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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.  
  30. /*
  31.  * Reader/Writer locks
  32.  */
  33.  
  34. /*
  35.  * These locks are not recursive.
  36.  * Neither readers nor writers will suffer starvation.
  37.  *
  38.  * If there is a writer followed by a reader waiting for the rwlock
  39.  * and the writer times out, all leading readers are automatically woken up
  40.  * and allowed in.
  41.  */
  42.  
  43. /*
  44.  * NOTE ON rwlock_holder_type
  45.  * This field is set on an attempt to acquire the exclusive mutex
  46.  * to the respective value depending whether the caller is a reader
  47.  * or a writer. The field is examined only if the thread had been
  48.  * previously blocked on the exclusive mutex. Thus it is save
  49.  * to store the rwlock type in the thread structure, because
  50.  * each thread can block on only one rwlock at a time.
  51.  */
  52.  
  53. #include <synch/rwlock.h>
  54. #include <synch/spinlock.h>
  55. #include <synch/mutex.h>
  56. #include <synch/waitq.h>
  57. #include <synch/synch.h>
  58. #include <list.h>
  59. #include <typedefs.h>
  60. #include <arch/asm.h>
  61. #include <arch.h>
  62. #include <proc/thread.h>
  63. #include <panic.h>
  64.  
  65. #define ALLOW_ALL       0
  66. #define ALLOW_READERS_ONLY  1
  67.  
  68. static void let_others_in(rwlock_t *rwl, int readers_only);
  69. static void release_spinlock(void *arg);
  70.  
  71. /** Initialize reader/writer lock
  72.  *
  73.  * Initialize reader/writer lock.
  74.  *
  75.  * @param rwl Reader/Writer lock.
  76.  */
  77. void rwlock_initialize(rwlock_t *rwl) {
  78.     spinlock_initialize(&rwl->lock);
  79.     mutex_initialize(&rwl->exclusive);
  80.     rwl->readers_in = 0;
  81. }
  82.  
  83. /** Acquire reader/writer lock for reading
  84.  *
  85.  * Acquire reader/writer lock for reading.
  86.  * Timeout and willingness to block may be specified.
  87.  *
  88.  * @param rwl Reader/Writer lock.
  89.  * @param usec Timeout in microseconds.
  90.  * @param trylock Switches between blocking and non-blocking mode.
  91.  *
  92.  * For exact description of possible combinations of
  93.  * 'usec' and 'trylock', see comment for waitq_sleep_timeout().
  94.  *
  95.  * @return See comment for waitq_sleep_timeout().
  96.  */
  97. int _rwlock_write_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock)
  98. {
  99.     ipl_t ipl;
  100.     int rc;
  101.    
  102.     ipl = interrupts_disable();
  103.     spinlock_lock(&THREAD->lock);
  104.     THREAD->rwlock_holder_type = RWLOCK_WRITER;
  105.     spinlock_unlock(&THREAD->lock);
  106.     interrupts_restore(ipl);
  107.  
  108.     /*
  109.      * Writers take the easy part.
  110.      * They just need to acquire the exclusive mutex.
  111.      */
  112.     rc = _mutex_lock_timeout(&rwl->exclusive, usec, trylock);
  113.     if (SYNCH_FAILED(rc)) {
  114.  
  115.         /*
  116.          * Lock operation timed out.
  117.          * The state of rwl is UNKNOWN at this point.
  118.          * No claims about its holder can be made.
  119.          */
  120.          
  121.         ipl = interrupts_disable();
  122.         spinlock_lock(&rwl->lock);
  123.         /*
  124.          * Now when rwl is locked, we can inspect it again.
  125.          * If it is held by some readers already, we can let
  126.          * readers from the head of the wait queue in.
  127.          */
  128.         if (rwl->readers_in)
  129.             let_others_in(rwl, ALLOW_READERS_ONLY);
  130.         spinlock_unlock(&rwl->lock);
  131.         interrupts_restore(ipl);
  132.     }
  133.    
  134.     return rc;
  135. }
  136.  
  137. /** Acquire reader/writer lock for writing
  138.  *
  139.  * Acquire reader/writer lock for writing.
  140.  * Timeout and willingness to block may be specified.
  141.  *
  142.  * @param rwl Reader/Writer lock.
