Re: Atomics for heap_parallelscan_nextpage()

On 2017-08-16 09:57:35 -0700, Peter Geoghegan wrote:
> On Wed, Aug 16, 2017 at 9:55 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2017-08-16 11:16:58 -0400, Tom Lane wrote:
> >> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
> >> > A couple of 32-bit x86 buildfarm members don't seem to be happy with
> >> > this. I'll investigate, but if anyone has a clue, I'm all ears...
> >>
> >> dromedary's issue seems to be alignment:
> >>
> >> TRAP: UnalignedPointer("(((uintptr_t) ((uintptr_t)(ptr)) + ((8) - 1)) & ~((uintptr_t) ((8) - 1))) != (uintptr_t)(ptr)", File: "../../../../src/include/port/atomics.h", Line: 452)
> >> 2017-08-16 11:11:38.558 EDT [75693:3] LOG: server process (PID 76277) was terminated by signal 6: Abort trap
> >> 2017-08-16 11:11:38.558 EDT [75693:4] DETAIL: Failed process was running: select count(*) from a_star;
> >>
> >> Not sure if this is your bug or if it's exposing a pre-existing
> >> deficiency in the atomics code, viz, failure to ensure that
> >> pg_atomic_uint64 is actually a 64-bit-aligned type. Andres?
> >
> > I suspect it's the former. Suspect that the shared memory that holds
> > the "parallel desc" isn't properly aligned:
>
> Clang has an -fsanitize=alignment option that may be useful here.

Given that we're crashing in an assert that does nothing but check for
alignment, before the memory is actually used in a "dangerous" manner, I
don't quite see how?

Greetings,

Andres Freund