Replace known_assigned_xids_lck with memory barriers.

This lock was introduced before memory barrier support was added,
and it is only used to guarantee proper memory ordering when
KnownAssignedXidsAdd() appends to the array without a lock.  Now
that such memory barrier support exists, we can remove the lock and
use barriers instead.

Suggested-by: Tom Lane
Author: Michail Nikolaev
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/CANtu0oh0si%3DjG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA%40mail.gmail.com

Author: nathan-bossart

src/backend/storage/ipc/procarray.c

@@ -61,7 +61,6 @@
 #include "port/pg_lfind.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
-#include "storage/spin.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/rel.h"
@@ -82,7 +81,6 @@ typedef struct ProcArrayStruct
 	int			numKnownAssignedXids;	/* current # of valid entries */
 	int			tailKnownAssignedXids;	/* index of oldest valid element */
 	int			headKnownAssignedXids;	/* index of newest element, + 1 */
-	slock_t		known_assigned_xids_lck;	/* protects head/tail pointers */
 
 	/*
 	 * Highest subxid that has been removed from KnownAssignedXids array to
@@ -441,7 +439,6 @@ CreateSharedProcArray(void)
 		procArray->numKnownAssignedXids = 0;
 		procArray->tailKnownAssignedXids = 0;
 		procArray->headKnownAssignedXids = 0;
-		SpinLockInit(&procArray->known_assigned_xids_lck);
 		procArray->lastOverflowedXid = InvalidTransactionId;
 		procArray->replication_slot_xmin = InvalidTransactionId;
 		procArray->replication_slot_catalog_xmin = InvalidTransactionId;
@@ -4533,22 +4530,19 @@ KnownAssignedTransactionIdsIdleMaintenance(void)
  * during normal running).  Compressing unused entries out of the array
  * likewise requires exclusive lock.  To add XIDs to the array, we just insert
  * them into slots to the right of the head pointer and then advance the head
- * pointer.  This wouldn't require any lock at all, except that on machines
- * with weak memory ordering we need to be careful that other processors
- * see the array element changes before they see the head pointer change.
- * We handle this by using a spinlock to protect reads and writes of the
- * head/tail pointers.  (We could dispense with the spinlock if we were to
- * create suitable memory access barrier primitives and use those instead.)
- * The spinlock must be taken to read or write the head/tail pointers unless
- * the caller holds ProcArrayLock exclusively.
+ * pointer.  This doesn't require any lock at all, but on machines with weak
+ * memory ordering, we need to be careful that other processors see the array
+ * element changes before they see the head pointer change.  We handle this by
+ * using memory barriers when reading or writing the head/tail pointers (unless
+ * the caller holds ProcArrayLock exclusively).
  *
  * Algorithmic analysis:
  *
  * If we have a maximum of M slots, with N XIDs currently spread across
  * S elements then we have N <= S <= M always.
  *
- *	* Adding a new XID is O(1) and needs little locking (unless compression
- *		must happen)
+ *	* Adding a new XID is O(1) and needs no lock (unless compression must
+ *		happen)
  *	* Compressing the array is O(S) and requires exclusive lock
  *	* Removing an XID is O(logS) and requires exclusive lock
  *	* Taking a snapshot is O(S) and requires shared lock
@@ -4778,22 +4772,15 @@ KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
 	pArray->numKnownAssignedXids += nxids;
 
 	/*
-	 * Now update the head pointer.  We use a spinlock to protect this
-	 * pointer, not because the update is likely to be non-atomic, but to
-	 * ensure that other processors see the above array updates before they
-	 * see the head pointer change.
-	 *
-	 * If we're holding ProcArrayLock exclusively, there's no need to take the
-	 * spinlock.
+	 * Now update the head pointer.  We use a write barrier to ensure that
+	 * other processors see the above array updates before they see the head
+	 * pointer change.  The barrier isn't required if we're holding
+	 * ProcArrayLock exclusively.
 	 */
-	if (exclusive_lock)
-		pArray->headKnownAssignedXids = head;
-	else
-	{
-		SpinLockAcquire(&pArray->known_assigned_xids_lck);
-		pArray->headKnownAssignedXids = head;
-		SpinLockRelease(&pArray->known_assigned_xids_lck);
-	}
+	if (!exclusive_lock)
+		pg_write_barrier();
+
+	pArray->headKnownAssignedXids = head;
 }
 
 /*
@@ -4815,20 +4802,15 @@ KnownAssignedXidsSearch(TransactionId xid, bool remove)
 	int			tail;
 	int			result_index = -1;
 
-	if (remove)
-	{
-		/* we hold ProcArrayLock exclusively, so no need for spinlock */
-		tail = pArray->tailKnownAssignedXids;
-		head = pArray->headKnownAssignedXids;
-	}
-	else
-	{
-		/* take spinlock to ensure we see up-to-date array contents */
-		SpinLockAcquire(&pArray->known_assigned_xids_lck);
-		tail = pArray->tailKnownAssignedXids;
-		head = pArray->headKnownAssignedXids;
-		SpinLockRelease(&pArray->known_assigned_xids_lck);
-	}
+	tail = pArray->tailKnownAssignedXids;
+	head = pArray->headKnownAssignedXids;
+
+	/*
+	 * Only the startup process removes entries, so we don't need the read
+	 * barrier in that case.
+	 */
+	if (!remove)
+		pg_read_barrier();		/* pairs with KnownAssignedXidsAdd */
 
 	/*
 	 * Standard binary search.  Note we can ignore the KnownAssignedXidsValid
@@ -5066,13 +5048,11 @@ KnownAssignedXidsGetAndSetXmin(TransactionId *xarray, TransactionId *xmin,
 	 * cannot enter and then leave the array while we hold ProcArrayLock.  We
 	 * might miss newly-added xids, but they should be >= xmax so irrelevant
 	 * anyway.
-	 *
-	 * Must take spinlock to ensure we see up-to-date array contents.
 	 */
-	SpinLockAcquire(&procArray->known_assigned_xids_lck);
 	tail = procArray->tailKnownAssignedXids;
 	head = procArray->headKnownAssignedXids;
-	SpinLockRelease(&procArray->known_assigned_xids_lck);
+
+	pg_read_barrier();			/* pairs with KnownAssignedXidsAdd */
 
 	for (i = tail; i < head; i++)
 	{
@@ -5119,10 +5099,10 @@ KnownAssignedXidsGetOldestXmin(void)
 	/*
 	 * Fetch head just once, since it may change while we loop.
 	 */
-	SpinLockAcquire(&procArray->known_assigned_xids_lck);
 	tail = procArray->tailKnownAssignedXids;
 	head = procArray->headKnownAssignedXids;
-	SpinLockRelease(&procArray->known_assigned_xids_lck);
+
+	pg_read_barrier();			/* pairs with KnownAssignedXidsAdd */
 
 	for (i = tail; i < head; i++)
 	{