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