go RWMutex

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for normal mutex, it’s like a talking pillow, whoever has it has the right to execute code
it’s good for writing data, but not so good for reading data
for read intensive processes, we want different threads to be able to read at the same time, and keep only one thread able to write
and when there’s data-write, we wanna block data-read
- this is where RWMutex comes in
if a reader is reading while a writer is writing, it causes data race
when a go routine has mutex.RLock(), it means “i’m a reader, other readers feel free to jump in when i’m reading, but writers hold up”