It's any blocking operation on a shared resource.

WTF? Even if it's not the only thing that blocks thread creation doesn't mean preclude it from being one of the things that blocks thread creation.

And that operation doesn't "obtain a lock", it enters a mutex.

WTF? You of all people should know they're the same thing. "A mutex is also a common name for a program object that negotiates mutual exclusion among threads, also called a lock."

The "other operation" in this case is CLONE, not thread creation.

WTF? The clone is part of the thread creation. So if the clone blocks, thread creation blocks too.

it isn't a bug, nor limitation, nor caveat--it is a requirement--implemented correctly and WAD.

I agree that it's not a bug, but you're wrong on the other two.

It's definitely a limitation. The OP's code didn't work.

The behaviour of a program can never be caveat any more than it can be blue, so that's a weird thing to list.