From: Nathan Bossart <nathan@postgresql.org>
Subject: [PATCH v4 2/2] Convert SISeg->maxMsgNum to an atomic variable.
Date: Tue, 22 Sep 2026 14:36:42 -0500
Currently, this variable is a uint32 protected by a spinlock. The
spinlock exists only to provide memory barriers, so by converting
the variable to an atomic and using the barrier-providing accessors
in the spinlock's place, we can remove the spinlock.
Author: Yura Sokolov <y.sokolov@postgrespro.ru>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Tested-by: solai v <solai.cdac@gmail.com>
Discussion: https://postgr.es/m/30aa0030-f694-44ef-a19d-6ef7ddb69374%40postgrespro.ru
Discussion: https://postgr.es/m/alAJeRRzehDjLaF1%40nathan
---
src/backend/storage/ipc/sinvaladt.c | 51 ++++++++++-------------------
1 file changed, 17 insertions(+), 34 deletions(-)
diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.cindex b29b4bcc5be..bc5f9537710 100644--- a/src/backend/storage/ipc/sinvaladt.c+++ b/src/backend/storage/ipc/sinvaladt.c@@ -24,7 +24,6 @@
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "storage/sinvaladt.h"
-#include "storage/spin.h"
#include "storage/subsystems.h"
/*
@@ -87,19 +86,10 @@
* has no need to touch anyone's ProcState, except in the infrequent cases
* when SICleanupQueue is needed. The only point of overlap is that
* the writer wants to change maxMsgNum while readers need to read it.
- * We deal with that by having a spinlock that readers must take for just- * long enough to read maxMsgNum, while writers take it for just long enough- * to write maxMsgNum. (The exact rule is that you need the spinlock to- * read maxMsgNum if you are not holding SInvalWriteLock, and you need the- * spinlock to write maxMsgNum unless you are holding both locks.)- *- * Note: since maxMsgNum is a uint32 and hence presumably atomically readable/- * writable, the spinlock might seem unnecessary. The reason it is needed- * is to provide a memory barrier: we need to be sure that messages written- * to the array are actually there before maxMsgNum is increased, and that- * readers will see that data after fetching maxMsgNum. Multiprocessors- * that have weak memory-ordering guarantees can fail without the memory- * barrier instructions that are included in the spinlock sequences.+ * We deal with that by making maxMsgNum an atomic variable. (The exact rule+ * is that you need to use a barrier-providing accessor to read maxMsgNum if+ * you are not holding SInvalWriteLock, and you need a barrier-providing+ * accessor to write maxMsgNum unless you are holding both locks.)
*/
@@ -169,11 +159,9 @@ typedef struct SISeg
* General state information
*/
uint32 minMsgNum; /* oldest message still needed */
- uint32 maxMsgNum; /* next message number to be assigned */+ pg_atomic_uint32 maxMsgNum; /* next message number to be assigned */
uint32 nextThreshold; /* # of messages to call SICleanupQueue */
- slock_t msgnumLock; /* spinlock protecting maxMsgNum */-
/*
* Circular buffer holding shared-inval messages
*/
@@ -244,11 +232,10 @@ SharedInvalShmemInit(void *arg)
{
int i;
- /* Clear message counters, init spinlock */+ /* Clear message counters */
shmInvalBuffer->minMsgNum = 0;
- shmInvalBuffer->maxMsgNum = 0;+ pg_atomic_init_u32(&shmInvalBuffer->maxMsgNum, 0);
shmInvalBuffer->nextThreshold = CLEANUP_MIN;
- SpinLockInit(&shmInvalBuffer->msgnumLock);
/* The buffer[] array is initially all unused, so we need not fill it */
@@ -306,7 +293,7 @@ SharedInvalBackendInit(bool sendOnly)
/* mark myself active, with all extant messages already read */
stateP->procPid = MyProcPid;
- stateP->nextMsgNum = segP->maxMsgNum;+ stateP->nextMsgNum = pg_atomic_read_u32(&segP->maxMsgNum);
stateP->resetState = false;
stateP->signaled = false;
stateP->hasMessages = false;
@@ -402,7 +389,7 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
*/
for (;;)
{
- numMsgs = segP->maxMsgNum - segP->minMsgNum;+ numMsgs = pg_atomic_read_u32(&segP->maxMsgNum) - segP->minMsgNum;
if (numMsgs + nthistime > MAXNUMMESSAGES ||
numMsgs >= segP->nextThreshold)
SICleanupQueue(true, nthistime);
@@ -413,17 +400,15 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
/*
* Insert new message(s) into proper slot of circular buffer
*/
- max = segP->maxMsgNum;+ max = pg_atomic_read_u32(&segP->maxMsgNum);
while (nthistime-- > 0)
{
segP->buffer[max % MAXNUMMESSAGES] = *data++;
max++;
}
- /* Update current value of maxMsgNum using spinlock */- SpinLockAcquire(&segP->msgnumLock);- segP->maxMsgNum = max;- SpinLockRelease(&segP->msgnumLock);+ /* Update current value of maxMsgNum using barrier */+ pg_atomic_write_membarrier_u32(&segP->maxMsgNum, max);
/*
* Now that the maxMsgNum change is globally visible, we give everyone
@@ -509,10 +494,8 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
*/
stateP->hasMessages = false;
- /* Fetch current value of maxMsgNum using spinlock */- SpinLockAcquire(&segP->msgnumLock);- max = segP->maxMsgNum;- SpinLockRelease(&segP->msgnumLock);+ /* Fetch current value of maxMsgNum using barrier */+ max = pg_atomic_read_membarrier_u32(&segP->maxMsgNum);
if (stateP->resetState)
{
@@ -601,7 +584,7 @@ SICleanupQueue(bool callerHasWriteLock, int minFree)
* Note that the thresholds are clamped at zero rather than allowed to
* wrap around.
*/
- min = segP->maxMsgNum;+ min = pg_atomic_read_u32(&segP->maxMsgNum);
minsig = (min > SIG_THRESHOLD) ? min - SIG_THRESHOLD : 0;
lowbound = (min + minFree > MAXNUMMESSAGES) ?
min + minFree - MAXNUMMESSAGES : 0;
@@ -648,7 +631,7 @@ SICleanupQueue(bool callerHasWriteLock, int minFree)
if (min >= MSGNUMWRAPAROUND)
{
segP->minMsgNum -= MSGNUMWRAPAROUND;
- segP->maxMsgNum -= MSGNUMWRAPAROUND;+ pg_atomic_fetch_sub_u32(&segP->maxMsgNum, MSGNUMWRAPAROUND);
for (i = 0; i < segP->numProcs; i++)
segP->procState[segP->pgprocnos[i]].nextMsgNum -= MSGNUMWRAPAROUND;
}
@@ -657,7 +640,7 @@ SICleanupQueue(bool callerHasWriteLock, int minFree)
* Determine how many messages are still in the queue, and set the
* threshold at which we should repeat SICleanupQueue().
*/
- numMsgs = segP->maxMsgNum - segP->minMsgNum;+ numMsgs = pg_atomic_read_u32(&segP->maxMsgNum) - segP->minMsgNum;
if (numMsgs < CLEANUP_MIN)
segP->nextThreshold = CLEANUP_MIN;
else
--
2.55.0
--ARdL8blrvHVCiCuh--
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Cc: nathan@postgresql.org
Subject: Re: [PATCH v4 2/2] Convert SISeg->maxMsgNum to an atomic variable.
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