hung backends stuck in spinlock heavy endless loop

On my workstation today (running vanilla 9.4.0) I was testing some new
code that does aggressive parallel loading to a couple of tables. It
ran ok several dozen times and froze up with no external trigger.
There were at most 8 active backends that were stuck (the loader is
threaded to a cap) -- each query typically resolves in a few seconds
but they were hung for 30 minutes+.  Had to do restart immediate as
backends were not responding to cancel...but I snapped a 'perf top'
before I did so.  The results were interesting so I'm posting them
here.  So far I have not been able to reproduce...FYI

 61.03%  postgres                     [.] s_lock
 13.56%  postgres                     [.] LWLockRelease
 10.11%  postgres                     [.] LWLockAcquire
  4.02%  perf                         [.] 0x00000000000526d3
  1.65%  postgres                     [.] _bt_compare
  1.60%  libc-2.17.so                 [.] 0x0000000000081069
  0.66%  [kernel]                     [k] kallsyms_expand_symbol.constprop.1
  0.60%  [kernel]                     [k] format_decode
  0.57%  [kernel]                     [k] number.isra.1
  0.47%  [kernel]                     [k] memcpy
  0.44%  postgres                     [.] ReleaseAndReadBuffer
  0.44%  postgres                     [.] FunctionCall2Coll
  0.41%  [kernel]                     [k] vsnprintf
  0.41%  [kernel]                     [k] module_get_kallsym
  0.32%  postgres                     [.] _bt_relandgetbuf
  0.31%  [kernel]                     [k] string.isra.5
  0.31%  [kernel]                     [k] strnlen
  0.31%  postgres                     [.] _bt_moveright
  0.28%  libc-2.17.so                 [.] getdelim
  0.22%  postgres                     [.] LockBuffer
  0.16%  [kernel]                     [k] seq_read
  0.16%  libc-2.17.so                 [.] __libc_calloc
  0.13%  postgres                     [.] _bt_checkpage
  0.09%  [kernel]                     [k] pointer.isra.15
  0.09%  [kernel]                     [k] update_iter
  0.08%  plugin_host                  [.] PyObject_GetAttr
  0.06%  [kernel]                     [k] strlcpy
  0.06%  [kernel]                     [k] seq_vprintf
  0.06%  [kernel]                     [k] copy_user_enhanced_fast_string
  0.06%  libc-2.17.so                 [.] _IO_feof
  0.06%  postgres                     [.] btoidcmp
  0.06%  [kernel]                     [k] page_fault
  0.06%  libc-2.17.so                 [.] free
  0.06%  libc-2.17.so                 [.] memchr
  0.06%  libpthread-2.17.so           [.] __pthread_mutex_unlock_usercnt

merlin