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  1. /*
  2.  * Copyright (C) 2001-2004 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 sync
  30.  * @{
  31.  */
  32.  
  33. /**
  34.  * @file
  35.  * @brief   Reader/Writer locks.
  36.  *
  37.  * A reader/writer lock can be held by multiple readers at a time.
  38.  * Or it can be exclusively held by a sole writer at a time.
  39.  *
  40.  * These locks are not recursive.
  41.  * Because a technique called direct hand-off is used and because
  42.  * waiting takes place in a single wait queue, neither readers
  43.  * nor writers will suffer starvation.
  44.  *
  45.  * If there is a writer followed by a reader waiting for the rwlock
  46.  * and the writer times out, all leading readers are automatically woken up
  47.  * and allowed in.
  48.  */
  49.  
  50. /*
  51.  * NOTE ON rwlock_holder_type
  52.  * This field is set on an attempt to acquire the exclusive mutex
  53.  * to the respective value depending whether the caller is a reader
  54.  * or a writer. The field is examined only if the thread had been
  55.  * previously blocked on the exclusive mutex. Thus it is save
  56.  * to store the rwlock type in the thread structure, because
  57.  * each thread can block on only one rwlock at a time.
  58.  */
  59.  
  60. #include <synch/rwlock.h>
  61. #include <synch/spinlock.h>
  62. #include <synch/mutex.h>
  63. #include <synch/waitq.h>
  64. #include <synch/synch.h>
  65. #include <adt/list.h>
  66. #include <typedefs.h>
  67. #include <arch/asm.h>
  68. #include <arch.h>
  69. #include <proc/thread.h>
  70. #include <panic.h>
  71.  
  72. #define ALLOW_ALL       0
  73. #define ALLOW_READERS_ONLY  1
  74.  
  75. static void let_others_in(rwlock_t *rwl, int readers_only);
  76. static void release_spinlock(void *arg);
  77.  
  78. /** Initialize reader/writer lock
  79.  *
  80.  * Initialize reader/writer lock.
  81.  *
  82.  * @param rwl Reader/Writer lock.
  83.  */
  84. void rwlock_initialize(rwlock_t *rwl) {
  85.     spinlock_initialize(&rwl->lock, "rwlock_t");
  86.     mutex_initialize(&rwl->exclusive);
  87.     rwl->readers_in = 0;
  88. }
  89.  
  90. /** Acquire reader/writer lock for reading
  91.  *
  92.  * Acquire reader/writer lock for reading.
  93.  * Timeout and willingness to block may be specified.
  94.  *
  95.  * @param rwl Reader/Writer lock.
  96.  * @param usec Timeout in microseconds.
  97.  * @param flags Specify mode of operation.
  98.  *
  99.  * For exact description of possible combinations of
  100.  * usec and flags, see comment for waitq_sleep_timeout().
  101.  *
  102.  * @return See comment for waitq_sleep_timeout().
  103.  */
  104. int _rwlock_write_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
  105. {
  106.     ipl_t ipl;
  107.     int rc;
  108.    
  109.     ipl = interrupts_disable();
  110.     spinlock_lock(&THREAD->lock);
  111.     THREAD->rwlock_holder_type = RWLOCK_WRITER;
  112.     spinlock_unlock(&THREAD->lock);
  113.     interrupts_restore(ipl);
  114.  
  115.     /*
  116.      * Writers take the easy part.
  117.      * They just need to acquire the exclusive mutex.
  118.      */
  119.     rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
  120.     if (SYNCH_FAILED(rc)) {
  121.  
  122.         /*
  123.          * Lock operation timed out or was interrupted.
  124.          * The state of rwl is UNKNOWN at this point.
  125.          * No claims about its holder can be made.
  126.          */
  127.          
  128.         ipl = interrupts_disable();
  129.         spinlock_lock(&rwl->lock);
  130.         /*
  131.          * Now when rwl is locked, we can inspect it again.
  132.          * If it is held by some readers already, we can let
  133.          * readers from the head of the wait queue in.
  134.          */
  135.         if (rwl->readers_in)
  136.             let_others_in(rwl, ALLOW_READERS_ONLY);
  137.         spinlock_unlock(&rwl->lock);
  138.         interrupts_restore(ipl);
  139.     }
  140.    
  141.     return rc;
  142. }
  143.  
