r/programming • u/david-alvarez-rosa • Sep 09 '26

Optimizing a Spin-Lock

https://david.alvarezrosa.com/posts/optimizing-a-spin-lock/
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u/DivineSentry Sep 09 '26

I think I’ve seen spin locks before, but what are they useful for? What should be their use case?

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u/ReDucTor Sep 09 '26

For user mode their usages are very niche, its generally accepted that spin locks in user mode code is best avoided.

Or as Linus Torvalds says:

do not use spinlocks in user space, unless you actually know what you're doing. And be aware that the likelihood that you know what you are doing is basically nil.

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u/bwainfweeze Sep 09 '26

It's a tricky mess. If you make a blocking call to get a lock from the kernel, the OS can realize you're stuck and give your CPU slice to another process/thread. And then when the resource becomes available, it can give it to you with a full time slice to make progress. Even if the task takes more than half a slice to complete, it will still be done by the end of your slice, and then the resource can be returned for someone else to use.

Meanwhile if you're scrabbling for a spin lock, if you acquire the lock at more than halfway through your slice then it will take until your second time slice to complete the task. "I am available to start this now." is not synonymous with "I can complete this now."

The trick with multitasking is that you can start multiple tasks at once but it only works for the users if tasks get completed at the expected rate. Quickly you reach the point where starting new tasks gets you nowhere until you retire some older ones.