public inbox for [email protected]
help / color / mirror / Atom feedFrom: Dilip Kumar <[email protected]>
To: Alvaro Herrera <[email protected]>
Cc: [email protected]
Subject: Re: pgsql: Improve performance of subsystems on top of SLRU
Date: Mon, 4 Mar 2024 11:44:48 +0530
Message-ID: <CAFiTN-sAPL1iCBfUOj1VimVBngO_0HyDNEgShWKL_-6RqbNDLg@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
<[email protected]>
On Mon, Mar 4, 2024 at 1:56 AM Alvaro Herrera <[email protected]> wrote:
>
> On 2024-Feb-28, Alvaro Herrera wrote:
>
> > Improve performance of subsystems on top of SLRU
>
> Coverity had the following complaint about this commit:
>
> ________________________________________________________________________________________________________
> *** CID NNNNNNN: Control flow issues (DEADCODE)
> /srv/coverity/git/pgsql-git/postgresql/src/backend/access/transam/multixact.c: 1375 in GetMultiXactIdMembers()
> 1369 * and acquire the lock of the new bank.
> 1370 */
> 1371 lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
> 1372 if (lock != prevlock)
> 1373 {
> 1374 if (prevlock != NULL)
> >>> CID 1592913: Control flow issues (DEADCODE)
> >>> Execution cannot reach this statement: "LWLockRelease(prevlock);".
> 1375 LWLockRelease(prevlock);
> 1376 LWLockAcquire(lock, LW_EXCLUSIVE);
> 1377 prevlock = lock;
> 1378 }
> 1379
> 1380 slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
>
> And I think it's correct that this is somewhat bogus, or at least
> confusing: the only way to have control back here on line 1371 after
> having executed once is via the "goto retry" line below; and there we
> release "prevlock" and set it to NULL beforehand, so it's impossible for
> prevlock to be NULL. Looking closer I think this code is all confused,
> so I suggest to rework it as shown in the attached patch.
>
> I'll have a look at the other places where we use this "prevlock" coding
> pattern tomorrow.
+ /* Acquire the bank lock for the page we need. */
lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
- if (lock != prevlock)
- {
- if (prevlock != NULL)
- LWLockRelease(prevlock);
- LWLockAcquire(lock, LW_EXCLUSIVE);
- prevlock = lock;
- }
+ LWLockAcquire(lock, LW_EXCLUSIVE);
This part is definitely an improvement.
I am not sure about the other changes, I mean that makes the code much
simpler but now we are not releasing the 'MultiXactOffsetCtl' related
bank lock, and later in the following loop, we are comparing that lock
against 'MultiXactMemberCtl' related bank lock. This makes code
simpler because now in the loop we are sure that we are always holding
the lock but I do not like comparing the bank locks for 2 different
SLRUs, although there is no problem as there would not be a common
lock address, anyway, I do not have any strong objection to what you
have done here.
--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com
view thread (6+ messages) latest in thread
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: [email protected]
Cc: [email protected], [email protected], [email protected]
Subject: Re: pgsql: Improve performance of subsystems on top of SLRU
In-Reply-To: <CAFiTN-sAPL1iCBfUOj1VimVBngO_0HyDNEgShWKL_-6RqbNDLg@mail.gmail.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox