Remove volatiles from {procarray,volatile}.c and fix memory ordering issue.

The use of volatiles in procarray.c largely originated from the time
when postgres did not have reliable compiler and memory
barriers. That's not the case anymore, so we can do better.

Several of the functions in procarray.c can be bottlenecks, and
removal of volatile yields mildly better code.

The new state, with explicit memory barriers, is also more
correct. The previous use of volatile did not actually deliver
sufficient guarantees on weakly ordered machines, in particular the
logic in GetNewTransactionId() does not look safe.  It seems unlikely
to be a problem in practice, but worth fixing.

Thomas and I independently wrote a patch for this.

Reported-By: Andres Freund and Thomas Munro
Author: Andres Freund, with cherrypicked changes from a patch by Thomas Munro
Discussion:
    https://postgr.es/m/20181005172955.wyjb4fzcdzqtaxjq@alap3.anarazel.de
    https://postgr.es/m/CAEepm=1nff0x=7i3YQO16jLA2qw-F9O39YmUew4oq-xcBQBs0g@mail.gmail.com

src/backend/access/transam/varsup.c

@@ -186,20 +186,23 @@ GetNewTransactionId(bool isSubXact)
 	 * latestCompletedXid is present in the ProcArray, which is essential for
 	 * correct OldestXmin tracking; see src/backend/access/transam/README.
 	 *
-	 * XXX by storing xid into MyPgXact without acquiring ProcArrayLock, we
-	 * are relying on fetch/store of an xid to be atomic, else other backends
-	 * might see a partially-set xid here.  But holding both locks at once
-	 * would be a nasty concurrency hit.  So for now, assume atomicity.
-	 *
 	 * Note that readers of PGXACT xid fields should be careful to fetch the
 	 * value only once, rather than assume they can read a value multiple
-	 * times and get the same answer each time.
+	 * times and get the same answer each time.  Note we are assuming that
+	 * TransactionId and int fetch/store are atomic.
 	 *
 	 * The same comments apply to the subxact xid count and overflow fields.
 	 *
-	 * A solution to the atomic-store problem would be to give each PGXACT its
-	 * own spinlock used only for fetching/storing that PGXACT's xid and
-	 * related fields.
+	 * Use of a write barrier prevents dangerous code rearrangement in this
+	 * function; other backends could otherwise e.g. be examining my subxids
+	 * info concurrently, and we don't want them to see an invalid
+	 * intermediate state, such as an incremented nxids before the array entry
+	 * is filled.
+	 *
+	 * Other processes that read nxids should do so before reading xids
+	 * elements with a pg_read_barrier() in between, so that they can be sure
+	 * not to read an uninitialized array element; see
+	 * src/backend/storage/lmgr/README.barrier.
 	 *
 	 * If there's no room to fit a subtransaction XID into PGPROC, set the
 	 * cache-overflowed flag instead.  This forces readers to look in
@@ -211,31 +214,20 @@ GetNewTransactionId(bool isSubXact)
 	 * window *will* include the parent XID, so they will deliver the correct
 	 * answer later on when someone does have a reason to inquire.)
 	 */
+	if (!isSubXact)
+		MyPgXact->xid = xid;	/* LWLockRelease acts as barrier */
+	else
 	{
-		/*
-		 * Use volatile pointer to prevent code rearrangement; other backends
-		 * could be examining my subxids info concurrently, and we don't want
-		 * them to see an invalid intermediate state, such as incrementing
-		 * nxids before filling the array entry.  Note we are assuming that
-		 * TransactionId and int fetch/store are atomic.
-		 */
-		volatile PGPROC *myproc = MyProc;
-		volatile PGXACT *mypgxact = MyPgXact;
+		int			nxids = MyPgXact->nxids;
 
