use LW_SHARED in dsa_get_total_size()
On Tue, Dec 02, 2025 at 10:31:33AM -0600, Nathan Bossart wrote:
On Tue, Dec 02, 2025 at 02:28:29PM +0530, Rahila Syed wrote:
Thank you for highlighting the discussions. I'm unsure about the best
approach here, but I think it would be safe to stay consistent with the
rest of the code in dsa.c, especially since it's unclear that the use of
LW_EXCLUSIVE for reading values in dsa is a mistake.Okay. I switched to LW_EXCLUSIVE and will consider starting a new thread
to use LW_SHARED when possible in dsa.c.
Here is a patch that switches to LW_SHARED in dsa_get_total_size() and
dsa_get_total_size_from_handle(). Another candidate was dsa_dump(), but
that function appears to do some things that require more than a shared
lock. In any case, it's just a debugging function, and I found no uses
in-tree or elsewhere.
--
nathan
Attachments:
v1-0001-use-LW_SHARED-in-dsa.c-when-appropriate.patchtext/plain; charset=us-asciiDownload
From f25ccc54e25f6db9b7b07b0b1ca771f7c758620f Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 2 Dec 2025 11:16:17 -0600
Subject: [PATCH v1 1/1] use LW_SHARED in dsa.c when appropriate
---
src/backend/utils/mmgr/dsa.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/src/backend/utils/mmgr/dsa.c b/src/backend/utils/mmgr/dsa.c
index c8a72686177..4b6bcffea28 100644
--- a/src/backend/utils/mmgr/dsa.c
+++ b/src/backend/utils/mmgr/dsa.c
@@ -1043,7 +1043,7 @@ dsa_get_total_size(dsa_area *area)
{
size_t size;
- LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
+ LWLockAcquire(DSA_AREA_LOCK(area), LW_SHARED);
size = area->control->total_segment_size;
LWLockRelease(DSA_AREA_LOCK(area));
@@ -1075,7 +1075,7 @@ dsa_get_total_size_from_handle(dsa_handle handle)
control = (dsa_area_control *) dsm_segment_address(segment);
- LWLockAcquire(&control->lock, LW_EXCLUSIVE);
+ LWLockAcquire(&control->lock, LW_SHARED);
size = control->total_segment_size;
LWLockRelease(&control->lock);
--
2.39.5 (Apple Git-154)
On Tue, Dec 02, 2025 at 10:31:33AM -0600, Nathan Bossart wrote:
On Tue, Dec 02, 2025 at 02:28:29PM +0530, Rahila Syed wrote:
Thank you for highlighting the discussions. I'm unsure about the best
approach here, but I think it would be safe to stay consistent with the
rest of the code in dsa.c, especially since it's unclear that the use of
LW_EXCLUSIVE for reading values in dsa is a mistake.Okay. I switched to LW_EXCLUSIVE and will consider starting a new thread
to use LW_SHARED when possible in dsa.c.Here is a patch that switches to LW_SHARED in dsa_get_total_size() and
dsa_get_total_size_from_handle().
I think this is a good idea. Exclusive here is not necessarily for the
size lookups.
A specific case I can think of is the work I have been investigating to move
pg_stat_statements query text to shared memory, concurrent backends may
need to check the size of the DSA concurrently, and this will prevent backends
from being blocked when doing so.
If it turns out a backend needs to modify the DSA, they will require
an Exclusive
lock, but that will be after the lookup.
--
Sami Imseih
Amazon Web Services (AWS)