[GHC] #13944: Introduce synchronized FFI

[GHC]

#13944: Introduce synchronized FFI
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        Reporter:  winter            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I don't see how this could work (or I've misunderstood your proposal).
 Consider that you have a thread with some reference to some heap object
 (call it `A`) sitting in another capability's nursery. Let's say that it
 enters a `synchronized` call and while the call is running a GC is
 started. During this GC another other capability evacuates `A` (producing
 a new object, say `A'`) and the nursery in which it lived is freed. Now we
 have a situation where the thread who made the call has a reference to
 `A`, an object which existed in a no-longer-existing heap block, which it
 may attempt to dereference once the call completes. The distinguishing
 feature of the `safe` call mechanism is that we suspend the running
 thread, saving all of its state back to its TSO, where they can be handled
 by the garbage collector.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13944#comment:3>
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