multithreading - CoreData deadlock with background thread -

i using classic multiple mocs sharing same nspersistentstorecoordinator understand performs better parent/child contexts according this:
http://floriankugler.com/blog/2013/4/29/concurrent-core-data-stack-performance-shootout

i being careful creating contexts , using them on thread/queue creates them.

the deadlock this:

main thread executing fetch request , hung on mutex in [nsmanagedobjectcontext executefetchrequest:error:]

#0 0x3ad840fc in __psynch_mutexwait () #1 0x3accd128 in pthread_mutex_lock () #2 0x328eee90 in -[_pflock lock] () #3 0x328ff350 in -[nspersistentstorecoordinator executerequest:withcontext:error:] () #4 0x328fdf16 in -[nsmanagedobjectcontext executefetchrequest:error:] () 

background thread hung on lock put on persistent store coordinator prior calling [coordinator managedobjectidforurirepresentation:objector].

#0 0x3ad840fc in __psynch_mutexwait () #1 0x3accd128 in pthread_mutex_lock () #2 0x328eee90 in -[_pflock lock] () #3 0x00093180 in -[datamanager getobjectbyuri:context:]  

i understand difficult debug post ideas appreciated. btw...this happens rarely.