-		if (!isSubXact)
-			mypgxact->xid = xid;
-		else
+		if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
 		{
-			int			nxids = mypgxact->nxids;
-
-			if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
-			{
-				myproc->subxids.xids[nxids] = xid;
-				mypgxact->nxids = nxids + 1;
-			}
-			else
-				mypgxact->overflowed = true;
+			MyProc->subxids.xids[nxids] = xid;
+			pg_write_barrier();
+			MyPgXact->nxids = nxids + 1;
 		}
+		else
+			MyPgXact->overflowed = true;
 	}
 
 	LWLockRelease(XidGenLock);

src/backend/storage/ipc/procarray.c

@@ -61,6 +61,7 @@
 #include "utils/rel.h"
 #include "utils/snapmgr.h"
 
+#define UINT32_ACCESS_ONCE(var)		 ((uint32)(*((volatile uint32 *)&(var))))
 
 /* Our shared memory area */
 typedef struct ProcArrayStruct
@@ -483,7 +484,7 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
 static void
 ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 {
-	volatile PROC_HDR *procglobal = ProcGlobal;
+	PROC_HDR   *procglobal = ProcGlobal;
 	uint32		nextidx;
 	uint32		wakeidx;
 
@@ -1077,16 +1078,17 @@ TransactionIdIsInProgress(TransactionId xid)
 	for (i = 0; i < arrayP->numProcs; i++)
 	{
 		int			pgprocno = arrayP->pgprocnos[i];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId pxid;
+		int			pxids;
 
 		/* Ignore my own proc --- dealt with it above */
 		if (proc == MyProc)
 			continue;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		pxid = pgxact->xid;
+		pxid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 		if (!TransactionIdIsValid(pxid))
 			continue;
@@ -1111,10 +1113,12 @@ TransactionIdIsInProgress(TransactionId xid)
 		/*
 		 * Step 2: check the cached child-Xids arrays
 		 */
-		for (j = pgxact->nxids - 1; j >= 0; j--)
+		pxids = pgxact->nxids;
+		pg_read_barrier();		/* pairs with barrier in GetNewTransactionId() */
+		for (j = pxids - 1; j >= 0; j--)
 		{
 			/* Fetch xid just once - see GetNewTransactionId */
-			TransactionId cxid = proc->subxids.xids[j];
+			TransactionId cxid = UINT32_ACCESS_ONCE(proc->subxids.xids[j]);
 
 			if (TransactionIdEquals(cxid, xid))
 			{
@@ -1233,12 +1237,12 @@ TransactionIdIsActive(TransactionId xid)
 	for (i = 0; i < arrayP->numProcs; i++)
 	{
 		int			pgprocno = arrayP->pgprocnos[i];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId pxid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		pxid = pgxact->xid;
+		pxid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 		if (!TransactionIdIsValid(pxid))
 			continue;
@@ -1321,8 +1325,8 @@ GetOldestXmin(Relation rel, int flags)
 	int			index;
 	bool		allDbs;
 
-	volatile TransactionId replication_slot_xmin = InvalidTransactionId;
-	volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
+	TransactionId replication_slot_xmin = InvalidTransactionId;
+	TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
 
 	/*
 	 * If we're not computing a relation specific limit, or if a shared
@@ -1349,8 +1353,8 @@ GetOldestXmin(Relation rel, int flags)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		if (pgxact->vacuumFlags & (flags & PROCARRAY_PROC_FLAGS_MASK))
 			continue;
@@ -1360,7 +1364,7 @@ GetOldestXmin(Relation rel, int flags)
 			proc->databaseId == 0)	/* always include WalSender */
 		{
 			/* Fetch xid just once - see GetNewTransactionId */
-			TransactionId xid = pgxact->xid;
+			TransactionId xid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 			/* First consider the transaction's own Xid, if any */
 			if (TransactionIdIsNormal(xid) &&
@@ -1374,14 +1378,18 @@ GetOldestXmin(Relation rel, int flags)
 			 * have an Xmin but not (yet) an Xid; conversely, if it has an
 			 * Xid, that could determine some not-yet-set Xmin.
 			 */
-			xid = pgxact->xmin; /* Fetch just once */
+			xid = UINT32_ACCESS_ONCE(pgxact->xmin);
 			if (TransactionIdIsNormal(xid) &&
 				TransactionIdPrecedes(xid, result))
 				result = xid;
 		}
 	}
 
