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  1. /*
  2.  * Copyright (c) 2008 Jakub Jermar
  3.  * All rights reserved.
  4.  *
  5.  * Redistribution and use in source and binary forms, with or without
  6.  * modification, are permitted provided that the following conditions
  7.  * are met:
  8.  *
  9.  * - Redistributions of source code must retain the above copyright
  10.  *   notice, this list of conditions and the following disclaimer.
  11.  * - Redistributions in binary form must reproduce the above copyright
  12.  *   notice, this list of conditions and the following disclaimer in the
  13.  *   documentation and/or other materials provided with the distribution.
  14.  * - The name of the author may not be used to endorse or promote products
  15.  *   derived from this software without specific prior written permission.
  16.  *
  17.  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  18.  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  19.  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  20.  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  21.  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  22.  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23.  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24.  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25.  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  26.  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27.  */
  28.  
  29. /** @addtogroup libc
  30.  * @{
  31.  */
  32. /** @file
  33.  */
  34.  
  35. #include <futex.h>
  36. #include <atomic.h>
  37. #include <libc.h>
  38. #include <stdio.h>
  39. #include <sys/types.h>
  40. #include <kernel/synch/synch.h>
  41.  
  42. /*
  43.  * Note about race conditions.
  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:
  46.  *
  47.  * (wq->missed_wakeups == 1) && (futex->count = 1)
  48.  *
  49.  * Scenario 1 (wait queue timeout vs. futex_up()):
  50.  * 1. assume wq->missed_wakeups == 0 && futex->count == -1
  51.  *    (ie. thread A sleeping, thread B in the critical section)
  52.  * 2. A receives timeout and gets removed from the wait queue
  53.  * 3. B wants to leave the critical section and calls futex_up()
  54.  * 4. B thus changes futex->count from -1 to 0
  55.  * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
  56.  * 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
  58.  *    from 0 to 1
  59.  *
  60.  * Scenario 2 (conditional down operation vs. futex_up)
  61.  * 1. assume wq->missed_wakeups == 0 && futex->count == 0
  62.  *    (i.e. thread A is in the critical section)
  63.  * 2. thread B performs futex_trydown() operation and changes futex->count from
  64.  *    0 to -1
  65.  *    B is now obliged to call SYS_FUTEX_SLEEP syscall
  66.  * 3. A wants to leave the critical section and does futex_up()
  67.  * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP
  68.  *    syscall
  69.  * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
  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
  74.  *
  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.
  78.  *
  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.
  82.  */
  83.  
  84. /** Initialize futex counter.
  85.  *
  86.  * @param futex     Futex.
  87.  * @param val       Initialization value.
  88.  */
  89. void futex_initialize(futex_t *futex, int val)
  90. {
  91.     atomic_set(futex, val);
  92. }
  93.  
  94. int futex_down(futex_t *futex)
  95. {
  96.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
  97. }
  98.  
  99. int futex_trydown(futex_t *futex)
  100. {
  101.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
  102.         SYNCH_FLAGS_NON_BLOCKING);
  103. }
  104.  
  105. /** Try to down the futex.
  106.  *
  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().
  112.  *
  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).
  118.  */
  119. int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
  120. {
  121.     int rc;
  122.    
  123.     while (atomic_predec(futex) < 0) {
  124.         rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count,
  125.             (sysarg_t) usec, (sysarg_t) flags);
  126.        
  127.         switch (rc) {
  128.         case ESYNCH_OK_ATOMIC:
  129.             /*
  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.
  134.              */
  135.             atomic_inc(futex);
  136.             break;
  137.  
  138.         case ESYNCH_TIMEOUT:
  139.             atomic_inc(futex);
  140.             return ESYNCH_TIMEOUT;
  141.             break;
  142.  
  143.         case ESYNCH_WOULD_BLOCK:
  144.             /*
  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.
  149.              */
  150.             atomic_inc(futex);
  151.             return ESYNCH_WOULD_BLOCK;
  152.             break;
  153.  
  154.         case ESYNCH_OK_BLOCKED:
  155.             /*
  156.              * Enter the critical section.
  157.              * The futex counter has already been incremented for
  158.              * us.
  159.              */
  160.             return ESYNCH_OK_BLOCKED;
  161.             break;
  162.         default:
  163.             return rc;
  164.         }
  165.     }
  166.  
  167.     /*
  168.      * Enter the critical section.
  169.      */
  170.     return ESYNCH_OK_ATOMIC;
  171. }
  172.  
  173. /** Up the futex.
  174.  *
  175.  * @param futex     Futex.
  176.  *
  177.  * @return      ENOENT if there is no such virtual address. Otherwise
  178.  *          zero.
  179.  */
  180. int futex_up(futex_t *futex)
  181. {
  182.     long val;
  183.    
  184.     val = atomic_postinc(futex);
  185.     if (val < 0)
  186.         return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
  187.        
  188.     return 0;
  189. }
  190.  
  191. /** @}
  192.  */
  193.