Question about indirectees of BLACKHOLE closures

[Ömer Sinan Ağacan]

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 <marlowsd at gmail.com>:
> Added comments: https://phabricator.haskell.org/D4517
>
> On 20 March 2018 at 14:58, Simon Marlow <marlowsd at gmail.com> wrote:
>>
>> 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
>>> _______________________________________________
>>> ghc-devs mailing list
>>> ghc-devs at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>>
>