-	/* fetch into volatile var while ProcArrayLock is held */
+	/*
+	 * Fetch into local variable while ProcArrayLock is held - the
+	 * LWLockRelease below is a barrier, ensuring this happens inside the
+	 * lock.
+	 */
 	replication_slot_xmin = procArray->replication_slot_xmin;
 	replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
 
@@ -1518,8 +1526,8 @@ GetSnapshotData(Snapshot snapshot)
 	int			count = 0;
 	int			subcount = 0;
 	bool		suboverflowed = false;
-	volatile TransactionId replication_slot_xmin = InvalidTransactionId;
-	volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
+	TransactionId replication_slot_xmin = InvalidTransactionId;
+	TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
 
 	Assert(snapshot != NULL);
 
@@ -1585,7 +1593,7 @@ GetSnapshotData(Snapshot snapshot)
 		for (index = 0; index < numProcs; index++)
 		{
 			int			pgprocno = pgprocnos[index];
-			volatile PGXACT *pgxact = &allPgXact[pgprocno];
+			PGXACT	   *pgxact = &allPgXact[pgprocno];
 			TransactionId xid;
 
 			/*
@@ -1597,13 +1605,13 @@ GetSnapshotData(Snapshot snapshot)
 				continue;
 
 			/* Update globalxmin to be the smallest valid xmin */
-			xid = pgxact->xmin; /* fetch just once */
+			xid = UINT32_ACCESS_ONCE(pgxact->xmin);
 			if (TransactionIdIsNormal(xid) &&
 				NormalTransactionIdPrecedes(xid, globalxmin))
 				globalxmin = xid;
 
 			/* Fetch xid just once - see GetNewTransactionId */
-			xid = pgxact->xid;
+			xid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 			/*
 			 * If the transaction has no XID assigned, we can skip it; it
@@ -1652,7 +1660,9 @@ GetSnapshotData(Snapshot snapshot)
 
 					if (nxids > 0)
 					{
-						volatile PGPROC *proc = &allProcs[pgprocno];
+						PGPROC	   *proc = &allProcs[pgprocno];
+
+						pg_read_barrier();	/* pairs with GetNewTransactionId */
 
 						memcpy(snapshot->subxip + subcount,
 							   (void *) proc->subxids.xids,
@@ -1702,7 +1712,11 @@ GetSnapshotData(Snapshot snapshot)
 	}
 
 
-	/* fetch into volatile var while ProcArrayLock is held */
+	/*
+	 * Fetch into local variable while ProcArrayLock is held - the
+	 * LWLockRelease below is a barrier, ensuring this happens inside the
+	 * lock.
+	 */
 	replication_slot_xmin = procArray->replication_slot_xmin;
 	replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
 
@@ -1810,8 +1824,8 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId xid;
 
 		/* Ignore procs running LAZY VACUUM */
@@ -1836,7 +1850,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
 		/*
 		 * Likewise, let's just make real sure its xmin does cover us.
 		 */
-		xid = pgxact->xmin;		/* fetch just once */
+		xid = UINT32_ACCESS_ONCE(pgxact->xmin);
 		if (!TransactionIdIsNormal(xid) ||
 			!TransactionIdPrecedesOrEquals(xid, xmin))
 			continue;
@@ -1872,7 +1886,7 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 {
 	bool		result = false;
 	TransactionId xid;
-	volatile PGXACT *pgxact;
+	PGXACT	   *pgxact;
 
