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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 <arch/asm.h>
  67. #include <arch.h>
  68. #include <proc/thread.h>
  69. #include <panic.h>
  70.  
  71. #define ALLOW_ALL       0
  72. #define ALLOW_READERS_ONLY  1
  73.  
  74. static void let_others_in(rwlock_t *rwl, int readers_only);
  75. static void release_spinlock(void *arg);
  76.  
  77. /** Initialize reader/writer lock
  78.  *
  79.  * Initialize reader/writer lock.
  80.  *
  81.  * @param rwl Reader/Writer lock.
  82.  */
  83. void rwlock_initialize(rwlock_t *rwl) {
  84.     spinlock_initialize(&rwl->lock, "rwlock_t");
  85.     mutex_initialize(&rwl->exclusive);
  86.     rwl->readers_in = 0;
  87. }
  88.  
  89. /** Acquire reader/writer lock for reading
  90.  *
  91.  * Acquire reader/writer lock for reading.
  92.  * Timeout and willingness to block may be specified.
  93.  *
  94.  * @param rwl Reader/Writer lock.
  95.  * @param usec Timeout in microseconds.
  96.  * @param flags Specify mode of operation.
  97.  *
  98.  * For exact description of possible combinations of
  99.  * usec and flags, see comment for waitq_sleep_timeout().
  100.  *
  101.  * @return See comment for waitq_sleep_timeout().
  102.  */
  103. int _rwlock_write_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
  104. {
  105.     ipl_t ipl;
  106.     int rc;
  107.    
  108.     ipl = interrupts_disable();
  109.     spinlock_lock(&THREAD->lock);
  110.     THREAD->rwlock_holder_type = RWLOCK_WRITER;
  111.     spinlock_unlock(&THREAD->lock);
  112.     interrupts_restore(ipl);
  113.  
  114.     /*
  115.      * Writers take the easy part.
  116.      * They just need to acquire the exclusive mutex.
  117.      */
  118.     rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
  119.     if (SYNCH_FAILED(rc)) {
  120.  
  121.         /*
  122.          * Lock operation timed out or was interrupted.
  123.          * The state of rwl is UNKNOWN at this point.
  124.          * No claims about its holder can be made.
  125.          */
  126.          
  127.         ipl = interrupts_disable();
  128.         spinlock_lock(&rwl->lock);
  129.         /*
  130.          * Now when rwl is locked, we can inspect it again.
  131.          * If it is held by some readers already, we can let
  132.          * readers from the head of the wait queue in.
  133.          */
  134.         if (rwl->readers_in)
  135.             let_others_in(rwl, ALLOW_READERS_ONLY);
  136.         spinlock_unlock(&rwl->lock);
  137.         interrupts_restore(ipl);
  138.     }
  139.    
  140.     return rc;
  141. }
  142.  
  143. /** Acquire reader/writer lock for writing
  144.  *
  145.  * Acquire reader/writer lock for writing.
  146.  * Timeout and willingness to block may be specified.
  147.  *
  148.  * @param rwl Reader/Writer lock.
  149.  * @param usec Timeout in microseconds.
  150.  * @param flags Select mode of operation.
  151.  *
  152.  * For exact description of possible combinations of
  153.  * usec and flags, see comment for waitq_sleep_timeout().
  154.  *
  155.  * @return See comment for waitq_sleep_timeout().
  156.  */
  157. int _rwlock_read_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
  158. {
  159.     int rc;
  160.     ipl_t ipl;
  161.    
  162.     ipl = interrupts_disable();
  163.     spinlock_lock(&THREAD->lock);
  164.     THREAD->rwlock_holder_type = RWLOCK_READER;
  165.     spinlock_unlock(&THREAD->lock);
  166.  
  167.     spinlock_lock(&rwl->lock);
  168.  
  169.     /*
  170.      * Find out whether we can get what we want without blocking.