  144. /** Acquire reader/writer lock for writing
  145.  *
  146.  * Acquire reader/writer lock for writing.
  147.  * Timeout and willingness to block may be specified.
  148.  *
  149.  * @param rwl Reader/Writer lock.
  150.  * @param usec Timeout in microseconds.
  151.  * @param flags Select mode of operation.
  152.  *
  153.  * For exact description of possible combinations of
  154.  * usec and flags, see comment for waitq_sleep_timeout().
  155.  *
  156.  * @return See comment for waitq_sleep_timeout().
  157.  */
  158. int _rwlock_read_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
  159. {
  160.     int rc;
  161.     ipl_t ipl;
  162.    
  163.     ipl = interrupts_disable();
  164.     spinlock_lock(&THREAD->lock);
  165.     THREAD->rwlock_holder_type = RWLOCK_READER;
  166.     spinlock_unlock(&THREAD->lock);
  167.  
  168.     spinlock_lock(&rwl->lock);
  169.  
  170.     /*
  171.      * Find out whether we can get what we want without blocking.
  172.      */
  173.     rc = mutex_trylock(&rwl->exclusive);
  174.     if (SYNCH_FAILED(rc)) {
  175.  
  176.         /*
  177.          * 'exclusive' mutex is being held by someone else.
  178.          * If the holder is a reader and there is no one
  179.          * else waiting for it, we can enter the critical
  180.          * section.
  181.          */
  182.  
  183.         if (rwl->readers_in) {
  184.             spinlock_lock(&rwl->exclusive.sem.wq.lock);
  185.             if (list_empty(&rwl->exclusive.sem.wq.head)) {
  186.                 /*
  187.                  * We can enter.
  188.                  */
  189.                 spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  190.                 goto shortcut;
  191.             }
  192.             spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  193.         }
  194.  
  195.         /*
  196.          * In order to prevent a race condition when a reader
  197.          * could block another reader at the head of the waitq,
  198.          * we register a function to unlock rwl->lock
  199.          * after this thread is put asleep.
  200.          */
  201.         #ifdef CONFIG_SMP
  202.         thread_register_call_me(release_spinlock, &rwl->lock);
  203.         #else
  204.         thread_register_call_me(release_spinlock, NULL);
  205.         #endif
  206.                  
  207.         rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
  208.         switch (rc) {
  209.         case ESYNCH_WOULD_BLOCK:
  210.             /*
  211.              * release_spinlock() wasn't called
  212.              */
  213.             thread_register_call_me(NULL, NULL);
  214.             spinlock_unlock(&rwl->lock);
  215.         case ESYNCH_TIMEOUT:
  216.         case ESYNCH_INTERRUPTED:
  217.             /*
  218.              * The sleep timed out.
  219.              * We just restore interrupt priority level.
  220.              */
  221.         case ESYNCH_OK_BLOCKED:    
  222.             /*
  223.              * We were woken with rwl->readers_in already incremented.
  224.              * Note that this arrangement avoids race condition between
  225.              * two concurrent readers. (Race is avoided if 'exclusive' is
  226.              * locked at the same time as 'readers_in' is incremented.
  227.              * Same time means both events happen atomically when
  228.              * rwl->lock is held.)
  229.              */
  230.             interrupts_restore(ipl);
  231.             break;
  232.         case ESYNCH_OK_ATOMIC:
  233.             panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC\n");
  234.             break;
  235.         default:
  236.             panic("invalid ESYNCH\n");
  237.             break;
  238.         }
  239.         return rc;
  240.     }
  241.  
  242. shortcut:
  243.  
  244.     /*
  245.      * We can increment readers_in only if we didn't go to sleep.
  246.      * For sleepers, rwlock_let_others_in() will do the job.
  247.      */
  248.     rwl->readers_in++;
  249.    
  250.     spinlock_unlock(&rwl->lock);
  251.     interrupts_restore(ipl);
  252.  
  253.     return ESYNCH_OK_ATOMIC;
  254. }
  255.  
  256. /** Release reader/writer lock held by writer
  257.  *
  258.  * Release reader/writer lock held by writer.
  259.  * Handoff reader/writer lock ownership directly
  260.  * to waiting readers or a writer.
  261.  *
  262.  * @param rwl Reader/Writer lock.
  263.  */
  264. void rwlock_write_unlock(rwlock_t *rwl)
  265. {
  266.     ipl_t ipl;
  267.    
