Question about indirectees of BLACKHOLE closures

[Simon Marlow]

Hi Omer,

On 20 March 2018 at 13:05, Ömer Sinan Ağacan <omeragacan at 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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