Re: Question about indirectees of BLACKHOLE closures
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
> ___
> ghc-devs mailing list
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>
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Re: Question about indirectees of BLACKHOLE closures
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_MessageBl
>> ackHole));
>>
>> 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@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>
>
___
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Re: Question about indirectees of BLACKHOLE closures
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
>>
Re: Question about indirectees of BLACKHOLE closures
Hi Omer,
As per my understanding, a BLACKHOLE can point to a THUNK when an exception
is thrown. An exception walks up the stack and overwrites the blackholes
pointed to by the update frames as it walks with an stg_raise closure. That
way, if any concurrent thread happens to evaluate a thunk that was walked,
it'll evaluate the thunk which will blow up as well thereby throwing the
exception on the other thread(s) too.
Definition of stg_raise:
https://github.com/ghc/ghc/blob/ba5797937e575ce6119de6c07703e90dda2557e8/rts/Exception.cmm#L424-L427
raiseExceptionHelper dealing with update frames:
https://github.com/ghc/ghc/blob/d9d463289fe20316cff12a8f0dbf414db678fa72/rts/Schedule.c#L2864-L2875
In general, yes, you can think that a BLACKHOLE will point to a non-THUNK
object assuming that everything went right.
Hope that helps,
Rahul
On Fri, Mar 23, 2018 at 5:48 PM, Ömer Sinan Ağacan
wrote:
> 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) {
> >>>
Re: Question about indirectees of BLACKHOLE closures
Hi Rahul,
I'm still confused. The code that walks the stack and updates UPDATE_FRAMEs
only makes indirections point to the "raise" closure, not to thunks or anything
else (I also don't understand why this is needed but I guess that's another
topic). I still don't see how can a BLACKHOLE point to a THUNK.
Ömer
2018-03-23 18:51 GMT+03:00 Rahul Muttineni :
> Hi Omer,
>
> As per my understanding, a BLACKHOLE can point to a THUNK when an exception
> is thrown. An exception walks up the stack and overwrites the blackholes
> pointed to by the update frames as it walks with an stg_raise closure. That
> way, if any concurrent thread happens to evaluate a thunk that was walked,
> it'll evaluate the thunk which will blow up as well thereby throwing the
> exception on the other thread(s) too.
>
> Definition of stg_raise:
> https://github.com/ghc/ghc/blob/ba5797937e575ce6119de6c07703e90dda2557e8/rts/Exception.cmm#L424-L427
>
> raiseExceptionHelper dealing with update frames:
> https://github.com/ghc/ghc/blob/d9d463289fe20316cff12a8f0dbf414db678fa72/rts/Schedule.c#L2864-L2875
>
> In general, yes, you can think that a BLACKHOLE will point to a non-THUNK
> object assuming that everything went right.
>
> Hope that helps,
> Rahul
>
> On Fri, Mar 23, 2018 at 5:48 PM, Ömer Sinan Ağacan
> wrote:
>>
>> 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_BLOC
Re: Question about indirectees of BLACKHOLE closures
I think I can at least answer the why: we're talking about threads
referring to suspended computations within a thread whose stack is being
"unwound". Those computations won't be resumable after the unwind (which
makes their context go away). So they have to be overwritten with something
to cause the referencing threads to abort if they need the
no-longer-computable results of those suspended computations.
On Sat, Mar 24, 2018 at 3:27 PM, Ömer Sinan Ağacan
wrote:
> Hi Rahul,
>
> I'm still confused. The code that walks the stack and updates UPDATE_FRAMEs
> only makes indirections point to the "raise" closure, not to thunks or
> anything
> else (I also don't understand why this is needed but I guess that's another
> topic). I still don't see how can a BLACKHOLE point to a THUNK.
>
> Ömer
>
> 2018-03-23 18:51 GMT+03:00 Rahul Muttineni :
> > Hi Omer,
> >
> > As per my understanding, a BLACKHOLE can point to a THUNK when an
> exception
> > is thrown. An exception walks up the stack and overwrites the blackholes
> > pointed to by the update frames as it walks with an stg_raise closure.
> That
> > way, if any concurrent thread happens to evaluate a thunk that was
> walked,
> > it'll evaluate the thunk which will blow up as well thereby throwing the
> > exception on the other thread(s) too.
> >
> > Definition of stg_raise:
> > https://github.com/ghc/ghc/blob/ba5797937e575ce6119de6c07703e9
> 0dda2557e8/rts/Exception.cmm#L424-L427
> >
> > raiseExceptionHelper dealing with update frames:
> > https://github.com/ghc/ghc/blob/d9d463289fe20316cff12a8f0dbf41
> 4db678fa72/rts/Schedule.c#L2864-L2875
> >
> > In general, yes, you can think that a BLACKHOLE will point to a non-THUNK
> > object assuming that everything went right.
