Date: Wed, 16 Aug 2023 13:07:15 -0700
From: Nathan Bossart <nathandbossart@gmail.com>
To: Michail Nikolaev <michail.nikolaev@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
	Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: Replace known_assigned_xids_lck by memory barrier
Message-ID: <20230816200715.GB2908487@nathanxps13>
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On Wed, Aug 16, 2023 at 09:29:10PM +0200, Michail Nikolaev wrote:
> As answer: probably we need to change
> "If we know that we're holding ProcArrayLock exclusively, we don't
> need the read barrier."
> to
> "If we're removing xid, we don't need the read barrier because only
> the startup process can remove and add xids to KnownAssignedXids"

Ah, that explains it.  v5 of the patch is attached.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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	filename="v5-0001-Replace-known_assigned_xids_lck-with-memory-barri.patch"

From 6038182ac96226f0eec43b1c6ab2e060e8b1e3a8 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Wed, 16 Aug 2023 10:52:56 -0700
Subject: [PATCH v5 1/1] Replace known_assigned_xids_lck with memory barriers.

Suggested-by: Tom Lane
Author: Michail Nikolaev
Discussion: https://postgr.es/m/CANtu0oh0si%3DjG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA%40mail.gmail.com
---
 src/backend/storage/ipc/procarray.c | 72 +++++++++++------------------
 1 file changed, 26 insertions(+), 46 deletions(-)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 2a3da49b8f..401e816275 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/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;
@@ -4561,14 +4558,11 @@ 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:
  *
@@ -4806,22 +4800,15 @@ 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;
 }
 
 /*
@@ -4843,20 +4830,15 @@ 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
@@ -5094,13 +5076,11 @@ 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++)
 	{
@@ -5147,10 +5127,10 @@ 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++)
 	{
-- 
2.25.1


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