netdev - Re: [PATCH] wireguard: queueing: Fix implicit type conversion

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Date:   Fri, 29 Oct 2021 08:30:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>, jiasheng@...as.ac.cn
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] wireguard: queueing: Fix implicit type conversion



On 10/29/21 7:27 AM, Jason A. Donenfeld wrote:
> On Fri, Oct 29, 2021 at 3:08 AM Jiasheng Jiang <jiasheng@...as.ac.cn> wrote:
>> It is universally accepted that the implicit type conversion is
>> terrible.
> 
> I'm not so sure about this, but either way, I think this needs a bit
> more justification and analysis to merge. cpumask_weight returns an
> unsigned, for example, and is used as a modulo operand later in the
> function. It looks like nr_cpumask_bits is also unsigned. And so on.
> So you're really trading one implicit type conversion package for
> another. If you're swapping these around, why? It can't be because,
> "it is universally accepted that the implicit type conversion is
> terrible," since you're adding more of it in a different form. Is your
> set of implicit type conversions semantically more proper? If so,
> please describe that. Alternatively, is there a way to harmonize
> everything into one type? Is there a minimal set of casts that enables
> that?
>

I agree with you.

Even standard iterators play/mix with signed/unsigned in plain sight.

extern unsigned int nr_cpu_ids;

unsigned int cpumask_next(int n, const struct cpumask *srcp);

int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool wrap);

#define for_each_cpu(cpu, mask)				\
	for ((cpu) = -1;				\
		(cpu) = cpumask_next((cpu), (mask)),	\
		(cpu) < nr_cpu_ids;)