Re: Atomics for heap_parallelscan_nextpage()

On August 16, 2017 3:09:27 PM PDT, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
>> On 08/17/2017 12:20 AM, Tom Lane wrote:
>>> Indeed, gaur fails with
>>> 2017-08-16 17:09:38.315 EDT [13043:11] PANIC: stuck spinlock
>detected at pg_atomic_compare_exchange_u64_impl, atomics.c:196
>
>> I was able to reproduce this locally, with --disable-atomics, but
>only
>> after hacking it to fill the struct with garbage, before initializing
>
>> it. IOW, it failed to fail, because the spinlock happened to be
>> initialized correctly by accident. Perhaps that's happening on
>piculet, too.
>
>Oh, right. HPPA is unique among our platforms, I think, in that the
>"unlocked" state of a spinlock is not "all zeroes". So if you're
>dealing
>with pre-zeroed memory, which shmem generally would be, failing to
>initialize a spinlock does not cause visible errors except on HPPA.
>
>I wonder whether it's sensible to have --enable-cassert have the effect
>of filling memory allocated by ShmemAlloc or the DSA code with junk (as
>palloc does) instead of leaving it at zeroes. It's not modeling the
>same kind of effect, since we have no shmem-freeing primitives, but
>it might be useful for this sort of thing.

We kind of do - crash restarts... So yes, that's probably a good idea.

Andres
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