#define LW_FLAG_HAS_WAITERS ((uint32) 1 << 30)
#define LW_FLAG_RELEASE_OK ((uint32) 1 << 29)
+#define LW_FLAG_LOCKED ((uint32) 1 << 28)
#define LW_VAL_EXCLUSIVE ((uint32) 1 << 24)
#define LW_VAL_SHARED 1
void
LWLockInitialize(LWLock *lock, int tranche_id)
{
- SpinLockInit(&lock->mutex);
pg_atomic_init_u32(&lock->state, LW_FLAG_RELEASE_OK);
#ifdef LOCK_DEBUG
pg_atomic_init_u32(&lock->nwaiters, 0);
pg_unreachable();
}
+/*
+ * Lock the LWLock's wait list against concurrent activity.
+ *
+ * NB: even though the wait list is locked, non-conflicting lock operations
+ * may still happen concurrently.
+ *
+ * Time spent holding mutex should be short!
+ */
+static void
+LWLockWaitListLock(LWLock *lock)
+{
+ uint32 old_state;
+#ifdef LWLOCK_STATS
+ lwlock_stats *lwstats;
+ uint32 delays = 0;
+
+ lwstats = get_lwlock_stats_entry(lock);
+#endif
+
+ while (true)
+ {
+ /* always try once to acquire lock directly */
+ old_state = pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_LOCKED);
+ if (!(old_state & LW_FLAG_LOCKED))
+ break; /* got lock */
+
+ /* and then spin without atomic operations until lock is released */
+ {
+ SpinDelayStatus delayStatus = init_spin_delay(&lock->state);
+
+ while (old_state & LW_FLAG_LOCKED)
+ {
+ perform_spin_delay(&delayStatus);
+ old_state = pg_atomic_read_u32(&lock->state);
+ }
+#ifdef LWLOCK_STATS
+ delays += delayStatus.delays;
+#endif
+ finish_spin_delay(&delayStatus);
+ }
+
+ /*
+ * Retry. The lock might obviously already be re-acquired by the time
+ * we're attempting to get it again.
+ */
+ }
+
+#ifdef LWLOCK_STATS
+ lwstats->spin_delay_count += delays;
+#endif
+}
+
+/*
+ * Unlock the LWLock's wait list.
+ *
+ * Note that it can be more efficient to manipulate flags and release the
+ * locks in a single atomic operation.
+ */
+static void
+LWLockWaitListUnlock(LWLock *lock)
+{
+ uint32 old_state PG_USED_FOR_ASSERTS_ONLY;
+
+ old_state = pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_LOCKED);
+
+ Assert(old_state & LW_FLAG_LOCKED);
+}
+
/*
* Wakeup all the lockers that currently have a chance to acquire the lock.
*/
bool wokeup_somebody = false;
dlist_head wakeup;
dlist_mutable_iter iter;
-#ifdef LWLOCK_STATS
- lwlock_stats *lwstats;
-
- lwstats = get_lwlock_stats_entry(lock);
-#endif
dlist_init(&wakeup);
new_release_ok = true;
- /* Acquire mutex. Time spent holding mutex should be short! */
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ /* lock wait list while collecting backends to wake up */
+ LWLockWaitListLock(lock);
dlist_foreach_modify(iter, &lock->waiters)
{
Assert(dlist_is_empty(&wakeup) || pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS);
- /* Unset both flags at once if required */
- if (!new_release_ok && dlist_is_empty(&wakeup))
- pg_atomic_fetch_and_u32(&lock->state,
- ~(LW_FLAG_RELEASE_OK | LW_FLAG_HAS_WAITERS));
- else if (!new_release_ok)
- pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_RELEASE_OK);
- else if (dlist_is_empty(&wakeup))
- pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_HAS_WAITERS);
- else if (new_release_ok)
- pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);
+ /* unset required flags, and release lock, in one fell swoop */
+ {
+ uint32 old_state;
+ uint32 desired_state;
+
+ old_state = pg_atomic_read_u32(&lock->state);
+ while (true)
+ {
+ desired_state = old_state;
+
+ /* compute desired flags */
+
+ if (new_release_ok)
+ desired_state |= LW_FLAG_RELEASE_OK;
+ else
+ desired_state &= ~LW_FLAG_RELEASE_OK;
- /* We are done updating the shared state of the lock queue. */
- SpinLockRelease(&lock->mutex);
+ if (dlist_is_empty(&wakeup))
+ desired_state &= ~LW_FLAG_HAS_WAITERS;
+
+ desired_state &= ~LW_FLAG_LOCKED; /* release lock */
+
+ if (pg_atomic_compare_exchange_u32(&lock->state, &old_state,
+ desired_state))
+ break;
+ }
+ }
/* Awaken any waiters I removed from the queue. */
dlist_foreach_modify(iter, &wakeup)
* that happens before the list unlink happens, the list would end up
* being corrupted.
