Re: [lkp-robot] [x86/kconfig] 81d3871900: BUG:unable_to_handle_kernel

[Linus Torvalds]

On Tue, Oct 17, 2017 at 3:33 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>
> It looks like a compiler bug. The code of slob_units() try to read two
> bytes at ffff88001c4afffe. It's valid. But the compiler generates
> wrong code that try to read four bytes.
>
> static slobidx_t slob_units(slob_t *s)
> {
>   if (s->units > 0)
>     return s->units;
>   return 1;
> }
>
> s->units is defined as two bytes in this setup.
>
> Wrongly generated code for this part.
>
> 'mov 0x0(%rbp), %ebp'
>
> %ebp is four bytes.
>
> I guess that this wrong four bytes read cross over the valid memory
> boundary and this issue happend.

Hmm. I can see why the compiler would do that (16-bit accesses are
slow), but it's definitely wrong.

Does it work ok if that slob_units() code is written as

  static slobidx_t slob_units(slob_t *s)
  {
     int units = READ_ONCE(s->units);

     if (units > 0)
         return units;
     return 1;
  }

which might be an acceptable workaround for now?

                   Linus