You're correct -- the problem with userspace spinlocks is that the holding thread can be scheduled off. You can prevent this to some degree (probabilistically)

You're correct -- the problem with userspace spinlocks is that the holding thread can be scheduled off. You can prevent this to some degree (probabilistically) with isolcpus and thread pinning, but that usually doesn't prevent hardware interrupts from running on those cores (which kernel spinlocks can avoid!). This isn't really solvable without running in kernel context (to have the elevated permissions necessary to mask interrupts).