linux-kernel - Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

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Date:   Thu, 3 Nov 2022 16:34:04 +0100
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Yury Norov <yury.norov@...il.com>, x86@...nel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        openrisc@...ts.librecores.org,
        Michael Ellerman <mpe@...erman.id.au>,
        "open list:LINUX FOR POWERPC PA SEMI PWRFICIENT" 
        <linuxppc-dev@...ts.ozlabs.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

On Thu, Nov 03, 2022 at 04:02:12PM +0100, Borislav Petkov wrote:
> On Thu, Nov 03, 2022 at 01:59:45PM +0100, Andrew Jones wrote:
> > The patch I'm proposing ensures cpumask_next()'s range, which is actually
> > [-1, nr_cpus_ids - 1),
> 
> Lemme make sure I understand it correctly: on the upper boundary, if you
> supply for n the value nr_cpu_ids - 2, then it will return potentially
> the last bit if the mask is set, i.e., the one at position (nr_cpu_ids - 1).
> 
> If you supply nr_cpus_ids - 1, then it'll return nr_cpu_ids to signal no
> further bits set.
> 
> Yes, no?

Yes

> 
> > I'll send a v4 with another stab at the commit message.
> 
> Yes, and it is still an unreadable mess: "A kernel compiled with commit
> ... but not its revert... " Nope.
> 
> First make sure cpumask_next()'s valid accepted range has been settled
> upon, has been explicitly documented in a comment above it and then I'll
> take a patch that fixes whatever is there to fix.

That's fair, but I'll leave that to Yury.

> 
> Callers should not have to filter values before passing them in - the
> function either returns an error or returns the next bit in the mask.

That's reasonable, but cpumask folk probably need to discuss it because
not all cpumask functions have a return value where an error may be
placed.

> 
> This thing:
> 
> 	if (*pos == nr_cpu_ids)
> 
> but then to pass in pos - 1:
> 
> 	*pos = cpumask_next(*pos - 1
> 
> looks to me like the interface needs more cooking.

Indeed, but that's less of an issue with cpumask_next() than with
the way cpuinfo implements its start and next seq ops (next
unconditionally increments *pos and then calls start and start
must use *pos - 1 since the first time its called it needs to use
-1).

Thanks,
drew