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Changeset 4db6eaf in mainline for uspace/lib/libc/generic/futex.c


Ignore:
Timestamp:
2008-01-06T14:05:15Z (14 years ago)
Author:
Jakub Jermar <jakub@…>
Branches:
lfn, master
Children:
10d6b858
Parents:
b3c38750
Message:

Fix type in declaration of VFS node rwlock.
Introduce a dedicated type for futexes. Make the existing futex code use this
newly introduced type and fix formatting in futex.c.

File:
1 edited

Legend:

Unmodified
Added
Removed
  • uspace/lib/libc/generic/futex.c

    rb3c38750 r4db6eaf  
    11/*
    2  * Copyright (c) 2006 Jakub Jermar
     2 * Copyright (c) 2008 Jakub Jermar
    33 * All rights reserved.
    44 *
     
    4242/*
    4343 * Note about race conditions.
    44  * Because of non-atomic nature of operations performed sequentially on the futex
    45  * counter and the futex wait queue, there is a race condition:
     44 * Because of non-atomic nature of operations performed sequentially on the
     45 * futex counter and the futex wait queue, there is a race condition:
    4646 *
    4747 * (wq->missed_wakeups == 1) && (futex->count = 1)
     
    5555 * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
    5656 * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
    57  * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
     57 * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it
     58 *    from 0 to 1
    5859 *
    5960 * Scenario 2 (conditional down operation vs. futex_up)
    6061 * 1. assume wq->missed_wakeups == 0 && futex->count == 0
    6162 *    (i.e. thread A is in the critical section)
    62  * 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
     63 * 2. thread B performs futex_trydown() operation and changes futex->count from
     64 *    0 to -1
    6365 *    B is now obliged to call SYS_FUTEX_SLEEP syscall
    6466 * 3. A wants to leave the critical section and does futex_up()
    65  * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
     67 * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP
     68 *    syscall
    6669 * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
    67  * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
    68  * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
     70 * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from
     71 *    0 to 1
     72 * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it
     73 *    from 0 to 1
    6974 *
    70  * Both scenarios allow two threads to be in the critical section simultaneously.
    71  * One without kernel intervention and the other through wq->missed_wakeups being 1.
     75 * Both scenarios allow two threads to be in the critical section
     76 * simultaneously. One without kernel intervention and the other through
     77 * wq->missed_wakeups being 1.
    7278 *
    73  * To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
    74  * in the wait queue, fixes the futex counter and RETRIES the whole operation again.
    75  *
     79 * To mitigate this problem, futex_down_timeout() detects that the syscall
     80 * didn't sleep in the wait queue, fixes the futex counter and RETRIES the
     81 * whole operation again.
    7682 */
    7783
    7884/** Initialize futex counter.
    7985 *
    80  * @param futex Futex.
    81  * @param val Initialization value.
     86 * @param futex         Futex.
     87 * @param val           Initialization value.
    8288 */
    83 void futex_initialize(atomic_t *futex, int val)
     89void futex_initialize(futex_t *futex, int val)
    8490{
    8591        atomic_set(futex, val);
    8692}
    8793
    88 int futex_down(atomic_t *futex)
     94int futex_down(futex_t *futex)
    8995{
    9096        return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
    9197}
    9298
    93 int futex_trydown(atomic_t *futex)
     99int futex_trydown(futex_t *futex)
    94100{
    95         return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
     101        return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
     102            SYNCH_FLAGS_NON_BLOCKING);
    96103}
    97104
    98105/** Try to down the futex.
    99106 *
    100  * @param futex Futex.
    101  * @param usec Microseconds to wait. Zero value means sleep without timeout.
    102  * @param flags Select mode of operation. See comment for waitq_sleep_timeout().
     107 * @param futex         Futex.
     108 * @param usec          Microseconds to wait. Zero value means sleep without
     109 *                      timeout.
     110 * @param flags         Select mode of operation. See comment for
     111 *                      waitq_sleep_timeout().
    103112 *
    104  * @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
    105  *         and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
    106  *         not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
    107  *         operation could not be carried out atomically (if requested so).
     113 * @return              ENOENT if there is no such virtual address. One of
     114 *                      ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or
     115 *                      ESYNCH_TIMEOUT if the lock was not acquired because of
     116 *                      a timeout or ESYNCH_WOULD_BLOCK if the operation could
     117 *                      not be carried out atomically (if requested so).
    108118 */
    109 int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
     119int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
    110120{
    111121        int rc;
    112122       
    113123        while (atomic_predec(futex) < 0) {
    114                 rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
     124                rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count,
     125                    (sysarg_t) usec, (sysarg_t) flags);
    115126               
    116127                switch (rc) {
    117128                case ESYNCH_OK_ATOMIC:
    118129                        /*
    119                          * Because of a race condition between timeout and futex_up()
    120                          * and between conditional futex_down_timeout() and futex_up(),
    121                          * we have to give up and try again in this special case.
     130                         * Because of a race condition between timeout and
     131                         * futex_up() and between conditional
     132                         * futex_down_timeout() and futex_up(), we have to give
     133                         * up and try again in this special case.
    122134                         */
    123135                        atomic_inc(futex);
     
    131143                case ESYNCH_WOULD_BLOCK:
    132144                        /*
    133                          * The conditional down operation should be implemented this way.
    134                          * The userspace-only variant tends to accumulate missed wakeups
    135                          * in the kernel futex wait queue.
     145                         * The conditional down operation should be implemented
     146                         * this way. The userspace-only variant tends to
     147                         * accumulate missed wakeups in the kernel futex wait
     148                         * queue.
    136149                         */
    137150                        atomic_inc(futex);
     
    142155                        /*
    143156                         * Enter the critical section.
    144                          * The futex counter has already been incremented for us.
     157                         * The futex counter has already been incremented for
     158                         * us.
    145159                         */
    146160                        return ESYNCH_OK_BLOCKED;
     
    159173/** Up the futex.
    160174 *
    161  * @param futex Futex.
     175 * @param futex         Futex.
    162176 *
    163  * @return ENOENT if there is no such virtual address. Otherwise zero.
     177 * @return              ENOENT if there is no such virtual address. Otherwise
     178 *                      zero.
    164179 */
    165 int futex_up(atomic_t *futex)
     180int futex_up(futex_t *futex)
    166181{
    167182        long val;
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