On 20 March 2018 at 13:05, Ömer Sinan Ağacan <omeragacan@gmail.com> wrote:

Hi,

I've been looking at BLACKHOLE closures and how the indirectee field is used
and I have a few questions:

Looking at evacuate for BLACKHOLE closures:

case BLACKHOLE:
{
StgClosure *r;
const StgInfoTable *i;
r = ((StgInd*)q)->indirectee;
if (GET_CLOSURE_TAG(r) == 0) {
i = r->header.info;
if (IS_FORWARDING_PTR(i)) {
r = (StgClosure *)UN_FORWARDING_PTR(i);
i = r->header.info;
}
if (i == &stg_TSO_info
|| i == &stg_WHITEHOLE_info
|| i == &stg_BLOCKING_QUEUE_CLEAN_info
|| i == &stg_BLOCKING_QUEUE_DIRTY_info) {
copy(p,info,q,sizeofW(StgInd),gen_no);
return;
}
ASSERT(i != &stg_IND_info);
}
q = r;
*p = r;
goto loop;
}

It seems like indirectee can be a TSO, WHITEHOLE, BLOCKING_QUEUE_CLEAN,
BLOCKING_QUEUE_DIRTY, and it can't be IND. I'm wondering what does it mean for
a BLACKHOLE to point to a

- TSO
- WHITEHOLE
- BLOCKING_QUEUE_CLEAN
- BLOCKING_QUEUE_DIRTY

That sounds right to me.

Is this documented somewhere or otherwise could someone give a few pointers on
where to look in the code?

Unfortunately I don't think we have good documentation for this, but you should look at the comments around messageBlackHole in Messages.c.

Secondly, I also looked at the BLACKHOLE entry code, and it seems like it has a
different assumption about what can indirectee field point to:

INFO_TABLE(stg_BLACKHOLE,1,0,BLACKHOLE,"BLACKHOLE","BLACKHOLE")
(P_ node)
{
W_ r, info, owner, bd;
P_ p, bq, msg;

TICK_ENT_DYN_IND(); /* tick */

retry:
p = StgInd_indirectee(node);
if (GETTAG(p) != 0) {
return (p);
}

info = StgHeader_info(p);
if (info == stg_IND_info) {
// This could happen, if e.g. we got a BLOCKING_QUEUE that has
// just been replaced with an IND by another thread in
// wakeBlockingQueue().
goto retry;
}

if (info == stg_TSO_info ||
info == stg_BLOCKING_QUEUE_CLEAN_info ||
info == stg_BLOCKING_QUEUE_DIRTY_info)
{
("ptr" msg) = ccall allocate(MyCapability() "ptr",
BYTES_TO_WDS(SIZEOF_MessageBlackHole));

SET_HDR(msg, stg_MSG_BLACKHOLE_info, CCS_SYSTEM);
MessageBlackHole_tso(msg) = CurrentTSO;
MessageBlackHole_bh(msg) = node;

(r) = ccall messageBlackHole(MyCapability() "ptr", msg "ptr");

if (r == 0) {
goto retry;
} else {
StgTSO_why_blocked(CurrentTSO) = BlockedOnBlackHole::I16;
StgTSO_block_info(CurrentTSO) = msg;
jump stg_block_blackhole(node);
}
}
else
{
ENTER(p);
}
}

The difference is, when the tag of indirectee is 0, evacuate assumes that
indirectee can't point to an IND, but BLACKHOLE entry code thinks it's possible
and there's even a comment about why. (I don't understand the comment yet) I'm
wondering if this code is correct, and why. Again any pointers would be
appreciated.

Taking a quick look at the code, my guess is that:

- a BLOCKING_QUEUE gets overwritten by an IND in wakeBlockingQueue()

- but when this happens, the indirectee of the BLACKHOLE will also be overwritten to point to the value

At runtime a thread might see an intermediate state because these mutations are happening in another thread, so we might follow the indirectee and see the IND. But this state can't be observed by the GC, because all mutator threads have stopped at a safe point.

Cheers

Simon

Thanks,

Ömer
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