[GHC] #11713: STM Finalizers

[GHC]

#11713: STM Finalizers
-------------------------------------+-------------------------------------
        Reporter:  mc.schroeder      |                Owner:  mc.schroeder
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Runtime System    |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2011
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by mc.schroeder):

 Actually, instead of simply locking all TVars involved in a finalizing
 transaction, my implementation "freezes" them, meaning they can still be
 read by other transactions (preserving read-parallelism). Only when
 another transaction tries to commit a write to a frozen TVar is its thread
 blocked. It is woken up again when the finalizer that froze the TVar has
 finished.

 Of course, this could take a while (potentially forever), and all
 transactions waiting on frozen TVars are stalled in the meantime.
 However, I would say that this is exactly the point of finalizers: they
 create a serialization point.

 As for the potential of deadlocks due to nested transactions:
 a deadlock could only occur if the inner transaction writes a TVar used by
 the outer transaction. In my opinion, this is a rather artifical scenario,
 that is unlikely to happen by accident. It will also always be detected by
 the runtime system, which would then gracefully abort the transaction with
 a BlockedIndefinitelyOnSTM exception.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11713#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler