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  1. /*
  2.  * Copyright (c) 2006 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 <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 futex
  45.  * 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 from 0 to 1
  58.  *
  59.  * Scenario 2 (conditional down operation vs. futex_up)
  60.  * 1. assume wq->missed_wakeups == 0 && futex->count == 0
  61.  *    (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.  *    B is now obliged to call SYS_FUTEX_SLEEP syscall
  64.  * 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
  66.  * 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
  69.  *
  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.
  72.  *
  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.  *
  76.  */
  77.  
  78. /** Initialize futex counter.
  79.  *
  80.  * @param futex Futex.
  81.  * @param val Initialization value.
  82.  */
  83. void futex_initialize(atomic_t *futex, int val)
  84. {
  85.     atomic_set(futex, val);
  86. }
  87.  
  88. int futex_down(atomic_t *futex)
  89. {
  90.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
  91. }
  92.  
  93. int futex_trydown(atomic_t *futex)
  94. {
  95.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
  96. }
  97.  
  98. /** Try to down the futex.
  99.  *
  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().
  103.  *
  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).
  108.  */
  109. int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
  110. {
  111.     int rc;
  112.    
  113.     while (atomic_predec(futex) < 0) {
  114.         rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
  115.        
  116.         switch (rc) {
  117.         case ESYNCH_OK_ATOMIC:
  118.             /*
  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.
  122.              */
  123.             atomic_inc(futex);
  124.             break;
  125.  
  126.         case ESYNCH_TIMEOUT:
  127.             atomic_inc(futex);
  128.             return ESYNCH_TIMEOUT;
  129.             break;
  130.  
  131.         case ESYNCH_WOULD_BLOCK:
  132.             /*
  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.
  136.              */
  137.             atomic_inc(futex);
  138.             return ESYNCH_WOULD_BLOCK;
  139.             break;
  140.  
  141.         case ESYNCH_OK_BLOCKED:
  142.             /*
  143.              * Enter the critical section.
  144.              * The futex counter has already been incremented for us.
  145.              */
  146.             return ESYNCH_OK_BLOCKED;
  147.             break;
  148.         default:
  149.             return rc;
  150.         }
  151.     }
  152.  
  153.     /*
  154.      * Enter the critical section.
  155.      */
  156.     return ESYNCH_OK_ATOMIC;
  157. }
  158.  
  159. /** Up the futex.
  160.  *
  161.  * @param futex Futex.
  162.  *
  163.  * @return ENOENT if there is no such virtual address. Otherwise zero.
  164.  */
  165. int futex_up(atomic_t *futex)
  166. {
  167.     long val;
  168.    
  169.     val = atomic_postinc(futex);
  170.     if (val < 0)
  171.         return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
  172.        
  173.     return 0;
  174. }
  175.  
  176. /** @}
  177.  */
  178.