[GHC] #15626: Optimise wakeups for STM

[GHC]

#15626: Optimise wakeups for STM
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D4961     |         Wiki Page:
-------------------------------------+-------------------------------------
 As discovered while debugging #15544, the previous attempt at optimising
 STM wakeups (Phab:D4961, merged as
 502640c90c3d0fbb6c46257be14fdc7e3c694c6c) broke other things (see comment
 17) and we're going to revert it. This is the tracking ticket for this
 issue.

 To summarize the problem: when a thread from capability 2 is in wait list
 of a TVar and we update the TVar in capability 1 we send a wakeup message
 to capability 2. If before capability 2 actually wake ups the thread
 capability 1 updates the TVar again we send a message again, causing
 redundant wakeup message sending. Ideally we should somehow mark the TSO
 as "awoken" and not send wakeup messages more than once. Phab:D4961 did
 exactly this, but in a wrong way (updated a field of TSO that shouldn't
 have been updated).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15626>
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