[horizion2025]

My concurrency knowledge is a bit rusty but aren't spinlocks only supposed to be used for very brief waits like in the hundreds of cycles (or situations where you can't block... like internal o/s scheduling structures in SMP setups)? If so how much does all this back off and starvation of higher priority threads even matter? If it is longer then you should use a locking primitive (except for in those low level os structures!) where most of the things discussed are not an issue. Would love to hear the use cases where spin locks are needed in eg user space, I dont doubt they occur.

[Lectem]

That's how they are supposed to work indeed! But spin locks aren't the only spin loops you may find, and allocator for example do spin. And for example under an allocation heavy code (that you should avoid too, but happens due to 3rd parties in real life), this can trigger contention, so you need contention to not be the worse type of contention.

[imtringued]

How can you guarantee that the OS doesn't preempt your thread in the middle of the spinlock? Suddenly your 100 cycle spinlock turns into millions or billions of wasted cycles, because the other threads that are trying to acquire the same lock are spinning and didn't bother informing the OS scheduler that they need the thread that is holding the spinlock, which also didn't inform the OS, to finish its business ASAP.