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> Outside of hard realtime code, there's zero reason to use spin locks.

On some common architectures, releasing a spin lock is cheaper than releasing a mutex.

On all architectures, releasing a mutex requires at least a branch (to see if you need to wake up sleeping threads) that you don’t need with a pure spinlock.

But if you don’t have a guarantee the lock owner won’t be preempted, well, spinning for a whole timeslot is quite a bit more expensive…

Spin locks are a tough sell in a preemptable context. Say you have two processes that share some memory location. They both briefly access it for a very short time, so you protect it with a spin lock. Well, what happens in the case when one of the threads is preempted while holding the lock? The other thread would try to aquire it, and just spin for its entire timeslot. No bueno. When you call spin_lock() in the kernel it actually disables preemption until you call spin_unlock() to avoid this. You can't disable preemption from userspace. There might be a use for a spin lock if you have a process that is SCHED_RR, but I haven't seen it.

You are not wrong in general but note that there are ways to disable preemption in userspace in practice, be it SCHED_FIFO[1] or isolcpu with cpu pinning.

[1] you better be careful with spinlocks and priorities here as you can livelock forever.

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