[GHC] #8209: Race condition in setNumCapabilities

[GHC]

#8209: Race condition in setNumCapabilities
-------------------------------------+----------------------------------
        Reporter:  akio              |            Owner:  simonmar
            Type:  bug               |           Status:  new
        Priority:  highest           |        Milestone:  7.8.1
       Component:  Runtime System    |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash     |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+----------------------------------

Comment (by akio):

 I found another error (reproducible with the same code).

 {{{moreCapabilities}}} calls {{{memcpy}}} to copy {{{Capability}}}s. This
 means the {{{lock}}} field is also copied. If the {{{lock}}} field of the
 old {{{Capability}}} is held by another thread, the new copy of {{{lock}}}
 is also in the held state. However it will never be released, because the
 actual lock held is the old {{{lock}}}, not the new one. The task running
 {{{setNumCapabilities()}}} then deadlocks trying to acquire this lock. (By
 the way copying a mutex seems to be an undefined behavior according to the
 pthread spec)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8209#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler