I'm usually on the other side of these primitives when I write code as a consumer of them, but it's very interesting to read about the nuances related to their implementations:

https://probablydance.com/2019/12/30/measuring-mutexes-spinlocks-and-how-bad-the-linux-scheduler-really-is/

The problem was that there was a thread that spent several milliseconds trying to acquire a spinlock at a time when no other thread was holding the spinlock.