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

From: Thomas Gleixner
Date: Sat Sep 16 2017 - 14:13:19 EST

On Sat, 16 Sep 2017, Linus Torvalds wrote:
> On Sat, Sep 16, 2017 at 10:35 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

Right, I just got a bit frightened as I really was not aware about that
'opmtimization' which means that so far I just was lucky not to trip over