Write prep exercises teaching/testing: Mutexes, Pointers, Interfaces about immersive-go-course HOT 1 CLOSED

I've been thinking about the mutexes part of this, and I think we want to roughly cover:

• A simple memory model (the idea we have registers, L1/L2 caches, and main memory), their rough speeds costs and limits, why we have these, and the fact that several of these are unsynchronised per-core, but without either teaching or assuming "here's all the parts of a CPU and how a whole OS works".
• At a very high level, the concept of a memory barrier and a happens-before relationship.
• Simple single-variable examples of thread unsafety (e.g. increment a counter from 100 goroutines, see it doesn't reliably end at 100). The idea that a mutex can fix this.
• Multi-variable examples of thread unsafety - actually having to think about what the critical section is for a multi-statement operation, where atomics aren't sufficient (e.g. keeping a slice and a separate counter in sync or something to always have consistent views).
• WaitGroup as a useful abstraction.
• Channels as a useful way of signalling done-ness across goroutines (and passing data, I suppose).

I'm not sure which of these topics we care enough about to include (cc @sre-is-laura @Radha13 @nahratzah) :

• Atomics, CompareAndSwap
• Semaphores
• RWMutex

I think the concurrency chapter of golangbot is a sufficient introduction to how to do most of these things in Go (i.e. the single-variable example, WaitGroups, Channels), and has sufficient examples for the single-variable stuff.

I have yet to find a good resource for explaining the ideas of tiered memory, barriers, and happens-before relationships that isn't either super vague and overly "here's how you write the code without knowing why", or super deep into much wider CPU and/or OS design. I'd be interested if anyone else can find one, otherwise we may need to put one together (@nahratzah @sre-is-laura do you know of any, or could you be tempted to write one? If not, I can try to put one together).

I can come up with an example exercise for thinking about the critical section when dealing with multiple variables.

from immersive-go-course.