113 lines
2.5 KiB
Plaintext
113 lines
2.5 KiB
Plaintext
Reader Writer Locks pseudocode
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==============================
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pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
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pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
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pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
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struct pthread_rwlock_t {
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unsigned int lock:
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- internal mutex
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unsigned int writers_preferred;
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- locking mode: 0 recursive, readers preferred
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1 nonrecursive, writers preferred
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unsigned int readers;
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- number of read-only references various threads have
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pthread_t writer;
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- descriptor of the writer or 0
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unsigned int readers_wakeup;
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- 'all readers should wake up' futex.
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unsigned int writer_wakeup;
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- 'one writer should wake up' futex.
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unsigned int nr_readers_queued;
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- number of readers queued up.
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unsigned int nr_writers_queued;
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- number of writers queued up.
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}
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pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
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{
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lll_lock(rwlock->lock);
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for (;;) {
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if (!rwlock->writer && (!rwlock->nr_writers_queued ||
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!rwlock->writers_preferred))
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break;
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rwlock->nr_readers_queued++;
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val = rwlock->readers_wakeup;
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lll_unlock(rwlock->lock);
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futex_wait(&rwlock->readers_wakeup, val)
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lll_lock(rwlock->lock);
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}
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rwlock->readers++;
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lll_unlock(rwlock->lock);
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}
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pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
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{
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int result = EBUSY;
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lll_lock(rwlock->lock);
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if (!rwlock->writer && (!rwlock->nr_writers_queued ||
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!rwlock->writers_preferred))
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rwlock->readers++;
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lll_unlock(rwlock->lock);
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return result;
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}
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pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
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{
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lll_lock(rwlock->lock);
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for (;;) {
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if (!rwlock->writer && !rwlock->readers)
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break;
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rwlock->nr_writers_queued++;
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val = rwlock->writer_wakeup;
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lll_unlock(rwlock->lock);
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futex_wait(&rwlock->writer_wakeup, val);
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lll_lock(rwlock->lock);
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rwlock->nr_writers_queued--;
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}
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rwlock->writer = pthread_self();
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lll_unlock(rwlock->lock);
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}
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pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
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{
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lll_lock(rwlock->lock);
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if (rwlock->writer)
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rwlock->writer = 0;
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else
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rwlock->readers--;
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if (!rwlock->readers) {
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if (rwlock->nr_writers_queued) {
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++rwlock->writer_wakeup;
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lll_unlock(rwlock->lock);
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futex_wake(&rwlock->writer_wakeup, 1);
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return;
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} else
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if (rwlock->nr_readers_queued) {
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++rwlock->readers_wakeup;
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lll_unlock(rwlock->lock);
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futex_wake(&rwlock->readers_wakeup, MAX_INT);
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return;
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}
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}
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lll_unlock(rwlock->lock);
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}
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