 
 104 
 
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/** Try to down the futex.
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/** Try to down the futex.
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 *
 106 
 *
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 * @param futex Futex.
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 * @param futex     Futex.
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 * @param usec Microseconds to wait. Zero value means sleep without timeout.
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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 waitq_sleep_timeout().
 110 
 * @param flags     Select mode of operation. See comment for
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 *          waitq_sleep_timeout().
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 *
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 *
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 * @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
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 * @return      ENOENT if there is no such virtual address. One of
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 *     and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
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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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 *     not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
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 *          a timeout or ESYNCH_WOULD_BLOCK if the operation could
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 *     operation could not be carried out atomically (if requested so).
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 *          not be carried out atomically (if requested so).
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 */
 118 
 */
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int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
 119 
int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
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{
 120 
{
111 
    int rc;
 121 
    int rc;
112 
   
 122 
   
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    while (atomic_predec(futex) < 0) {
 123 
    while (atomic_predec(futex) < 0) {
114 
        rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
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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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 126 
       
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        switch (rc) {
 127 
        switch (rc) {
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        case ESYNCH_OK_ATOMIC:
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        case ESYNCH_OK_ATOMIC:
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            /*
 129 
            /*
119 
             * Because of a race condition between timeout and futex_up()
 130 
             * Because of a race condition between timeout and
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 131 
             * futex_up() and between conditional
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             * and between conditional futex_down_timeout() and futex_up(),
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             * futex_down_timeout() and futex_up(), we have to give
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             * we have to give up and try again in this special case.
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             * up and try again in this special case.
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             */
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             */
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            atomic_inc(futex);
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            atomic_inc(futex);
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            break;
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            break;
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 137 
 
126 
        case ESYNCH_TIMEOUT:
 138 
        case ESYNCH_TIMEOUT:
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            return ESYNCH_TIMEOUT;
 140 
            return ESYNCH_TIMEOUT;
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            break;
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            break;
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 142 
 
131 
        case ESYNCH_WOULD_BLOCK:
 143 
        case ESYNCH_WOULD_BLOCK:
132 
            /*
 144 
            /*
133 
             * The conditional down operation should be implemented this way.
 145 
             * The conditional down operation should be implemented
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             * The userspace-only variant tends to accumulate missed wakeups
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             * this way. The userspace-only variant tends to
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             * in the kernel futex wait queue.
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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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             */
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            atomic_inc(futex);
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            atomic_inc(futex);
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            return ESYNCH_WOULD_BLOCK;
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            return ESYNCH_WOULD_BLOCK;
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            break;
 152 
            break;
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 153 
 
141 
        case ESYNCH_OK_BLOCKED:
 154 
        case ESYNCH_OK_BLOCKED:
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            /*
 155 
            /*
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             * Enter the critical section.
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             * Enter the critical section.
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             * The futex counter has already been incremented for us.
 157 
             * The futex counter has already been incremented for
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 158 
             * us.
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             */
 159 
             */
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            return ESYNCH_OK_BLOCKED;
 160 
            return ESYNCH_OK_BLOCKED;
147 
            break;
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            break;
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        default:
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        default:
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            return rc;
 163 
            return rc;
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    return ESYNCH_OK_ATOMIC;
 170 
    return ESYNCH_OK_ATOMIC;
157 
}
 171 
}
158 
 
 172 
 
159 
/** Up the futex.
 173 
/** Up the futex.
160 
 *
 174 
 *
161 
 * @param futex Futex.
 175 
 * @param futex     Futex.
162 
 *
 176 
 *
163 
 * @return ENOENT if there is no such virtual address. Otherwise zero.
 177 
 * @return      ENOENT if there is no such virtual address. Otherwise
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 178 
 *          zero.
164 
 */
 179 
 */
165 
int futex_up(atomic_t *futex)
 180 
int futex_up(futex_t *futex)
166 
{
 181 
{
167 
    long val;
 182 
    long val;
168 
   
 183 
   
169 
    val = atomic_postinc(futex);
 184 
    val = atomic_postinc(futex);
170 
    if (val < 0)
 185 
    if (val < 0)