 	Assert(TransactionIdIsNormal(xmin));
 	Assert(proc != NULL);
@@ -1888,7 +1902,7 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 	 * can't go backwards.  Also, make sure it's running in the same database,
 	 * so that the per-database xmin cannot go backwards.
 	 */
-	xid = pgxact->xmin;			/* fetch just once */
+	xid = UINT32_ACCESS_ONCE(pgxact->xmin);
 	if (proc->databaseId == MyDatabaseId &&
 		TransactionIdIsNormal(xid) &&
 		TransactionIdPrecedesOrEquals(xid, xmin))
@@ -1995,11 +2009,11 @@ GetRunningTransactionData(void)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId xid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		xid = pgxact->xid;
+		xid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 		/*
 		 * We don't need to store transactions that don't have a TransactionId
@@ -2039,8 +2053,8 @@ GetRunningTransactionData(void)
 		for (index = 0; index < arrayP->numProcs; index++)
 		{
 			int			pgprocno = arrayP->pgprocnos[index];
-			volatile PGPROC *proc = &allProcs[pgprocno];
-			volatile PGXACT *pgxact = &allPgXact[pgprocno];
+			PGPROC	   *proc = &allProcs[pgprocno];
+			PGXACT	   *pgxact = &allPgXact[pgprocno];
 			int			nxids;
 
 			/*
@@ -2050,6 +2064,9 @@ GetRunningTransactionData(void)
 			nxids = pgxact->nxids;
 			if (nxids > 0)
 			{
+				/* barrier not really required, as XidGenLock is held, but ... */
+				pg_read_barrier();	/* pairs with GetNewTransactionId */
+
 				memcpy(&xids[count], (void *) proc->subxids.xids,
 					   nxids * sizeof(TransactionId));
 				count += nxids;
@@ -2131,11 +2148,11 @@ GetOldestActiveTransactionId(void)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		TransactionId xid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		xid = pgxact->xid;
+		xid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 		if (!TransactionIdIsNormal(xid))
 			continue;
@@ -2229,11 +2246,11 @@ GetOldestSafeDecodingTransactionId(bool catalogOnly)
 		for (index = 0; index < arrayP->numProcs; index++)
 		{
 			int			pgprocno = arrayP->pgprocnos[index];
-			volatile PGXACT *pgxact = &allPgXact[pgprocno];
+			PGXACT	   *pgxact = &allPgXact[pgprocno];
 			TransactionId xid;
 
 			/* Fetch xid just once - see GetNewTransactionId */
-			xid = pgxact->xid;
+			xid = UINT32_ACCESS_ONCE(pgxact->xid);
 
 			if (!TransactionIdIsNormal(xid))
 				continue;
@@ -2283,8 +2300,8 @@ GetVirtualXIDsDelayingChkpt(int *nvxids)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		if (pgxact->delayChkpt)
 		{
@@ -2323,8 +2340,8 @@ HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids, int nvxids)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 		VirtualTransactionId vxid;
 
 		GET_VXID_FROM_PGPROC(vxid, *proc);
@@ -2433,8 +2450,8 @@ BackendXidGetPid(TransactionId xid)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		if (pgxact->xid == xid)
 		{
@@ -2505,8 +2522,8 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		if (proc == MyProc)
 			continue;
@@ -2517,7 +2534,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
 		if (allDbs || proc->databaseId == MyDatabaseId)
 		{
 			/* Fetch xmin just once - might change on us */
-			TransactionId pxmin = pgxact->xmin;
+			TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
 
 			if (excludeXmin0 && !TransactionIdIsValid(pxmin))
 				continue;
@@ -2602,8 +2619,8 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		/* Exclude prepared transactions */
 		if (proc->pid == 0)
@@ -2613,7 +2630,7 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 			proc->databaseId == dbOid)
 		{
 			/* Fetch xmin just once - can't change on us, but good coding */
-			TransactionId pxmin = pgxact->xmin;
+			TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
 
 			/*
 			 * We ignore an invalid pxmin because this means that backend has
@@ -2661,7 +2678,7 @@ CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
 		VirtualTransactionId procvxid;
 
