Re: Lock on ShmemVariableCache fields?

HI,

On Oct 31, 2022, 10:48 +0800, Japin Li <japinli(at)hotmail(dot)com>, wrote:
>
> Hi, hackers
>
> The VariableCacheData says nextOid and oidCount are protected by
> OidGenLock. However, we update them without holding the lock on
> OidGenLock in BootStrapXLOG(). Same as nextXid, for other fields
> that are protected by XidGenLock, it holds the lock, see
> SetTransactionIdLimit().
>
> void
> BootStrapXLOG(void)
> {
> [...]
>
> ShmemVariableCache->nextXid = checkPoint.nextXid;
> ShmemVariableCache->nextOid = checkPoint.nextOid;
> ShmemVariableCache->oidCount = 0;
>
> [...]
>
> SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
>
> [...]
> }
>
> I also find a similar code in StartupXLOG(). Why we don't hold the lock
> on OidGenLock when updating ShmemVariableCache->nextOid and
> ShmemVariableCache->oidCount?
>
> If the lock is unnecessary, I think adding some comments is better.
>
> --
> Regrads,
> Japin Li.
> ChengDu WenWu Information Technology Co.,Ltd.
>
>
As its name BootStrapXLOG, it’s used in BootStrap mode to initialize the template database.
The process doesn’t speak SQL and the database is not ready.
There won’t be  concurrent access to variables.

Regards,
Zhang Mingli
>