Comment by colonwqbang
5 hours ago
Does there exist any platform which has multithreading but not atomics? Such a platform would be quite impractical as you can't really implement locks or any other threading primitive without atomics.
5 hours ago
Does there exist any platform which has multithreading but not atomics? Such a platform would be quite impractical as you can't really implement locks or any other threading primitive without atomics.
> Does there exist any platform which has multithreading but not atomics?
Yes. Also, almost every platform I know that supports multi threading and atomics doesn’t support atomics between /all/ possible masters. Consider a microcontroller with, say, two Arm cores (multithreaded, atomic-supporting) and a DMA engine.
Certainly such systems can pretty readily exist. You merely need atomic reads/writes in order to implement locks.
You can't create userspace locks which is a bummer, but the OS has the capability of enforcing locks. That's basically how early locking worked.
The main thing needed to make a correct lock is interrupt protection. Something every OS has.
To go fast, you need atomic operations. It especially becomes important if you are dealing with multiple cores. However, for a single core system atomics aren't needed for the OS to create locks.
> You merely need atomic reads/writes in order to implement locks.
Nit: while it's possible to implement one with just atomic reads and writes, it's generally not trivial/efficient/ergonomic to do so without an atomic composite read-write operation, like a compare-and-swap.
I wrote "multithreaded" but I really meant "multicore". If two cores are contending for a lock I don't see how irq protection help. As long as there is only one core, I agree.
On most multicore systems you can pin the IRQ handling to a single core. Pinning locking interrupts to a single core would be how you handle this.