*
- * The barrier pairs with the SpinLockAcquire() when enqueing for
+ * The barrier pairs with the LWLockWaitListLock() when enqueing for
* another lock.
*/
pg_write_barrier();
static void
LWLockQueueSelf(LWLock *lock, LWLockMode mode)
{
-#ifdef LWLOCK_STATS
- lwlock_stats *lwstats;
-
- lwstats = get_lwlock_stats_entry(lock);
-#endif
-
/*
* If we don't have a PGPROC structure, there's no way to wait. This
* should never occur, since MyProc should only be null during shared
if (MyProc->lwWaiting)
elog(PANIC, "queueing for lock while waiting on another one");
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ LWLockWaitListLock(lock);
/* setting the flag is protected by the spinlock */
pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);
dlist_push_tail(&lock->waiters, &MyProc->lwWaitLink);
/* Can release the mutex now */
- SpinLockRelease(&lock->mutex);
+ LWLockWaitListUnlock(lock);
#ifdef LOCK_DEBUG
pg_atomic_fetch_add_u32(&lock->nwaiters, 1);
lwstats->dequeue_self_count++;
#endif
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ LWLockWaitListLock(lock);
/*
* Can't just remove ourselves from the list, but we need to iterate over
pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_HAS_WAITERS);
}
- SpinLockRelease(&lock->mutex);
+ /* XXX: combine with fetch_and above? */
+ LWLockWaitListUnlock(lock);
/* clear waiting state again, nice for debugging */
if (found)
{
bool mustwait;
uint64 value;
-#ifdef LWLOCK_STATS
- lwlock_stats *lwstats;
-
- lwstats = get_lwlock_stats_entry(lock);
-#endif
/*
* Test first to see if it the slot is free right now.
*result = false;
/*
- * Read value using spinlock as we can't rely on atomic 64 bit
- * reads/stores. TODO: On platforms with a way to do atomic 64 bit
- * reads/writes the spinlock could be optimized away.
+ * Read value using the lwlock's wait list lock, as we can't generally
+ * rely on atomic 64 bit reads/stores. TODO: On platforms with a way to
+ * do atomic 64 bit reads/writes the spinlock should be optimized away.
*/
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ LWLockWaitListLock(lock);
value = *valptr;
- SpinLockRelease(&lock->mutex);
+ LWLockWaitListUnlock(lock);
if (value != oldval)
{
{
dlist_head wakeup;
dlist_mutable_iter iter;
-#ifdef LWLOCK_STATS
- lwlock_stats *lwstats;
-
- lwstats = get_lwlock_stats_entry(lock);
-#endif
PRINT_LWDEBUG("LWLockUpdateVar", lock, LW_EXCLUSIVE);
dlist_init(&wakeup);
- /* Acquire mutex. Time spent holding mutex should be short! */
-#ifdef LWLOCK_STATS
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
+ LWLockWaitListLock(lock);
Assert(pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE);
}
/* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
+ LWLockWaitListUnlock(lock);
/*
* Awaken any waiters I removed from the queue.
void
LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val)
{
-#ifdef LWLOCK_STATS
- lwlock_stats *lwstats;
+ LWLockWaitListLock(lock);
- lwstats = get_lwlock_stats_entry(lock);
- lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
-#else
- SpinLockAcquire(&lock->mutex);
-#endif
/*
* Set the variable's value before releasing the lock, that prevents race
* a race condition wherein a new locker acquires the lock, but hasn't yet
* set the variables value.
*/
*valptr = val;
- SpinLockRelease(&lock->mutex);
+ LWLockWaitListUnlock(lock);
LWLockRelease(lock);
}
projects / postgresql.git / commitdiff