ghc 7.10.1 hard lock on exit with shake, OS X 10.10

[Evan Laforge]

After I upgraded to 7.10.1 I started noticing that my shakefile would
lock up on exit.  It's after the 'main' function exits, and none of
the shake tests have a problem, so presumably it's a GHC thing, that
shake somehow causes to happen.  Only kill -9 gets it to quit.  Here's
a stack trace from the OS X sampler:

Call graph:
    2801 Thread_909901   DispatchQueue_2:
com.apple.libdispatch-manager  (serial)
      2801 _dispatch_mgr_thread  (in libdispatch.dylib) + 52  [0x7fff8828ca6a]
        2801 kevent64  (in libsystem_kernel.dylib) + 10  [0x7fff90ec0232]

Total number in stack (recursive counted multiple, when >=5):

Sort by top of stack, same collapsed (when >= 5):
        kevent64  (in libsystem_kernel.dylib)        2801


I know there aren't a lot of details here, but does this sound
familiar to anyone?  I can't see anything on
https://ghc.haskell.org/trac/ghc/wiki/Status/GHC-7.10.2 that looks
like this.  Is there any way I can get more information I can get to
report this?

It used to be frequent (once in 10 runs maybe), but later became quite
infrequent (once in a couple hundred runs, maybe).  I downgraded to
7.8.4 and it hasn't happened again.