  171.      */
  172.     rc = mutex_trylock(&rwl->exclusive);
  173.     if (SYNCH_FAILED(rc)) {
  174.  
  175.         /*
  176.          * 'exclusive' mutex is being held by someone else.
  177.          * If the holder is a reader and there is no one
  178.          * else waiting for it, we can enter the critical
  179.          * section.
  180.          */
  181.  
  182.         if (rwl->readers_in) {
  183.             spinlock_lock(&rwl->exclusive.sem.wq.lock);
  184.             if (list_empty(&rwl->exclusive.sem.wq.head)) {
  185.                 /*
  186.                  * We can enter.
  187.                  */
  188.                 spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  189.                 goto shortcut;
  190.             }
  191.             spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  192.         }
  193.  
  194.         /*
  195.          * In order to prevent a race condition when a reader
  196.          * could block another reader at the head of the waitq,
  197.          * we register a function to unlock rwl->lock
  198.          * after this thread is put asleep.
  199.          */
  200.         #ifdef CONFIG_SMP
  201.         thread_register_call_me(release_spinlock, &rwl->lock);
  202.         #else
  203.         thread_register_call_me(release_spinlock, NULL);
  204.         #endif
  205.                  
  206.         rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
  207.         switch (rc) {
  208.         case ESYNCH_WOULD_BLOCK:
  209.             /*
  210.              * release_spinlock() wasn't called
  211.              */
  212.             thread_register_call_me(NULL, NULL);
  213.             spinlock_unlock(&rwl->lock);
  214.         case ESYNCH_TIMEOUT:
  215.         case ESYNCH_INTERRUPTED:
  216.             /*
  217.              * The sleep timed out.
  218.              * We just restore interrupt priority level.
  219.              */
  220.         case ESYNCH_OK_BLOCKED:    
  221.             /*
  222.              * We were woken with rwl->readers_in already
  223.              * incremented.
  224.              *
  225.              * Note that this arrangement avoids race condition
  226.              * between two concurrent readers. (Race is avoided if
  227.              * 'exclusive' is locked at the same time as
  228.              * 'readers_in' is incremented. Same time means both
  229.              * events happen atomically when rwl->lock is held.)
  230.              */
  231.             interrupts_restore(ipl);
  232.             break;
  233.         case ESYNCH_OK_ATOMIC:
  234.             panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC\n");
  235.             break;
  236.         default:
  237.             panic("invalid ESYNCH\n");
  238.             break;
  239.         }
  240.         return rc;
  241.     }
  242.  
  243. shortcut:
  244.  
  245.     /*
  246.      * We can increment readers_in only if we didn't go to sleep.
  247.      * For sleepers, rwlock_let_others_in() will do the job.
  248.      */
  249.     rwl->readers_in++;
  250.    
  251.     spinlock_unlock(&rwl->lock);
  252.     interrupts_restore(ipl);
  253.  
  254.     return ESYNCH_OK_ATOMIC;
  255. }
  256.  
  257. /** Release reader/writer lock held by writer
  258.  *
  259.  * Release reader/writer lock held by writer.
  260.  * Handoff reader/writer lock ownership directly
  261.  * to waiting readers or a writer.
  262.  *
  263.  * @param rwl Reader/Writer lock.
  264.  */
  265. void rwlock_write_unlock(rwlock_t *rwl)
  266. {
  267.     ipl_t ipl;
  268.    
  269.     ipl = interrupts_disable();
  270.     spinlock_lock(&rwl->lock);
  271.     let_others_in(rwl, ALLOW_ALL);
  272.     spinlock_unlock(&rwl->lock);
  273.     interrupts_restore(ipl);
  274.    
  275. }
  276.  
  277. /** Release reader/writer lock held by reader
  278.  *
  279.  * Release reader/writer lock held by reader.
  280.  * Handoff reader/writer lock ownership directly
  281.  * to a waiting writer or don't do anything if more
  282.  * readers poses the lock.
  283.  *
  284.  * @param rwl Reader/Writer lock.
