[Ded7xSEoPKYNsDd]

Correct me if I'm wrong, but I think a compiler that doesn't do link-time-optimization has to assume that the _spinlock() function can write to mutexp (e.g. through a global variable), and therefore it cannot reorder anything around it.

[wuch]

I think you are looking at the side that does the reading. I was actually thinking about the other side that does the writing (inside pthread_mutex_init), where those two things are not separated by any function calls and are more readily reordered. Not to mention that it would be problematic on architectures with relaxed memory model either way.