Feedback-ID: i0fe9450f:Fastmail
Date: Wed, 17 Sep 2025 11:35:56 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Nathan Bossart <nathan@postgresql.org>
Cc: pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Move named LWLock tranche requests to shared memory.
Message-ID: <aMoejB3iTWy1SxfF@paquier.xyz>
References: <E1uwodL-000EuJ-2j@gemulon.postgresql.org>
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Ni Nathan,

On Thu, Sep 11, 2025 at 09:15:12PM +0000, Nathan Bossart wrote:
> Move named LWLock tranche requests to shared memory.
>=20
> In EXEC_BACKEND builds, GetNamedLWLockTranche() can segfault when
> called outside of the postmaster process, as it might access
> NamedLWLockTrancheRequestArray, which won't be initialized.  Given
> the lack of reports, this is apparently unusual, presumably because
> it is usually called from a shmem_startup_hook like this:

Since this commit has been merged, batta has kept failing.  Here is
the first failure:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=3Dbatta&dt=3D2025-0=
9-12%2002%3A05%3A01

I use this animal with a specific configuration:
shared_preload_libraries =3D 'pg_stat_statements'
compute_query_id =3D regress
regress_dump_restore
wal_consistency_checking
--enable-injection-points

The recovery tests 013_crash_restart.pl, 022_crash_temp_files.pl and
041_checkpoint_at_promote.pl stress some restart scenarios, not all
use injection points.  I could not get a backtrace from the host.

However, I have come up with the following change in 013 that's able
to reproduce what I think is the same crash:
--- a/src/test/recovery/t/013_crash_restart.pl
+++ b/src/test/recovery/t/013_crash_restart.pl
@@ -21,6 +21,8 @@ my $psql_timeout =3D IPC::Run::timer($PostgreSQL::Test::U=
tils::timeout_default);
=20
 my $node =3D PostgreSQL::Test::Cluster->new('primary');
 $node->init(allows_streaming =3D> 1);
+$node->append_conf('postgresql.conf',
+                   "shared_preload_libraries =3D 'pg_stat_statements'");
 $node->start();
=20
And here is the backtrace:
#0  0x000055fcdf6bc97a in NumLWLocksForNamedTranches () at lwlock.c:385
385 numLocks +=3D NamedLWLockTrancheRequestArray[i].num_lwlocks;
(gdb) bt
#0  0x000055fcdf6bc97a in NumLWLocksForNamedTranches () at lwlock.c:385=20
#1  0x000055fcdf6bc9b3 in LWLockShmemSize () at lwlock.c:400=20
#2  0x000055fcdf65bda5 in CalculateShmemSize (num_semaphores=3D0x7ffcaf7a78=
e4) at ipci.c:130=20
#3  0x000055fcdf65c0b1 in CreateSharedMemoryAndSemaphores () at ipci.c:210=
=20
#4  0x000055fcdf42830c in PostmasterStateMachine () at postmaster.c:3223=20
#5  0x000055fcdf42703f in process_pm_child_exit () at postmaster.c:2558=20
#6  0x000055fcdf425729 in ServerLoop () at postmaster.c:1696=20
#7  0x000055fcdf424be1 in PostmasterMain (argc=3D4, argv=3D0x55fd0a8faa10) =
at postmaster.c:1403=20
#8  0x000055fcdef80a19 in main (argc=3D4, argv=3D0x55fd0a8faa10) at main.c:=
231
(gdb) p i
$3 =3D 0
(gdb) p NamedLWLockTrancheRequestArray[0]
Cannot access memory at address 0x7f15ee4ccc08

Thanks,
--
Michael

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