Comment by colonwqbang
7 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.
7 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.
Yes but "atomic" operations with the DMA engine are accomplished through interrupts (atomic) or memory mapped IO configuration (atomic).
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.