Spinning around: Please don't – Common problems with spin locks

https://www.siliceum.com/en/blog/post/spinning-around/

40•bdash•4h ago

Comments

gafferongames•1h ago

Great article! Thanks for posting this.

CamperBob2•1h ago

Sheesh. Can something this complicated ever truly be said to work?

nh23423fefe•1h ago

Isn't it the opposite? The complication is evidence of function. The simple code doesn't work.

kelnos•41m ago

That assertion feels suspiciously like a logical fallacy.

bluGill•1h ago

You can limit yourself to the performance of a 1mhz 6502 with no OS if you don't like it. Even MSDos on a 8086 with 640K ram allows for things that require complexity of this type (not spin locks, but the tricks needed to make "terminate stay resident" work are evil in a similar way)

yjftsjthsd-h•59m ago

I don't think that's fair. You can go fast, just not more than one task at a time.

bluGill•42m ago

Modern CPUs (since around 2000) go faster in large part because they have multiple cores that can do more than one thing in a time. If your program needs to go faster using more cores is often your best answer and then you will need these tricks. (SIMD or the GPU are also common answers that might or might not be better for your problem)

yjftsjthsd-h•31m ago

Modern CPUs can do 4-5 GHz singled threaded. (Sometimes you can even get a higher clock speed by disabling other cores.) This somewhat outpaces "a 1mhz 6502" even without parallelization.

bluGill•26m ago

They can, but nobody runs a single process on such CPUs. They run some form of OS which implements spinlock, mutexes, and all these other complex things.

I suppose someplace someone is running an embedded system without an OS on such a processor - but I'd expect they are still using extra cores and so have all of the above tricks someplace.

jcranmer•1h ago

The basic rule of writing your own cross-thread datastructures like mutexes or condition variables is... don't, unless you have very good reason not to. If you're in that rare circumstance where you know the library you're using isn't viable for some reason, then the next best rule is to use your OS's version of a futex as the atomic primitive, since it's going to solve most of the pitfalls for you automatically.

The only time I've manually written my own spin lock was when I had to coordinate between two different threads, one of which was running 16-bit code, so using any library was out of the question, and even relying on syscalls was sketchy because making sure the 16-bit code is in the right state to call a syscall itself is tricky. Although in this case, since I didn't need to care about things like fairness (only two threads are involved), the spinlock core ended up being simple:

    "thunk_spin:",
        "xchg cx, es:[{in_rv}]",
        "test cx, cx",
        "jnz thunk_has_data",
        "pause",
        "jmp thunk_spin",
    "thunk_has_data:",

kccqzy•56m ago

Another time when writing a quick and dirty spinlock is reasonable is inside a logging library. A logging library would normally use a full-featured mutex, but what if we want the mutex implementation to be able to log? Say the mutex can log that it is non recursive yet the same thread is acquiring it twice; or that it has detected a deadlock. The solution is to introduce a special subset of the logging library to use a spinlock.

wizzwizz4•4m ago

I'm not sure how a spinlock solves this problem. Wouldn't that just cause the process to hang busy?

fasterik•33m ago

As always: use standard libraries, profile, then write your own if the data indicates it's necessary. The following is an interesting talk where the author used a spinlock to speed up a real-time physics solver significantly.

Dennis Gustafsson – Parallelizing the physics solver – BSC 2025 https://www.youtube.com/watch?v=Kvsvd67XUKw