linux-kernel - Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208

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Date:   Sat, 16 Sep 2017 10:55:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
        LKML <linux-kernel@...r.kernel.org>,
        Don Zickus <dzickus@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG:
 unable to handle kernel NULL pointer dereference at 0000000000000208

On Sat, Sep 16, 2017 at 10:35 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Don't bother. I found it already. On UP we have:
>
> #define for_each_cpu(cpu, mask)               \
>         for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
>
> which is a total fail as it breaks any code which uses for_each_cpu() or
> any of the other variants on UP by assuming that all cpumask have bit 0
> set.

It's fairly fundamental. UP assumes that all CPU masks are always that
"one CPU set". Not just here - everywhere.

I guess we could somehow try to move away from that, but really, the
assumption of fixed masks ends up simplifying the code generation a
lot, so it made tons of sense back when UP was a primary target.

So it's an approach that is somewhat historical, but I'm not sure it's
worth re-visiting that old decision. People should simply not expect
to traverse over empty masks in anything that is UP.

So I suspect your perf fix is the right one, and maybe we could/should
just make people more aware of the empty cpumask issue with UP.

            Linus