nptl/DESIGN-rwlock.txt

   1 Reader Writer Locks pseudocode
   2 ==============================
   3
   4         pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
   5         pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
   6         pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
   7
   8 struct pthread_rwlock_t {
   9
  10    unsigned int lock:
  11          - internal mutex
  12
  13    unsigned int writers_preferred;
  14          - locking mode: 0 recursive, readers preferred
  15                          1 nonrecursive, writers preferred
  16
  17    unsigned int readers;
  18          - number of read-only references various threads have
  19
  20    pthread_t writer;
  21          - descriptor of the writer or 0
  22
  23    unsigned int readers_wakeup;
  24          - 'all readers should wake up' futex.
  25
  26    unsigned int writer_wakeup;
  27          - 'one writer should wake up' futex.
  28
  29    unsigned int nr_readers_queued;
  30          - number of readers queued up.
  31
  32    unsigned int nr_writers_queued;
  33          - number of writers queued up.
  34 }
  35
  36 pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
  37 {
  38   lll_lock(rwlock->lock);
  39   for (;;) {
  40     if (!rwlock->writer && (!rwlock->nr_writers_queued ||
  41                                         !rwlock->writers_preferred))
  42         break;
  43
  44     rwlock->nr_readers_queued++;
  45     val = rwlock->readers_wakeup;
  46     lll_unlock(rwlock->lock);
  47
  48     futex_wait(&rwlock->readers_wakeup, val)
  49
  50     lll_lock(rwlock->lock);
  51   }
  52   rwlock->readers++;
  53   lll_unlock(rwlock->lock);
  54 }
  55
  56 pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
  57 {
  58   int result = EBUSY;
  59   lll_lock(rwlock->lock);
  60   if (!rwlock->writer && (!rwlock->nr_writers_queued ||
  61                                         !rwlock->writers_preferred))
  62     rwlock->readers++;
  63   lll_unlock(rwlock->lock);
  64   return result;
  65 }
  66
  67 pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
  68 {
  69   lll_lock(rwlock->lock);
  70   for (;;) {
  71     if (!rwlock->writer && !rwlock->readers)
  72        break;
  73
  74     rwlock->nr_writers_queued++;
  75     val = rwlock->writer_wakeup;
  76     lll_unlock(rwlock->lock);
  77
  78     futex_wait(&rwlock->writer_wakeup, val);
  79
  80     lll_lock(rwlock->lock);
  81     rwlock->nr_writers_queued--;
  82   }
  83   rwlock->writer = pthread_self();
  84   lll_unlock(rwlock->lock);
  85 }
  86
  87 pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
  88 {
  89   lll_lock(rwlock->lock);
  90
  91   if (rwlock->writer)
  92     rwlock->writer = 0;
  93   else
  94     rwlock->readers--;
  95
  96   if (!rwlock->readers) {
  97     if (rwlock->nr_writers_queued) {
  98       ++rwlock->writer_wakeup;
  99       lll_unlock(rwlock->lock);
 100       futex_wake(&rwlock->writer_wakeup, 1);
 101       return;
 102     } else
 103       if (rwlock->nr_readers_queued) {
 104         ++rwlock->readers_wakeup;
 105         lll_unlock(rwlock->lock);
 106         futex_wake(&rwlock->readers_wakeup, MAX_INT);
 107         return;
 108       }
 109   }
 110
 111   lll_unlock(rwlock->lock);
 112 }