Re: [PATCH 5/5] cpumask: fix comment of cpumask_xxx

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Vernon Yang <vernon2gm@gmail.com>
Cc: tytso@mit.edu, Jason@zx2c4.com, davem@davemloft.net,
	edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
	jejb@linux.ibm.com, martin.petersen@oracle.com,
	yury.norov@gmail.com, andriy.shevchenko@linux.intel.com,
	linux@rasmusvillemoes.dk, james.smart@broadcom.com,
	dick.kennedy@broadcom.com, linux-kernel@vger.kernel.org,
	wireguard@lists.zx2c4.com, netdev@vger.kernel.org,
	linux-scsi@vger.kernel.org
Subject: Re: [PATCH 5/5] cpumask: fix comment of cpumask_xxx
Date: Mon, 6 Mar 2023 09:47:34 -0800	[thread overview]
Message-ID: <CAHk-=witXXeQuP9fgs4dDL2Ex0meXQiHJs+3JEfNdaPwngMVEg@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whVnaTBt2Xm-A+8SMc5-q5CuZBDU6rUZ8yC8GoAnbTBvw@mail.gmail.com>

On Mon, Mar 6, 2023 at 9:29 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The correct thing to do is always that
>
>    * Returns >= nr_cpu_ids if no cpus set.
>
> because nr_cpu_ids is always the *smallest* of the access sizes.
>
> Of course, right now Guenter seems to be reporting a problem with that
> optimization, so unless I figure out what is going on I'll just need
> to revert it anyway.

Ahh. And the reason is exactly that people do *not* follow that
"Returns >= nr_cpu_ids" rule.

The drivers/char/random.c code is very wrong, and does

             if (cpu == nr_cpumask_bits)
                             cpu = cpumask_first(&timer_cpus);

which fails miserably exactly because it doesn't use ">=".

Oh well.

I'll have to look for more of this pattern, but basically all those
"xyz_cpumask_bits" things were supposed to always be just internal to
that header file implementation, which is *exactly* why you have to
check the result for ">= nr_cpu_ids".

       Linus