  143.  * @param usec Timeout in microseconds.
  144.  * @param trylock Switches between blocking and non-blocking mode.
  145.  *
  146.  * For exact description of possible combinations of
  147.  * 'usec' and 'trylock', see comment for waitq_sleep_timeout().
  148.  *
  149.  * @return See comment for waitq_sleep_timeout().
  150.  */
  151. int _rwlock_read_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock)
  152. {
  153.     int rc;
  154.     ipl_t ipl;
  155.    
  156.     ipl = interrupts_disable();
  157.     spinlock_lock(&THREAD->lock);
  158.     THREAD->rwlock_holder_type = RWLOCK_READER;
  159.     spinlock_unlock(&THREAD->lock);
  160.  
  161.     spinlock_lock(&rwl->lock);
  162.  
  163.     /*
  164.      * Find out whether we can get what we want without blocking.
  165.      */
  166.     rc = mutex_trylock(&rwl->exclusive);
  167.     if (SYNCH_FAILED(rc)) {
  168.  
  169.         /*
  170.          * 'exclusive' mutex is being held by someone else.
  171.          * If the holder is a reader and there is no one
  172.          * else waiting for it, we can enter the critical
  173.          * section.
  174.          */
  175.  
  176.         if (rwl->readers_in) {
  177.             spinlock_lock(&rwl->exclusive.sem.wq.lock);
  178.             if (list_empty(&rwl->exclusive.sem.wq.head)) {
  179.                 /*
  180.                  * We can enter.
  181.                  */
  182.                 spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  183.                 goto shortcut;
  184.             }
  185.             spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  186.         }
  187.  
  188.         /*
  189.          * In order to prevent a race condition when a reader
  190.          * could block another reader at the head of the waitq,
  191.          * we register a function to unlock rwl->lock
  192.          * after this thread is put asleep.
  193.          */
  194.         thread_register_call_me(release_spinlock, &rwl->lock);
  195.                  
  196.         rc = _mutex_lock_timeout(&rwl->exclusive, usec, trylock);
  197.         switch (rc) {
  198.             case ESYNCH_WOULD_BLOCK:
  199.                 /*
  200.                  * release_spinlock() wasn't called
  201.                  */
  202.                 thread_register_call_me(NULL, NULL);                 
  203.                 spinlock_unlock(&rwl->lock);
  204.             case ESYNCH_TIMEOUT:
  205.                 /*
  206.                  * The sleep timeouted.
  207.                  * We just restore interrupt priority level.
  208.                  */
  209.             case ESYNCH_OK_BLOCKED:    
  210.                 /*
  211.                  * We were woken with rwl->readers_in already incremented.
  212.                  * Note that this arrangement avoids race condition between
  213.                  * two concurrent readers. (Race is avoided if 'exclusive' is
  214.                  * locked at the same time as 'readers_in' is incremented.
  215.                  * Same time means both events happen atomically when
  216.                  * rwl->lock is held.)
  217.                  */
  218.                 interrupts_restore(ipl);
  219.                 break;
  220.             case ESYNCH_OK_ATOMIC:
  221.                 panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC");
  222.                 break;
  223.             dafault:
  224.                 panic("invalid ESYNCH");
  225.                 break;
  226.         }
  227.         return rc;
  228.     }
  229.  
  230. shortcut:
  231.  
  232.     /*
  233.      * We can increment readers_in only if we didn't go to sleep.
  234.      * For sleepers, rwlock_let_others_in() will do the job.
  235.      */
  236.     rwl->readers_in++;
  237.    
  238.     spinlock_unlock(&rwl->lock);
  239.     interrupts_restore(ipl);
  240.  
  241.     return ESYNCH_OK_ATOMIC;
  242. }
  243.  
  244. /** Release reader/writer lock held by writer
  245.  *
  246.  * Release reader/writer lock held by writer.
  247.  * Handoff reader/writer lock ownership directly
  248.  * to waiting readers or a writer.
  249.  *
  250.  * @param rwl Reader/Writer lock.
  251.  */
  252. void rwlock_write_unlock(rwlock_t *rwl)
  253. {
  254.     ipl_t ipl;
  255.    