 		GET_VXID_FROM_PGPROC(procvxid, *proc);
@@ -2716,8 +2733,8 @@ MinimumActiveBackends(int min)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
-		volatile PGXACT *pgxact = &allPgXact[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
+		PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 		/*
 		 * Since we're not holding a lock, need to be prepared to deal with
@@ -2763,7 +2780,7 @@ CountDBBackends(Oid databaseid)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
 
 		if (proc->pid == 0)
 			continue;			/* do not count prepared xacts */
@@ -2793,7 +2810,7 @@ CountDBConnections(Oid databaseid)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
 
 		if (proc->pid == 0)
 			continue;			/* do not count prepared xacts */
@@ -2825,7 +2842,7 @@ CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conflictPending)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
 
 		if (databaseid == InvalidOid || proc->databaseId == databaseid)
 		{
@@ -2864,7 +2881,7 @@ CountUserBackends(Oid roleid)
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
 		int			pgprocno = arrayP->pgprocnos[index];
-		volatile PGPROC *proc = &allProcs[pgprocno];
+		PGPROC	   *proc = &allProcs[pgprocno];
 
 		if (proc->pid == 0)
 			continue;			/* do not count prepared xacts */
@@ -2927,8 +2944,8 @@ CountOtherDBBackends(Oid databaseId, int *nbackends, int *nprepared)
 		for (index = 0; index < arrayP->numProcs; index++)
 		{
 			int			pgprocno = arrayP->pgprocnos[index];
-			volatile PGPROC *proc = &allProcs[pgprocno];
-			volatile PGXACT *pgxact = &allPgXact[pgprocno];
+			PGPROC	   *proc = &allProcs[pgprocno];
+			PGXACT	   *pgxact = &allPgXact[pgprocno];
 
 			if (proc->databaseId != databaseId)
 				continue;
@@ -3017,6 +3034,7 @@ ProcArrayGetReplicationSlotXmin(TransactionId *xmin,
 #define XidCacheRemove(i) \
 	do { \
 		MyProc->subxids.xids[i] = MyProc->subxids.xids[MyPgXact->nxids - 1]; \
+		pg_write_barrier(); \
 		MyPgXact->nxids--; \
 	} while (0)
 
@@ -3044,6 +3062,11 @@ XidCacheRemoveRunningXids(TransactionId xid,
 	 * possible this could be relaxed since we know this routine is only used
 	 * to abort subtransactions, but pending closer analysis we'd best be
 	 * conservative.
+	 *
+	 * Note that we do not have to be careful about memory ordering of our own
+	 * reads wrt. GetNewTransactionId() here - only this process can modify
+	 * relevant fields of MyProc/MyPgXact.  But we do have to be careful about
+	 * our own writes being well ordered.
 	 */
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
@@ -3401,8 +3424,7 @@ ExpireOldKnownAssignedTransactionIds(TransactionId xid)
 static void
 KnownAssignedXidsCompress(bool force)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 	int			head,
 				tail;
 	int			compress_index;
@@ -3465,8 +3487,7 @@ static void
 KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
 					 bool exclusive_lock)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 	TransactionId next_xid;
 	int			head,
 				tail;
@@ -3583,8 +3604,7 @@ KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
 static bool
 KnownAssignedXidsSearch(TransactionId xid, bool remove)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 	int			first,
 				last;
 	int			head;
@@ -3738,8 +3758,7 @@ KnownAssignedXidsRemoveTree(TransactionId xid, int nsubxids,
 static void
 KnownAssignedXidsRemovePreceding(TransactionId removeXid)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 	int			count = 0;
 	int			head,
 				tail,
@@ -3924,8 +3943,7 @@ KnownAssignedXidsGetOldestXmin(void)
 static void
 KnownAssignedXidsDisplay(int trace_level)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 	StringInfoData buf;
 	int			head,
 				tail,
@@ -3963,8 +3981,7 @@ KnownAssignedXidsDisplay(int trace_level)
 static void
 KnownAssignedXidsReset(void)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile ProcArrayStruct *pArray = procArray;
+	ProcArrayStruct *pArray = procArray;
 
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);