Re: Atomics for heap_parallelscan_nextpage()

On 2017-08-16 11:16:58 -0400, Tom Lane wrote:
> Heikki Linnakangas <hlinnaka@iki.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:

void
ExecSeqScanInitializeDSM(SeqScanState *node,
						 ParallelContext *pcxt)
{
...
	pscan = shm_toc_allocate(pcxt->toc, node->pscan_len);
	heap_parallelscan_initialize(pscan,
								 node->ss.ss_currentRelation,
								 estate->es_snapshot);


/*
...
 * We allocate backwards from the end of the segment, so that the TOC entries
 * can grow forward from the start of the segment.
 */
extern void *
shm_toc_allocate(shm_toc *toc, Size nbytes)
{
	volatile shm_toc *vtoc = toc;
	Size		total_bytes;
	Size		allocated_bytes;
	Size		nentry;
	Size		toc_bytes;

	/* Make sure request is well-aligned. */
	nbytes = BUFFERALIGN(nbytes);
...
	return ((char *) toc) + (total_bytes - allocated_bytes - nbytes);
}


/*
 * Initialize a region of shared memory with a table of contents.
 */
shm_toc *
shm_toc_create(uint64 magic, void *address, Size nbytes)
{
	shm_toc    *toc = (shm_toc *) address;

	Assert(nbytes > offsetof(shm_toc, toc_entry));
	toc->toc_magic = magic;
	SpinLockInit(&toc->toc_mutex);
	toc->toc_total_bytes = nbytes;
	toc->toc_allocated_bytes = 0;
	toc->toc_nentry = 0;

	return toc;
}

Afaict shm_create/shm_toc_allocate don't actually guarantee that the end
of the toc's memory is suitably aligned.  But I didn't yet have any
coffee, so ...

- Andres