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_lock13.56%  postgres                     [.] LWLockRelease10.11%  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                     [.]
ReleaseAndReadBuffer0.44%  postgres                     [.] FunctionCall2Coll 0.41%  [kernel]                     [k]
vsnprintf0.41%  [kernel]                     [k] module_get_kallsym 0.32%  postgres                     [.]
_bt_relandgetbuf0.31%  [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.150.09%  [kernel]                     [k] update_iter 0.08%  plugin_host                  [.]
PyObject_GetAttr0.06%  [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