what does "__workq_kernreturn" indicate in an iOS crash log or when you pause execution of an app?

Question

When debugging on iOS, if I pause execution I often see multiple references to "__workq_kernreturn".

If my app happens to crash, I often see multiple threads identified as below:

Thread 19:
0   libsystem_kernel.dylib              0x332d0cd4 __workq_kernreturn + 8

Thread 20:
0   libsystem_kernel.dylib              0x332d0cd4 __workq_kernreturn + 8

Thread 21:
0   libsystem_kernel.dylib              0x332d0cd4 __workq_kernreturn + 8

Does "__workq_kernreturn" indicate a thread that is waiting to exit, or a thread that is deadlocked? Is this something to be worried about?

Answer 1

It's nothing to worry about unless there are lots of them. If there are lots, that suggests you may be spawning more threads than you probably should, but the workq_kernreturn is still not a problem itself. It usually means that the thread is finishing. The source for it is available at opensource.apple.com if you want to take a look at what it does.

The most common stack you're probably looking at is this one:

_workq_kernreturn
_pthread_wqthread
start_wqthread

Because of optimizations, you don't see the call to _pthread_workq_return() in there, but that's what's actually happening. _workq_kernreturn is just waiting for for the spinlock to finish so it can run the next thing on its queue (or exit).