From: Michael Paquier <michael@paquier.xyz>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Julien Rouhaud <rjuju123@gmail.com>
Cc: Sergei Kornilov <sk@zsrv.org>
Cc: Guillaume Lelarge <guillaume@lelarge.info>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Expose lock group leader pid in pg_stat_activity
Date: Mon, 16 Mar 2020 15:02:31 +0900
Message-ID: <20200316060231.GD2331@paquier.xyz> (raw)
In-Reply-To: <20200316054341.GG26184@telsasoft.com>
References: <20200116074912.GA98418@paquier.xyz>
<CAOBaU_Z6WcT8FJfcCMU_Bp+PTzUpTEY9BfBQE4gbGKpE5wpfwQ@mail.gmail.com>
<20200316042752.GE26184@telsasoft.com>
<20200316054341.GG26184@telsasoft.com>
On Mon, Mar 16, 2020 at 12:43:41AM -0500, Justin Pryzby wrote:
> I think I see. Julien's v3 patch did this:
> https://www.postgresql.org/message-id/attachment/106429/pgsa_leader_pid-v3.diff
> + if (proc->lockGroupLeader)
> + values[29] = Int32GetDatum(proc->lockGroupLeader->pid);
>
> ..which is racy because a proc with a leader might die and be replaced by
> another proc without a leader between 1 and 2.
>
> But the code since v4 does:
>
> + leader = proc->lockGroupLeader;
> + if (leader)
> + values[29] = Int32GetDatum(leader->pid);
>
> ..which is safe because PROCs are allocated in shared memory, so leader is for
> sure a non-NULL pointer to a PROC. leader->pid may be read inconsistently,
> which is what the comment says: "no extra lock is being held".
Yes, you have the correct answer here. As shaped, the code relies on
the state of a PGPROC entry in shared memory.
--
Michael
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To: pgsql-hackers@postgresql.org
Cc: michael@paquier.xyz, pryzby@telsasoft.com, rjuju123@gmail.com, sk@zsrv.org, guillaume@lelarge.info, pgsql-hackers@lists.postgresql.org
Subject: Re: Expose lock group leader pid in pg_stat_activity
In-Reply-To: <20200316060231.GD2331@paquier.xyz>
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