  256.     ipl = interrupts_disable();
  257.     spinlock_lock(&rwl->lock);
  258.     let_others_in(rwl, ALLOW_ALL);
  259.     spinlock_unlock(&rwl->lock);
  260.     interrupts_restore(ipl);
  261.    
  262. }
  263.  
  264. /** Release reader/writer lock held by reader
  265.  *
  266.  * Release reader/writer lock held by reader.
  267.  * Handoff reader/writer lock ownership directly
  268.  * to a waiting writer or don't do anything if more
  269.  * readers poses the lock.
  270.  *
  271.  * @param rwl Reader/Writer lock.
  272.  */
  273. void rwlock_read_unlock(rwlock_t *rwl)
  274. {
  275.     ipl_t ipl;
  276.  
  277.     ipl = interrupts_disable();
  278.     spinlock_lock(&rwl->lock);
  279.     if (!--rwl->readers_in)
  280.         let_others_in(rwl, ALLOW_ALL);
  281.     spinlock_unlock(&rwl->lock);
  282.     interrupts_restore(ipl);
  283. }
  284.  
  285.  
  286. /** Direct handoff
  287.  *
  288.  * Direct handoff of reader/writer lock ownership
  289.  * to waiting readers or a writer.
  290.  *
  291.  * Must be called with rwl->lock locked.
  292.  * Must be called with interrupts_disable()'d.
  293.  *
  294.  * @param rwl Reader/Writer lock.
  295.  * @param readers_only See the description below.
  296.  *
  297.  * If readers_only is false: (unlock scenario)
  298.  * Let the first sleeper on 'exclusive' mutex in, no matter
  299.  * whether it is a reader or a writer. If there are more leading
  300.  * readers in line, let each of them in.
  301.  *
  302.  * Otherwise: (timeout scenario)
  303.  * Let all leading readers in.
  304.  */
  305. void let_others_in(rwlock_t *rwl, int readers_only)
  306. {
  307.     rwlock_type_t type = RWLOCK_NONE;
  308.     thread_t *t = NULL;
  309.     int one_more = 1;
  310.    
  311.     spinlock_lock(&rwl->exclusive.sem.wq.lock);
  312.  
  313.     if (!list_empty(&rwl->exclusive.sem.wq.head))
  314.         t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
  315.     do {
  316.         if (t) {
  317.             spinlock_lock(&t->lock);
  318.             type = t->rwlock_holder_type;
  319.             spinlock_unlock(&t->lock);         
  320.         }
  321.    
  322.         /*
  323.          * If readers_only is true, we wake all leading readers
  324.          * if and only if rwl is locked by another reader.
  325.          * Assumption: readers_only ==> rwl->readers_in
  326.          */
  327.         if (readers_only && (type != RWLOCK_READER))
  328.             break;
  329.  
  330.  
  331.         if (type == RWLOCK_READER) {
  332.             /*
  333.              * Waking up a reader.
  334.              * We are responsible for incrementing rwl->readers_in for it.
  335.              */
  336.              rwl->readers_in++;
  337.         }
  338.  
  339.         /*
  340.          * Only the last iteration through this loop can increment
  341.          * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding
  342.          * iterations will wake up a thread.
  343.          */
  344.         /* We call the internal version of waitq_wakeup, which
  345.          * relies on the fact that the waitq is already locked.
  346.          */
  347.         _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);
  348.        
  349.         t = NULL;
  350.         if (!list_empty(&rwl->exclusive.sem.wq.head)) {
  351.             t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
  352.             if (t) {
  353.                 spinlock_lock(&t->lock);
  354.                 if (t->rwlock_holder_type != RWLOCK_READER)
  355.                     one_more = 0;
  356.                 spinlock_unlock(&t->lock); 
  357.             }
  358.         }
  359.     } while ((type == RWLOCK_READER) && t && one_more);
  360.  
  361.     spinlock_unlock(&rwl->exclusive.sem.wq.lock);
  362. }
  363.  
  364. /** Release spinlock callback
  365.  *
  366.  * This is a callback function invoked from the scheduler.
  367.  * The callback is registered in _rwlock_read_lock_timeout().
  368.  *
  369.  * @param arg Spinlock.
  370.  */
  371. void release_spinlock(void *arg)
  372. {
  373.     spinlock_unlock((spinlock_t *) arg);
  374. }
  375.