> >
> > Hope that helps,
> > Rahul
> >
> > On Fri, Mar 23, 2018 at 5:48 PM, Ömer Sinan Ağacan >
> > wrote:
> >>
> >> 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",
Re: Question about indirectees of BLACKHOLE closures
The raise closure is declared to be a THUNK:
https://phabricator.haskell.org/diffusion/GHC/browse/master/rts/Exception.cmm;60e29dc2611f5c1a01cfd9a870841927847a7b74$424
Another example of this is when an asynchronous exception is thrown, and we
update all the thunks/BLACKHOLEs pointed to by the update frames to point
to new thunks (actually AP_STACK closures) representing the frozen state of
evaluation of those thunks. For this, see rts/RaiseAsync.c.
Cheers
Simon
On 24 March 2018 at 19:27, Ömer Sinan Ağacan wrote:
> Hi Rahul,
>
> I'm still confused. The code that walks the stack and updates UPDATE_FRAMEs
> only makes indirections point to the "raise" closure, not to thunks or
> anything
> else (I also don't understand why this is needed but I guess that's another
> topic). I still don't see how can a BLACKHOLE point to a THUNK.
>
> Ömer
>
> 2018-03-23 18:51 GMT+03:00 Rahul Muttineni :
> > Hi Omer,
> >
> > As per my understanding, a BLACKHOLE can point to a THUNK when an
> exception
> > is thrown. An exception walks up the stack and overwrites the blackholes
> > pointed to by the update frames as it walks with an stg_raise closure.
> That
> > way, if any concurrent thread happens to evaluate a thunk that was
> walked,
> > it'll evaluate the thunk which will blow up as well thereby throwing the
> > exception on the other thread(s) too.
> >
> > Definition of stg_raise:
> > https://github.com/ghc/ghc/blob/ba5797937e575ce6119de6c07703e9
> 0dda2557e8/rts/Exception.cmm#L424-L427
> >
> > raiseExceptionHelper dealing with update frames:
> > https://github.com/ghc/ghc/blob/d9d463289fe20316cff12a8f0dbf41
> 4db678fa72/rts/Schedule.c#L2864-L2875
> >
> > In general, yes, you can think that a BLACKHOLE will point to a non-THUNK
> > object assuming that everything went right.
> >
> > Hope that helps,
> > Rahul
> >
> > On Fri, Mar 23, 2018 at 5:48 PM, Ömer Sinan Ağacan >
> > wrote:
> >>
> >> 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:
> >> >>>
> >> >>>
Re: Question about indirectees of BLACKHOLE closures
Simon Marlow writes: > The raise closure is declared to be a THUNK: > > https://phabricator.haskell.org/diffusion/GHC/browse/master/rts/Exception.cmm;60e29dc2611f5c1a01cfd9a870841927847a7b74$424 > > Another example of this is when an asynchronous exception is thrown, and we > update all the thunks/BLACKHOLEs pointed to by the update frames to point > to new thunks (actually AP_STACK closures) representing the frozen state of > evaluation of those thunks. For this, see rts/RaiseAsync.c. > This thread has answered a number of interesting questions. It would be a shame if these answers vanished into the abyss of the ghc-devs archives. Omer, do you think you could make sure that the discussion here is summarized in a Note (or ensure that the relevant notes reference one another, if they already exist)? Cheers, - Ben signature.asc Description: PGP signature ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Question about indirectees of BLACKHOLE closures
I still don't understand the whole story with blackholes but I'll update the comments around the BLACKHOLE stack frame and/or wiki pages once I get a better understanding. Ömer 2018-03-26 21:47 GMT+03:00 Ben Gamari : > Simon Marlow writes: > >> The raise closure is declared to be a THUNK: >> >> https://phabricator.haskell.org/diffusion/GHC/browse/master/rts/Exception.cmm;60e29dc2611f5c1a01cfd9a870841927847a7b74$424 >> >> Another example of this is when an asynchronous exception is thrown, and we >> update all the thunks/BLACKHOLEs pointed to by the update frames to point >> to new thunks (actually AP_STACK closures) representing the frozen state of >> evaluation of those thunks. For this, see rts/RaiseAsync.c. >> > This thread has answered a number of interesting questions. It would be > a shame if these answers vanished into the abyss of the ghc-devs > archives. > > Omer, do you think you could make sure that the discussion here is > summarized in a Note (or ensure that the relevant notes reference one > another, if they already exist)? > > Cheers, > > - Ben > ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
