Re: Question about indirectees of BLACKHOLE closures

23 Mar 2018

      Thanks Simon, that's really helpful.

A few more questions:

As far as I understand the difference between

- BLACKHOLE pointing to a TSO
- BLACKHOLE pointing to a BLOCKING_QUEUE

is that in the former we don't yet have any threads blocked by the BLACKHOLE
whereas in the latter we have and the blocking queue holds all those blocked
threads. Did I get this right?

Secondly, can a BLACKHOLE point to a THUNK? I'd expect no, because we BLACKHOLE
a closure when we're done evaluating it (assuming no eager blackholing), and
evaluation usually happens up to WHNF.

Thanks,

Ömer

2018-03-20 18:27 GMT+03:00 Simon Marlow :
...
Added comments: https://phabricator.haskell.org/D4517
On 20 March 2018 at 14:58, Simon Marlow  wrote:
...
Hi Omer,
On 20 March 2018 at 13:05, Ömer Sinan Ağacan  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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