Re: [PATCH net v3] net: stmmac: protect updates of 64-bit statistics counters

[Eric Dumazet]

On Wed, Mar 6, 2024 at 11:03 AM Petr Tesařík <petr@xxxxxxxxxxx> wrote:
>
> On Wed, 6 Mar 2024 10:01:53 +0100
> Petr Tesařík <petr@xxxxxxxxxxx> wrote:
>
> > On Wed, 6 Mar 2024 09:23:53 +0100
> > "Linux regression tracking (Thorsten Leemhuis)" <regressions@xxxxxxxxxxxxx> wrote:
> >
> > > On 28.02.24 12:03, Petr Tesařík wrote:
> > > > On Wed, 28 Feb 2024 07:19:56 +0100
> > > > "Linux regression tracking (Thorsten Leemhuis)" <regressions@xxxxxxxxxxxxx> wrote:
> > > >
> > > >> Net maintainers, chiming in here, as it seems handling this regression
> > > >> stalled.
> > > > Indeed, I was too busy with sandbox mode...
> > >
> > > Hmm, no reply in the past week to Petr's request for help from someone
> > > with more knowledge about the field. :-/
> > >
> > > So I guess this means that this won't be fixed for 6.8? Unfortunate, but
> > > well, that's how it it sometimes.
> >
> > For the record, I _can_ reproduce lockdep splats on my device, but they
> > don't make any sense to me. They seem to confirm Jisheng Zhang's
> > conclusion that lockdep conflates two locks which should have different
> > lock-classes.
> >
> > So far I have noticed only one issue: the per-cpu syncp's are not
> > initialized. I'll recompile and see if that's what confuses lockdep.
>
> That wasn't the issue. FTR the syncp was in fact initialized, because
> devm_netdev_alloc_pcpu_stats() is a macro that also takes care of the
> initialization of the syncp struct field.
>
> The problem is u64_stats_init().
>
> Commit 9464ca650008 ("net: make u64_stats_init() a function") changed
> it to an inline function. But that's wrong. It uses seqcount_init(),
> which in turn declares:
>
>         static struct lock_class_key __key;
>
> This assumes that each lock gets its own instance. But if
> u64_stats_init() is a function (albeit an inline one), all calls
> within the same file end up using the same instance.
>
> Eric, would it be OK to revert the above-mentioned commit?

Oh, nice !

Well, this would not be a revert, let's keep type safety checks if possible.

Thanks.