Re: [PATCH v2 2/3] nfsd: protect concurrent access to nfsd stats counters

J . Bruce Fields
2021-01-07T14:26:12Z

On Thu, Jan 07, 2021 at 09:17:42AM +0200, Amir Goldstein wrote:
> On Wed, Jan 6, 2021 at 10:50 PM J . Bruce Fields <bfields@fieldses.org> wrote:
> >
> > These three patches seem fine. To bikeshed just a bit more:
> >
> > On Wed, Jan 06, 2021 at 09:52:35AM +0200, Amir Goldstein wrote:
> > > /* We found a matching entry which is either in progress or done. */
> > > - nfsdstats.rchits++;
> > > + nfsd_stats_rc_hits_inc();
> >
> > Maybe make that something like
> >
> > nfsd_stats_inc(NFSD_STATS_RC_HITS);
> >
> > and then we could avoid boilerplate like the below?
> >
>
> It looks like boilerplate, but every helper is a bit different,
> so we would have to introduce several variants like:
>
> nfsd_net_stats_inc(nn, NFSD_NET_PAYLOAD_MISSES);
>
> Which is the way I started, but realized it opens a big hole for
> human mistakes in the form of:
>
> nfsd_net_stats_inc(nn, NFSD_STATS_RC_HITS);
> nfsd_stats_inc(NFSD_NET_PAYLOAD_MISSES);
>
> And we have no way to detect those errors at compile time
> because enum types are not strict types.
>
> Also, in the next patch, three more helpers become unique
> as they are made into helpers to account for both global and per-export
> stats (fh_stale, io_read, io_write).
> Making those helpers generic would require aligning the start of global
> stats enum values with the per-export enum values and that is a source
> of more human errors.
>
> See the end result of stats.h. All the helpers are unique and the only ones
> that remain generic are nfsd_stats_rc_*.
> Making those generic would also require checking the range of the index
> value is in the NFSD_STATS_RC_* range.
>
> I also tried a version with boilerplate defining macros, i.e.:
>
> NFSD_STATS_FUNCS(rc_hits, RC_HITS)
> NFSD_STATS_FUNCS(fh_stale, FH_STALE)
> NFSD_STATS_FUNCS(io_read, IO_READ)
>
> But when I realized in the end it can only serve the rc_* counters,
> I dropped it.

Yeah, OK. I'm not a fan of that kind of macro use, either. It's made
me waste time when "grep" doesn't turn up a function definition, for
example.

> Thoughts?

I've got no clever alternative, and you've tried a lot of alternatives,
so I'll trust your judgement. Thanks for the explanation.

--b.