  268.     ipl = interrupts_disable();
  269.     spinlock_lock(&rwl->lock);
  270.     let_others_in(rwl, ALLOW_ALL);
  271.     spinlock_unlock(&rwl->lock);
  272.     interrupts_restore(ipl);
  273.    
  274. }
  275.  
  276. /** Release reader/writer lock held by reader
  277.  *
  278.  * Release reader/writer lock held by reader.
  279.  * Handoff reader/writer lock ownership directly
  280.  * to a waiting writer or don't do anything if more
  281.  * readers poses the lock.
  282.  *
  283.  * @param rwl Reader/Writer lock.
  284.  */
  285. void rwlock_read_unlock(rwlock_t *rwl)
  286. {
  287.     ipl_t ipl;
  288.  
  289.     ipl = interrupts_disable();
  290.     spinlock_lock(&rwl->lock);
  291.     if (!--rwl->readers_in)
  292.         let_others_in(rwl, ALLOW_ALL);
  293.     spinlock_unlock(&rwl->lock);
  294.     interrupts_restore(ipl);
  295. }
  296.  
  297.  
  298. /** Direct handoff of reader/writer lock ownership.
  299.  *
  300.  * Direct handoff of reader/writer lock ownership
  301.  * to waiting readers or a writer.
  302.  *
  303.  * Must be called with rwl->lock locked.
  304.  * Must be called with interrupts_disable()'d.
  305.  *
  306.  * @param rwl Reader/Writer lock.
  307.  * @param readers_only See the description below.
  308.  *
  309.  * If readers_only is false: (unlock scenario)
  310.  * Let the first sleeper on 'exclusive' mutex in, no matter
  311.  * whether it is a reader or a writer. If there are more leading
  312.  * readers in line, let each of them in.
  313.  *
  314.  * Otherwise: (timeout scenario)
  315.  * Let all leading readers in.
  316.  */
  317. void let_others_in(rwlock_t *rwl, int readers_only)
  318. {
  319.     rwlock_type_t type = RWLOCK_NONE;
  320.     thread_t *t = NULL;
  321.     bool one_more = true;
  322.    
  323.     spinlock_lock(&rwl->exclusive.sem.wq.lock);
  324.  
  325.     if (!list_empty(&rwl->exclusive.sem.wq.head))
  326.         t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
  327.     do {
  328.         if (t) {
  329.             spinlock_lock(&t->lock);
  330.             type = t->rwlock_holder_type;
  331.             spinlock_unlock(&t->lock);         
  332.         }
  333.    
  334.         /*
  335.          * If readers_only is true, we wake all leading readers
  336.          * if and only if rwl is locked by another reader.
  337.          * Assumption: readers_only ==> rwl->readers_in
  338.          */
  339.         if (readers_only && (type != RWLOCK_READER))
  340.             break;
  341.  
  342.  
  343.         if (type == RWLOCK_READER) {
  344.             /*
  345.              * Waking up a reader.
  346.              * We are responsible for incrementing rwl->readers_in for it.
  347.              */
  348.              rwl->readers_in++;
  349.         }
  350.  
  351.         /*
  352.          * Only the last iteration through this loop can increment
  353.          * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding
  354.          * iterations will wake up a thread.
  355.          */
  356.         /* We call the internal version of waitq_wakeup, which
  357.          * relies on the fact that the waitq is already locked.
  358.          */
  359.         _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);
  360.        
  361.         t = NULL;
  362.         if (!list_empty(&rwl->exclusive.sem.wq.head)) {
  363.             t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
  364.             if (t) {
  365.                 spinlock_lock(&t->lock);
  366.                 if (t->rwlock_holder_type != RWLOCK_READER)
  367.                     one_more = false;
  368.                 spinlock_unlock(&t->lock); 
  369.             }
  370.         }
  371.     } while ((type == RWLOCK_READER) && t && one_more);
  372.  
  373.     spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  374. }
  375.  
  376. /** Release spinlock callback
  377.  *
  378.  * This is a callback function invoked from the scheduler.
  379.  * The callback is registered in _rwlock_read_lock_timeout().
  380.  *
  381.  * @param arg Spinlock.
  382.  */
  383. void release_spinlock(void *arg)
  384. {
  385.     spinlock_unlock((spinlock_t *) arg);
  386. }
  387.  
  388. /** @}
  389.  */
  390.