objective c - Show error [NSConditionLock lockWhenCondition:beforeDate:] when call background methods iOS -

when run multiple methods in background, app shows error below. says cannot execute background methods.

[nsconditionlock lockwhencondition:beforedate:]: deadlock (<nsconditionlock: 0x166ae1a0> '(null)')  *** break on _nslockerror() debug. 

why app getting error? how can solve issue?