Re: [CLOBBER_CACHE]Server crashed with segfault 11 while executing clusterdb

On Tue, Jul 6, 2021 at 11:06 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Amul Sul <sulamul(at)gmail(dot)com> writes:
> > On Tue, Apr 20, 2021 at 6:59 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> >> I don't mind RelationGetSmgr(index)->smgr_rnode alone or
> >> &variable->member alone and there's not the previous call to
> >> RelationGetSmgr just above. How about using a temporary variable?
> >>
> >> SMgrRelation srel = RelationGetSmgr(index);
> >> smgrwrite(srel, ...);
> >> log_newpage(srel->..);
>
> > Understood. Used a temporary variable for the place where
> > RelationGetSmgr() calls are placed too close or in a loop.
>
> [ squint... ] Doesn't this risk introducing exactly the sort of
> cache-clobber hazard we're trying to prevent? That is, the above is
> not safe unless you are *entirely* certain that there is not and never
> will be any possibility of a relcache flush before you are done using
> the temporary variable. Otherwise it can become a dangling pointer.
>

Yeah, there will a hazard, even if we sure right but cannot guarantee future
changes in any subroutine that could get call in between.

> The point of the static-inline function idea was to be cheap enough
> that it isn't worth worrying about this sort of risky optimization.
> Given that an smgr function is sure to involve some kernel calls,
> I doubt it's worth sweating over an extra test-and-branch beforehand.
> So where I was hoping to get to is that smgr objects are *only*
> referenced by RelationGetSmgr() calls and nobody ever keeps any
> other pointers to them across any non-smgr operations.
>

Ok, will revert changes added in the previous version, thanks.

Regards,
Amul