Avoid the use of a separate spinlock to protect a LWLock's wait queue.

Previously we used a spinlock, in adition to the atomically manipulated
->state field, to protect the wait queue. But it's pretty simple to
instead perform the locking using a flag in state.

Due to 6150a1b BufferDescs, on platforms (like PPC) with > 1 byte
spinlocks, increased their size above 64byte. As 64 bytes are the size
we pad allocated BufferDescs to, this can increase false sharing;
causing performance problems in turn. Together with the previous commit
this reduces the size to <= 64 bytes on all common platforms.

Author: Andres Freund
Discussion: CAA4eK1+ZeB8PMwwktf+3bRS0Pt4Ux6Rs6Aom0uip8c6shJWmyg@mail.gmail.com
    20160327121858.zrmrjegmji2ymnvr@alap3.anarazel.de

Author: anarazel

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);
 }

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;