postgres.git / summary / log / commit / refs
commit df3978c2340d79c6319f8239a69abbd27b1069df
Author: Nathan Bossart <nathan@postgresql.org>
Date: Thu Sep 24 19:47:20 2026 +0000
Convert SISeg->maxMsgNum to an atomic variable.
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 | 54 ++++++++++++++-----------------------
1 file changed, 20 insertions(+), 34 deletions(-)
diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.c
index 446a2003999..16880e7cc4a 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 +86,13 @@
* 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.) The
+ * barriers ensure that messages written to the array are actually there
+ * before maxMsgNum is increased, and that readers will see that data after
+ * fetching maxMsgNum.
*/
@@ -169,11 +162,9 @@ 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 +235,10 @@ 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 +296,7 @@ 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 +392,7 @@ 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 +403,15 @@ 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 +497,8 @@ 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)
{
@@ -598,7 +584,7 @@ SICleanupQueue(bool callerHasWriteLock, int minFree)
* backends here it is possible for them to keep sending messages without
* a problem even when they are the only active backend.
*/
- min = segP->maxMsgNum;
+ min = pg_atomic_read_u32(&segP->maxMsgNum);
/* clamp at zero to avoid underflow */
if (min > SIG_THRESHOLD)
@@ -654,7 +640,7 @@ 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;
}
@@ -663,7 +649,7 @@ 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
[parent: 25217d91b1a2]