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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. #include <futex.h>
  30. #include <atomic.h>
  31. #include <libc.h>
  32. #include <stdio.h>
  33. #include <types.h>
  34. #include <kernel/synch/synch.h>
  35.  
  36. /*
  37.  * Note about race conditions.
  38.  * Because of non-atomic nature of operations performed sequentially on the futex
  39.  * counter and the futex wait queue, there is a race condition:
  40.  *
  41.  * wq->missed_wakeups == 1 && futex->count = 1
  42.  *
  43.  * Scenario 1 (wait queue timeout vs. futex_up()):
  44.  * 1. assume wq->missed_wakeups == 0 && futex->count == -1
  45.  *    (ie. thread A sleeping, thread B in the critical section)
  46.  * 2. A receives timeout and gets removed from the wait queue
  47.  * 3. B wants to leave the critical section and calls futex_up()
  48.  * 4. B thus changes futex->count from -1 to 0
  49.  * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
  50.  * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
  51.  * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
  52.  *
  53.  * Scenario 2 (conditional down operation vs. futex_up)
  54.  * 1. assume wq->missed_wakeups == 0 && futex->count == 0
  55.  *    (i.e. thread A is in the critical section)
  56.  * 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
  57.  *    B is now obliged to call SYS_FUTEX_SLEEP syscall
  58.  * 3. A wants to leave the critical section and does futex_up()
  59.  * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
  60.  * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
  61.  * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
  62.  * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
  63.  *
  64.  * Both scenarios allow two threads to be in the critical section simultaneously.
  65.  * One without kernel intervention and the other through wq->missed_wakeups being 1.
  66.  *
  67.  * To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
  68.  * in the wait queue, fixes the futex counter and RETRIES the whole operation again.
  69.  *
  70.  */
  71.  
  72. /** Initialize futex counter.
  73.  *
  74.  * @param futex Futex.
  75.  * @param val Initialization value.
  76.  */
  77. void futex_initialize(atomic_t *futex, int val)
  78. {
  79.     atomic_set(futex, val);
  80. }
  81.  
  82. int futex_down(atomic_t *futex)
  83. {
  84.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_BLOCKING);
  85. }
  86.  
  87. int futex_trydown(atomic_t *futex)
  88. {
  89.     return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_NON_BLOCKING);
  90. }
  91.  
  92. /** Try to down the futex.
  93.  *
  94.  * @param futex Futex.
  95.  * @param usec Microseconds to wait. Zero value means sleep without timeout.
  96.  * @param trydown If usec is zero and trydown is non-zero, only conditional
  97.  *
  98.  * @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
  99.  *     and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
  100.  *     not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
  101.  *     operation could not be carried out atomically (if requested so).
  102.  */
  103. int futex_down_timeout(atomic_t *futex, uint32_t usec, int trydown)
  104. {
  105.     int rc;
  106.    
  107.     while (atomic_predec(futex) < 0) {
  108.         rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) trydown);
  109.        
  110.         switch (rc) {
  111.         case ESYNCH_OK_ATOMIC:
  112.             /*
  113.              * Because of a race condition between timeout and futex_up()
  114.              * and between conditional futex_down_timeout() and futex_up(),
  115.              * we have to give up and try again in this special case.
  116.              */
  117.             atomic_inc(futex);
  118.             break;
  119.  
  120.         case ESYNCH_TIMEOUT:
  121.             atomic_inc(futex);
  122.             return ESYNCH_TIMEOUT;
  123.             break;
  124.  
  125.         case ESYNCH_WOULD_BLOCK:
  126.             /*
  127.              * The conditional down operation should be implemented this way.
  128.              * The userspace-only variant tends to accumulate missed wakeups
  129.              * in the kernel futex wait queue.
  130.              */
  131.             atomic_inc(futex);
  132.             return ESYNCH_WOULD_BLOCK;
  133.             break;
  134.  
  135.         case ESYNCH_OK_BLOCKED:
  136.             /*
  137.              * Enter the critical section.
  138.              * The futex counter has already been incremented for us.
  139.              */
  140.             return ESYNCH_OK_BLOCKED;
  141.             break;
  142.         default:
  143.             return rc;
  144.         }
  145.     }
  146.  
  147.     /*
  148.      * Enter the critical section.
  149.      */
  150.     return ESYNCH_OK_ATOMIC;
  151. }
  152.  
  153. /** Up the futex.
  154.  *
  155.  * @param futex Futex.
  156.  *
  157.  * @return ENOENT if there is no such virtual address or futex. Otherwise zero.
  158.  */
  159. int futex_up(atomic_t *futex)
  160. {
  161.     long val;
  162.    
  163.     val = atomic_postinc(futex);
  164.     if (val < 0)
  165.         return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
  166.        
  167.     return 0;
  168. }
  169.