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author"Steven J. Hill" <sjhill@realitydiluted.com>2005-05-07 02:04:55 +0000
committer"Steven J. Hill" <sjhill@realitydiluted.com>2005-05-07 02:04:55 +0000
commit0b3366f6a93ac0f7e2028745ea557c0acd8722c6 (patch)
tree2a87cb353c106eb55a44437d3b4d5e0d60af1d36 /libpthread/nptl/DESIGN-rwlock.txt
parent2274f6b2ddc7d7ea919a8fa1f9c54ef1920cb180 (diff)
Import in NPTL code from glibc. For further information please
consult the 'README.NPTL' file.
Diffstat (limited to 'libpthread/nptl/DESIGN-rwlock.txt')
-rw-r--r--libpthread/nptl/DESIGN-rwlock.txt113
1 files changed, 113 insertions, 0 deletions
diff --git a/libpthread/nptl/DESIGN-rwlock.txt b/libpthread/nptl/DESIGN-rwlock.txt
new file mode 100644
index 000000000..810d1b8f3
--- /dev/null
+++ b/libpthread/nptl/DESIGN-rwlock.txt
@@ -0,0 +1,113 @@
+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);
+}