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
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