From: "Pengchengliu" <pengchengliu@tju.edu.cn>
To: "'Greg Nancarrow'" <gregn4422@gmail.com>
Cc: "'Andres Freund'" <andres@anarazel.de>,
	"'PostgreSQL-development'" <pgsql-hackers@postgresql.org>
References: 
 <CAJcOf-cJfLcgxwz8TeTZqWe3z2XdLkZAT8gHTwgHqHP9-Cb-Mw@mail.gmail.com>
 <AMkAGwA2DuDLiuCUBBPk1KoS.3.1621046249310.Hmail.2008202116@tju.edu.cn>
 <CAJcOf-eDcGVUx0+HjeYzj73AK+x2OOLEQLcfPn4ThEG9pwZpSw@mail.gmail.com>
In-Reply-To: 
 <CAJcOf-eDcGVUx0+HjeYzj73AK+x2OOLEQLcfPn4ThEG9pwZpSw@mail.gmail.com>
Subject: RE: Re: Parallel scan with SubTransGetTopmostTransaction assert
 coredump
Date: Tue, 18 May 2021 09:27:33 +0800
Message-ID: <000901d74b84$fa262890$ee7279b0$@tju.edu.cn>
MIME-Version: 1.0
Content-Type: text/plain;
	charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Thread-Index: AQKMVnF2uVfQC+5xVzv7kF3C/yFluQK3oWxFAlVwYVGpVexukA==
Content-Language: zh-cn
Archived-At: 
 <https://www.postgresql.org/message-id/000901d74b84%24fa262890%24ee7279b0%24%40tju.edu.cn>
Precedence: bulk

Hi Greg,
  > I actually think that the Assert in SubTransGetTopmostTransaction() =
is correct, but in the parallel-worker case, the snapshots are not being =
setup correctly.
 =20
   I agree with you that Assert in SubTransGetTopmostTransaction() is =
correct.  The root cause is Transaction Xmin are not being setup =
correctly in the parallel-worker.
  =20
   Actually I am very confused about ActiveSnapshot and =
TransactionSnapshot. I don't know why main process send ActiveSnapshot =
and TransactionSnapshot separately.  And what is exact difference =
between them?
   If you know that, could you explain that for me? It will be very =
appreciated.=20
   Before we know them exactly, I think we should not modify the =
TransactionSnapshot to ActiveSnapshot in main process. If it is, the =
main process should send ActiveSnapshot only. =20

Thanks
Pengcheng
  =20
  =20

-----Original Message-----
From: Greg Nancarrow <gregn4422@gmail.com>=20
Sent: 2021=E5=B9=B45=E6=9C=8817=E6=97=A5 20:59
To: =E5=88=98=E9=B9=8F=E7=A8=8B <pengchengliu@tju.edu.cn>
Cc: Andres Freund <andres@anarazel.de>; PostgreSQL-development =
<pgsql-hackers@postgresql.org>
Subject: Re: Re: Parallel scan with SubTransGetTopmostTransaction assert =
coredump

On Sat, May 15, 2021 at 12:37 PM =E5=88=98=E9=B9=8F=E7=A8=8B =
<pengchengliu@tju.edu.cn> wrote:
>
>
>   BTW, I test it in a high performance server. It is verly easily be =
reproduced. My colleague and me use different environment both can =
reproduce it.
>

Hi Pengcheng,

Although the issue won't reproduce easily in my system, I can certainly =
see how, for the snapshots used in the parallel worker case, the Active =
snapshot used is potentially an earlier snapshot that the Transaction =
snapshot. I don't know why it is getting a newer Transaction snapshot in =
InitializeParallelDSM(), when it has previously pushed the return value =
of GetTransactionSnapshot() as the Active snapshot.

So I too hope Tom or Robert can explain what is going on here and how to =
resolve it (as you requested them to, in your other post).

I actually think that the Assert in SubTransGetTopmostTransaction() is =
correct, but in the parallel-worker case, the snapshots are not being =
setup correctly.

Can you try the trivial change below and see if it prevents the =
coredump?

Regards,
Greg Nancarrow
Fujitsu Australia


diff --git a/src/backend/access/transam/parallel.c
b/src/backend/access/transam/parallel.c
index 14a8690019..870889053f 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -216,7 +216,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
  int i;
  FixedParallelState *fps;
  dsm_handle session_dsm_handle =3D DSM_HANDLE_INVALID;
- Snapshot transaction_snapshot =3D GetTransactionSnapshot();
+ Snapshot transaction_snapshot =3D GetActiveSnapshot();
  Snapshot active_snapshot =3D GetActiveSnapshot();

  /* We might be running in a very short-lived memory context. */