linux-kernel - Re: [lkp-robot] [x86/kconfig] 81d3871900: BUG:unable_to_handle

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Date:   Wed, 18 Oct 2017 06:40:48 -0400
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Slaby <jslaby@...e.cz>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
        linux-mm <linux-mm@...ck.org>, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [lkp-robot] [x86/kconfig] 81d3871900: BUG:unable_to_handle_kernel

On Tue, Oct 17, 2017 at 3:33 AM, Joonsoo Kim <iamjoonsoo.kim@....com> 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