diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 41b56546ba9e3679432a5c727523ef65d28a0749..2815d916f3ddd4b7844dc0fdc164b5161714b499 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -3455,8 +3455,7 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
 				newbuf,
 				vmbuffer = InvalidBuffer,
 				vmbuffer_new = InvalidBuffer;
-	bool		need_toast,
-				already_marked;
+	bool		need_toast;
 	Size		newtupsize,
 				pagefree;
 	bool		have_tuple_lock = false;
@@ -3898,8 +3897,8 @@ l2:
 	 * on the same page as the old, then we need to release the content lock
 	 * (but not the pin!) on the old tuple's buffer while we are off doing
 	 * TOAST and/or table-file-extension work.  We must mark the old tuple to
-	 * show that it's already being updated, else other processes may try to
-	 * update it themselves.
+	 * show that it's locked, else other processes may try to update it
+	 * themselves.
 	 *
 	 * We need to invoke the toaster if there are already any out-of-line
 	 * toasted values present, or if the new tuple is over-threshold.
@@ -3923,19 +3922,73 @@ l2:
 
 	if (need_toast || newtupsize > pagefree)
 	{
+		TransactionId xmax_lock_old_tuple;
+		uint16		infomask_lock_old_tuple,
+					infomask2_lock_old_tuple;
+
+		/*
+		 * To prevent concurrent sessions from updating the tuple, we have to
+		 * temporarily mark it locked, while we release the lock.
+		 *
+		 * To satisfy the rule that any xid potentially appearing in a buffer
+		 * written out to disk, we unfortunately have to WAL log this
+		 * temporary modification.  We can reuse xl_heap_lock for this
+		 * purpose.  If we crash/error before following through with the
+		 * actual update, xmax will be of an aborted transaction, allowing
+		 * other sessions to proceed.
+		 */
+
+		/*
+		 * Compute xmax / infomask appropriate for locking the tuple. This has
+		 * to be done separately from the lock, because the potentially
+		 * created multixact would otherwise be wrong.
+		 */
+		compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(oldtup.t_data),
+								  oldtup.t_data->t_infomask,
+								  oldtup.t_data->t_infomask2,
+								  xid, *lockmode, false,
+							  &xmax_lock_old_tuple, &infomask_lock_old_tuple,
+								  &infomask2_lock_old_tuple);
+
+		Assert(HEAP_XMAX_IS_LOCKED_ONLY(infomask_lock_old_tuple));
+
+		START_CRIT_SECTION();
+
 		/* Clear obsolete visibility flags ... */
 		oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
 		oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
 		HeapTupleClearHotUpdated(&oldtup);
 		/* ... and store info about transaction updating this tuple */
-		Assert(TransactionIdIsValid(xmax_old_tuple));
-		HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
-		oldtup.t_data->t_infomask |= infomask_old_tuple;
-		oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
+		Assert(TransactionIdIsValid(xmax_lock_old_tuple));
+		HeapTupleHeaderSetXmax(oldtup.t_data, xmax_lock_old_tuple);
+		oldtup.t_data->t_infomask |= infomask_lock_old_tuple;
+		oldtup.t_data->t_infomask2 |= infomask2_lock_old_tuple;
 		HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
-		/* temporarily make it look not-updated */
+
+		/* temporarily make it look not-updated, but locked */
 		oldtup.t_data->t_ctid = oldtup.t_self;
-		already_marked = true;
+
+		MarkBufferDirty(buffer);
+
+		if (RelationNeedsWAL(relation))
+		{
+			xl_heap_lock xlrec;
+			XLogRecPtr	recptr;
+
+			XLogBeginInsert();
+			XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
+
+			xlrec.offnum = ItemPointerGetOffsetNumber(&oldtup.t_self);
+			xlrec.locking_xid = xmax_lock_old_tuple;
+			xlrec.infobits_set = compute_infobits(oldtup.t_data->t_infomask,
+												  oldtup.t_data->t_infomask2);
+			XLogRegisterData((char *) &xlrec, SizeOfHeapLock);
+			recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK);
+			PageSetLSN(page, recptr);
+		}
+
+		END_CRIT_SECTION();
+
 		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 
 		/*
@@ -4006,7 +4059,6 @@ l2:
 	else
 	{
 		/* No TOAST work needed, and it'll fit on same page */
-		already_marked = false;
 		newbuf = buffer;
 		heaptup = newtup;
 	}
@@ -4093,18 +4145,16 @@ l2:
 
 	RelationPutHeapTuple(relation, newbuf, heaptup, false);		/* insert new tuple */
 
-	if (!already_marked)
-	{
-		/* Clear obsolete visibility flags ... */
-		oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
-		oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
-		/* ... and store info about transaction updating this tuple */
-		Assert(TransactionIdIsValid(xmax_old_tuple));
-		HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
-		oldtup.t_data->t_infomask |= infomask_old_tuple;
-		oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
-		HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
-	}
+
+	/* Clear obsolete visibility flags, possibly set by ourselves above... */
+	oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
+	oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
+	/* ... and store info about transaction updating this tuple */
+	Assert(TransactionIdIsValid(xmax_old_tuple));
+	HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
+	oldtup.t_data->t_infomask |= infomask_old_tuple;
+	oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
+	HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
 
 	/* record address of new tuple in t_ctid of old one */
 	oldtup.t_data->t_ctid = heaptup->t_self;