  285.  */
  286. void rwlock_read_unlock(rwlock_t *rwl)
  287. {
  288.     ipl_t ipl;
  289.  
  290.     ipl = interrupts_disable();
  291.     spinlock_lock(&rwl->lock);
  292.     if (!--rwl->readers_in)
  293.         let_others_in(rwl, ALLOW_ALL);
  294.     spinlock_unlock(&rwl->lock);
  295.     interrupts_restore(ipl);
  296. }
  297.  
  298.  
  299. /** Direct handoff of reader/writer lock ownership.
  300.  *
  301.  * Direct handoff of reader/writer lock ownership
  302.  * to waiting readers or a writer.
  303.  *
  304.  * Must be called with rwl->lock locked.
  305.  * Must be called with interrupts_disable()'d.
  306.  *
  307.  * @param rwl Reader/Writer lock.
  308.  * @param readers_only See the description below.
  309.  *
  310.  * If readers_only is false: (unlock scenario)
  311.  * Let the first sleeper on 'exclusive' mutex in, no matter
  312.  * whether it is a reader or a writer. If there are more leading
  313.  * readers in line, let each of them in.
  314.  *
  315.  * Otherwise: (timeout scenario)
  316.  * Let all leading readers in.
  317.  */
  318. void let_others_in(rwlock_t *rwl, int readers_only)
  319. {
  320.     rwlock_type_t type = RWLOCK_NONE;
  321.     thread_t *t = NULL;
  322.     bool one_more = true;
  323.    
  324.     spinlock_lock(&rwl->exclusive.sem.wq.lock);
  325.  
  326.     if (!list_empty(&rwl->exclusive.sem.wq.head))
  327.         t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t,
  328.             wq_link);
  329.     do {
  330.         if (t) {
  331.             spinlock_lock(&t->lock);
  332.             type = t->rwlock_holder_type;
  333.             spinlock_unlock(&t->lock);         
  334.         }
  335.    
  336.         /*
  337.          * If readers_only is true, we wake all leading readers
  338.          * if and only if rwl is locked by another reader.
  339.          * Assumption: readers_only ==> rwl->readers_in
  340.          */
  341.         if (readers_only && (type != RWLOCK_READER))
  342.             break;
  343.  
  344.  
  345.         if (type == RWLOCK_READER) {
  346.             /*
  347.              * Waking up a reader.
  348.              * We are responsible for incrementing rwl->readers_in
  349.              * for it.
  350.              */
  351.              rwl->readers_in++;
  352.         }
  353.  
  354.         /*
  355.          * Only the last iteration through this loop can increment
  356.          * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding
  357.          * iterations will wake up a thread.
  358.          */
  359.         /* We call the internal version of waitq_wakeup, which
  360.          * relies on the fact that the waitq is already locked.
  361.          */
  362.         _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);
  363.        
  364.         t = NULL;
  365.         if (!list_empty(&rwl->exclusive.sem.wq.head)) {
  366.             t = list_get_instance(rwl->exclusive.sem.wq.head.next,
  367.                 thread_t, wq_link);
  368.             if (t) {
  369.                 spinlock_lock(&t->lock);
  370.                 if (t->rwlock_holder_type != RWLOCK_READER)
  371.                     one_more = false;
  372.                 spinlock_unlock(&t->lock); 
  373.             }
  374.         }
  375.     } while ((type == RWLOCK_READER) && t && one_more);
  376.  
  377.     spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  378. }
  379.  
  380. /** Release spinlock callback
  381.  *
  382.  * This is a callback function invoked from the scheduler.
  383.  * The callback is registered in _rwlock_read_lock_timeout().
  384.  *
  385.  * @param arg Spinlock.
  386.  */
  387. void release_spinlock(void *arg)
  388. {
  389.     spinlock_unlock((spinlock_t *) arg);
  390. }
  391.  
  392. /** @}
  393.  */
  394.