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glibc/nptl/DESIGN-rwlock.txt
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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); | |
} |