diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 53ae7d5f31034fa67a563e13f471112cdb3e75d7..bb8b8bb29f621712e8cd04b70c72d6903d48b8b1 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -97,6 +97,7 @@ extern slock_t *ShmemLock;
#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
@@ -711,7 +712,6 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
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);
@@ -842,6 +842,74 @@ LWLockAttemptLock(LWLock *lock, LWLockMode mode)
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.
*/
@@ -852,22 +920,13 @@ LWLockWakeup(LWLock *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)
{
@@ -904,19 +963,33 @@ LWLockWakeup(LWLock *lock)
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)
@@ -933,7 +1006,7 @@ LWLockWakeup(LWLock *lock)
* 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();
@@ -950,12 +1023,6 @@ LWLockWakeup(LWLock *lock)
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
@@ -967,11 +1034,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
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);
@@ -986,7 +1049,7 @@ LWLockQueueSelf(LWLock *lock, LWLockMode mode)
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);
@@ -1015,11 +1078,7 @@ LWLockDequeueSelf(LWLock *lock)
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
@@ -1043,7 +1102,8 @@ LWLockDequeueSelf(LWLock *lock)
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)
@@ -1460,11 +1520,6 @@ LWLockConflictsWithVar(LWLock *lock,
{
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.
@@ -1484,17 +1539,13 @@ LWLockConflictsWithVar(LWLock *lock,
*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)
{
@@ -1668,22 +1719,12 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
{
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);
@@ -1706,7 +1747,7 @@ LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
}
/* We are done updating shared state of the lock itself. */
- SpinLockRelease(&lock->mutex);
+ LWLockWaitListUnlock(lock);
/*
* Awaken any waiters I removed from the queue.
@@ -1804,21 +1845,15 @@ LWLockRelease(LWLock *lock)
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);
}
diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index 5e6299af1d0d41cbbda3a546d33647d282b265bc..3db11e43f0da2a440f9f5bb5521c33fdcd591bc3 100644
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -57,15 +57,11 @@ typedef struct LWLockTranche
*/
typedef struct LWLock
{
- slock_t mutex; /* Protects LWLock and queue of PGPROCs */
uint16 tranche; /* tranche ID */
-
pg_atomic_uint32 state; /* state of exclusive/nonexclusive lockers */
-#ifdef LOCK_DEBUG
- pg_atomic_uint32 nwaiters; /* number of waiters */
-#endif
dlist_head waiters; /* list of waiting PGPROCs */
#ifdef LOCK_DEBUG
+ pg_atomic_uint32 nwaiters; /* number of waiters */
struct PGPROC *owner; /* last exclusive owner of the lock */
#endif
} LWLock;