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Re: [PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 2 Mar 2021 11:57:46 +0100
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On Tue, Mar 02, 2021 at 10:59:37AM +0100, Jan Beulich wrote:
> On 01.03.2021 19:00, Roger Pau Monné wrote:
> > On Mon, Mar 01, 2021 at 04:14:26PM +0100, Jan Beulich wrote:
> >> On 01.03.2021 15:01, Roger Pau Monné wrote:
> >>> On Mon, Mar 01, 2021 at 09:31:07AM +0100, Jan Beulich wrote:
> >>>> In this case the compiler is recognizing that no valid array indexes
> >>>> remain (in x2apic_cluster()'s access to per_cpu(cpu_2_logical_apicid,
> >>>> ...)), but oddly enough isn't really consistent about the checking it
> >>>> does (see the code comment).
> >>>
> >>> I assume this is because of the underlying per_cpu access to
> >>> __per_cpu_offset using cpu as the index, in which case wouldn't it be
> >>> better to place the BUG_ON there?
> >>
> >> Not sure, to be honest. It seemed more logical to me to place it
> >> next to where the issue is.
> >
> > I'm not sure whether I would prefer to place it in per_cpu directly to
> > avoid similar issues popping up in other parts of the code in the
> > future?
>
> That's going to be a lot of BUG_ON(), and hence a lot of "ud2"
> instances. Not something I'm keen on having. The more that it's
> not the per_cpu() which triggers the warning, but separate
> conditionals allowing the compiler to deduce value ranges of
> variables.
Right. I don't see much other way around this then. Did you check with
clang also?
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks, Roger.
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