From: Chao Li <li.evan.chao@gmail.com>
Message-Id: <9186C6D0-F7A9-482A-9183-89E530B57E36@gmail.com>
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Subject: Re: Optimize LISTEN/NOTIFY
Date: Fri, 26 Sep 2025 17:44:44 +0800
In-Reply-To: <d35a114d-204a-4b40-9284-06749532b724@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 17:32, Joel Jacobson <joel@compiler.org> wrote:
>=20
> On Fri, Sep 26, 2025, at 04:26, Chao Li wrote:
>=20
>> I think what you explained is partially correct.
>>=20
>> Based on my understanding, any backend process may call=20
>> SignalBackends(), which means that it=E2=80=99s possible that =
multiple backend=20
>> processes may call SignalBackends() concurrently.
>>=20
>> Looking at your code, between checking=20
>> QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) and set the flag to true, there =
is=20
>> a block of code (the =E2=80=9Cif-else=E2=80=9D) to run, so that =
it=E2=80=99s possible that=20
>> multiple backend processes have passed the=20
>> QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) check, then multiple signals =
will=20
>> be sent to a process, which will lead to duplicate timeout enabled in=20=

>> the receiver process.
>=20
> I don't see how that can happen; we're checking wakeup_pending_flag
> while holding an exclusive lock, so I don't see how multiple backend
> processes could be within the region where we check/set
> wakeup_pending_flag, at the same time?
>=20
> /Joel


I might miss the factor of holding an exclusive lock. I will revisit =
that part again.

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 17:32, Joel Jacobson =
&lt;joel@compiler.org&gt; wrote:</div><br =
class=3D"Apple-interchange-newline"><div><div>On Fri, Sep 26, 2025, at =
04:26, Chao Li wrote:<br><br><blockquote type=3D"cite">I think what you =
explained is partially correct.<br><br>Based on my understanding, any =
backend process may call <br>SignalBackends(), which means that it=E2=80=99=
s possible that multiple backend <br>processes may call SignalBackends() =
concurrently.<br><br>Looking at your code, between checking =
<br>QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) and set the flag to true, there =
is <br>a block of code (the =E2=80=9Cif-else=E2=80=9D) to run, so that =
it=E2=80=99s possible that <br>multiple backend processes have passed =
the <br>QUEUE_BACKEND_WAKEUP_PENDING_FLAG(i) check, then multiple =
signals will <br>be sent to a process, which will lead to duplicate =
timeout enabled in <br>the receiver process.<br></blockquote><br>I don't =
see how that can happen; we're checking wakeup_pending_flag<br>while =
holding an exclusive lock, so I don't see how multiple =
backend<br>processes could be within the region where we =
check/set<br>wakeup_pending_flag, at the same =
time?<br><br>/Joel<br></div></div></blockquote></div><div><br></div><div>I=
 might miss the factor of holding an exclusive lock. I will revisit that =
part again.</div><div><br></div>Best regards,<br><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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