From: Chao Li <li.evan.chao@gmail.com>
Message-Id: <52CC167F-763B-4ECA-B0B4-DAB381816828@gmail.com>
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Subject: Re: Optimize LISTEN/NOTIFY
Date: Fri, 26 Sep 2025 10:26:37 +0800
In-Reply-To: <baf71275-9083-41be-ae13-f80821541a1a@app.fastmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 Thomas Munro <thomas.munro@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>,
 Heikki Linnakangas <hlinnaka@iki.fi>,
 Rishu Bagga <rishu.postgres@gmail.com>
To: Joel Jacobson <joel@compiler.org>
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> On Sep 26, 2025, at 05:13, Joel Jacobson <joel@compiler.org> wrote:
>=20
> Hmm, I don't see how duplicate timeout could happen?
>=20
> Once we decide to defer the wakeup, wakeup_pending_flag remains set,
> which avoids further signals from notifiers, so I don't see how we =
could
> re-enter ProcessIncomingNotify(), since notifyInterruptPending is =
reset
> when ProcessIncomingNotify() is called, and notifyInterruptPending is
> only set when a signal is received (or set directly when in same
> process).
>=20


I think what you explained is partially correct.

Based on my understanding, any backend process may call =
SignalBackends(), which means that it=E2=80=99s possible that multiple =
backend processes may call SignalBackends() concurrently.

Looking at your code, between checking =
QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) and set the flag to true, there is =
a block of code (the =E2=80=9Cif-else=E2=80=9D) to run, so that it=E2=80=99=
s possible that multiple backend processes have passed the =
QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) check, then multiple signals will =
be sent to a process, which will lead to duplicate timeout enabled in =
the receiver process.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/


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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; =
charset=3Dutf-8"></head><body style=3D"overflow-wrap: break-word; =
-webkit-nbsp-mode: space; line-break: after-white-space;"><br =
id=3D"lineBreakAtBeginningOfMessage"><div><br><blockquote =
type=3D"cite"><div>On Sep 26, 2025, at 05:13, Joel Jacobson =
&lt;joel@compiler.org&gt; wrote:</div><div><div><br>Hmm, I don't see how =
duplicate timeout could happen?<br><br>Once we decide to defer the =
wakeup, wakeup_pending_flag remains set,<br>which avoids further signals =
from notifiers, so I don't see how we could<br>re-enter =
ProcessIncomingNotify(), since notifyInterruptPending is reset<br>when =
ProcessIncomingNotify() is called, and notifyInterruptPending is<br>only =
set when a signal is received (or set directly when in =
same<br>process).<br><br></div></div></blockquote></div><div><br></div><di=
v>I think what you explained is partially =
correct.</div><div><br></div><div>Based on my understanding, any backend =
process may call SignalBackends(), which means that it=E2=80=99s =
possible that multiple backend processes may call SignalBackends() =
concurrently.</div><div><br></div><div>Looking at your code, between =
checking QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) and set the flag to true, =
there is a block of code (the =E2=80=9Cif-else=E2=80=9D) to run, so that =
it=E2=80=99s possible that multiple backend processes have passed the =
QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) check, then multiple signals will =
be sent to a process, which will lead to duplicate timeout enabled in =
the receiver process.</div><div><br></div><div>Best regards,</div><div>
<div dir=3D"auto" style=3D"caret-color: rgb(0, 0, 0); color: rgb(0, 0, =
0); letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; text-decoration: none; overflow-wrap: =
break-word; -webkit-nbsp-mode: space; line-break: =
after-white-space;"><div>--</div><div>Chao Li (Evan)<br>HighGo Software =
Co., Ltd.<br>https://www.highgo.com/</div><div><br></div></div><br =
class=3D"Apple-interchange-newline"><br =
class=3D"Apple-interchange-newline">

</div>
<br></body></html>=

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