postgres.git / summary / log / commit / refs

commit    09a579abaca86bcd85ebc49b247d2ec58bdc6d3b
Author:   Nathan Bossart <nathan@postgresql.org>
Date:     Tue Sep 22 19:23:04 2026 +0000

    Convert SharedFileSet->refcnt to an atomic variable.
    
    Currently, this variable is an int protected by a spinlock.  By
    converting it to an atomic variable, we can remove the spinlock.
    Detaching becomes an atomic subtract, and attaching becomes a
    compare-and-exchange loop, since it must not resurrect a fileset
    whose reference count has already reached zero.
    
    Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
    Tested-by: solai v <solai.cdac@gmail.com>
    Discussion: https://postgr.es/m/alAJeRRzehDjLaF1%40nathan


 src/backend/storage/file/sharedfileset.c | 33 ++++++++++++++------------------
 src/include/storage/sharedfileset.h      |  5 ++---
 2 files changed, 16 insertions(+), 22 deletions(-)

diff --git a/src/backend/storage/file/sharedfileset.c b/src/backend/storage/file/sharedfileset.c
index d76bd72dc63..451916fc41c 100644
--- a/src/backend/storage/file/sharedfileset.c
+++ b/src/backend/storage/file/sharedfileset.c
@@ -38,8 +38,7 @@ void
 SharedFileSetInit(SharedFileSet *fileset, dsm_segment *seg)
 {
 	/* Initialize the shared fileset specific members. */
-	SpinLockInit(&fileset->mutex);
-	fileset->refcnt = 1;
+	pg_atomic_init_u32(&fileset->refcnt, 1);
 
 	/* Initialize the fileset. */
 	FileSetInit(&fileset->fs);
@@ -55,22 +54,20 @@ SharedFileSetInit(SharedFileSet *fileset, dsm_segment *seg)
 void
 SharedFileSetAttach(SharedFileSet *fileset, dsm_segment *seg)
 {
-	bool		success;
+	uint32		refcnt;
 
-	SpinLockAcquire(&fileset->mutex);
-	if (fileset->refcnt == 0)
-		success = false;
-	else
+	refcnt = pg_atomic_read_u32(&fileset->refcnt);
+	while (true)
 	{
-		++fileset->refcnt;
-		success = true;
-	}
-	SpinLockRelease(&fileset->mutex);
+		if (refcnt == 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("could not attach to a SharedFileSet that is already destroyed")));
 
-	if (!success)
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("could not attach to a SharedFileSet that is already destroyed")));
+		if (pg_atomic_compare_exchange_u32(&fileset->refcnt, &refcnt,
+										   refcnt + 1))
+			break;
+	}
 
 	/* Register our cleanup callback. */
 	on_dsm_detach(seg, SharedFileSetOnDetach, PointerGetDatum(fileset));
@@ -98,11 +95,9 @@ SharedFileSetOnDetach(dsm_segment *segment, Datum datum)
 	bool		unlink_all = false;
 	SharedFileSet *fileset = (SharedFileSet *) DatumGetPointer(datum);
 
-	SpinLockAcquire(&fileset->mutex);
-	Assert(fileset->refcnt > 0);
-	if (--fileset->refcnt == 0)
+	Assert(pg_atomic_read_u32(&fileset->refcnt) > 0);
+	if (pg_atomic_sub_fetch_u32(&fileset->refcnt, 1) == 0)
 		unlink_all = true;
-	SpinLockRelease(&fileset->mutex);
 
 	/*
 	 * If we are the last to detach, we delete the directory in all
diff --git a/src/include/storage/sharedfileset.h b/src/include/storage/sharedfileset.h
index 904396e7173..d89626ae64b 100644
--- a/src/include/storage/sharedfileset.h
+++ b/src/include/storage/sharedfileset.h
@@ -15,10 +15,10 @@
 #ifndef SHAREDFILESET_H
 #define SHAREDFILESET_H
 
+#include "port/atomics.h"
 #include "storage/dsm.h"
 #include "storage/fd.h"
 #include "storage/fileset.h"
-#include "storage/spin.h"
 
 /*
  * A set of temporary files that can be shared by multiple backends.
@@ -26,8 +26,7 @@
 typedef struct SharedFileSet
 {
 	FileSet		fs;
-	slock_t		mutex;			/* mutex protecting the reference count */
-	int			refcnt;			/* number of attached backends */
+	pg_atomic_uint32 refcnt;	/* number of attached backends */
 } SharedFileSet;
 
 extern void SharedFileSetInit(SharedFileSet *fileset, dsm_segment *seg);