[GHC] #15508: concprog001 fails with various errors

[GHC]

#15508: concprog001 fails with various errors
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        Reporter:  osa1              |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15571            |  Differential Rev(s):  Phab:D5051
       Wiki Page:                    |  (reverted), Phab:D5165
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Comment (by simonmar):

 > Why we can make a MessageBlackHole an IND without setting the
 indirectee? Is this used to remove (by skipping) a MessageBlackHole from
 the queue? (because actually removing would be impossible as we don't have
 a prev field in MessageBlackHole?)

 Yes, exactly.

 > As far as I understand, a BLACKHOLE can become a BLOCKING_QUEUE (in
 messageBlackHole()), and queue of a BLOCKING_QUEUE is for threads that are
 blocked on this BLACKHOLE.

 Just to be clear, A BLACKHOLE doesn't "become" a BLOCKING_QUEUE, the
 BLACKHOLE's indirectee field points to the BLOCKING_QUEUE.

 > Why do we have a bh field in messageBlackHole? We could pass the bh as a
 parameter to messageBlackHole() and bh would the blackhole that we just
 looked at to find the MessageBlackHole.

 A MessageBlackHole is sent to another capability, which needs to know
 which BLACKHOL to add it to.

 > Why do we need a list of BLOCKING_QUEUEs? Is this only to be able to
 implement checkBlockingQueues()? The comments around checkBlockingQueues()
 say

 See this comment in `messageBlackHole()`:

 {{{
         // All BLOCKING_QUEUES are linked in a list on owner->bq, so
         // that we can search through them in the event that there is
         // a collision to update a BLACKHOLE and a BLOCKING_QUEUE
         // becomes orphaned (see updateThunk()).
 }}}

 > but I don't understand how can owner of a BLACKHOLE not point to our
 capability if we BLACKHOLEd it.

 A BLACKHOLE can be updated by another thread at any time. The fact that it
 is a BLACKHOLE is not an exclusive ownership. We use compare-and-swap to
 install the BLACKHOLE in `threadPaused()`, but another thread might
 already be evaluating the thunk and will update it with the value later,
 and we don't use a compare-and-swap when updating because that would be
 too expensive.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15508#comment:19>
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