Hacker News Reader

Comment by lemevi | Hacker News Reader

Do you think it would be fine to use a singleton if the writes are all happening in a single thread and it's not critical that the reads be synchronized? Sharing state across go routines seems like something to be avoided. In vanilla Java it's always not easy to avoid sharing state.

View on HN

If you want your reads to ever work, then they need to be synchronized. Reading from an unfenced address during concurrent writes is undefined behavior for any CPU architecture you can think of, which means you’ll get stale reads _in a best-case scenario_. You can also get garbage reads (e.g. as your CPU interprets your read of a 64-bit pointer as two 32-bit reads), crashes, bees, etc.

The code you write is either thread-safe, used in a single-threaded context, or a pinless grenade.

There are exceptions to this rule--i.e. there are ways to not really be thread-safe but to have things work anyway--but they fall in the category of "you have to really, really know your CPU and be willing to write processor-specific code that just happens to work", so you can basically ignore them.

My favorite is the libdispatch abuse of cpuid to flood the pipeline on Intel CPUs for this problem: https://www.mikeash.com/pyblog/friday-qa-2014-06-06-secrets-...

But really, take Coda's advice. If you aren't synchronizing your reads, you can basically just assume your code is broken.

An interesting exception is Lamport's Bakery:

https://en.m.wikipedia.org/wiki/Lamport%27s_bakery_algorithm

Thread safety without synchronisation primitives. I'm not sure it's ever actually a good idea to use it, though.

EDIT: unless you count a fence as a synchronisation primitive.

If you're only reading/writing from one thread, then locking/unlocking a mutex should have an extremely small overhead. So if you plan on having such a pattern available for use in multithreaded contexts, a mutex or some other thread-safe abstraction couldn't hurt.

You would have to use sync.Atomic for all reads and writes. If you just use normal reads and writes it's in violation of the Go language memory model. It might work for now or it might start a nuclear war. You never know.

Reply on news.ycombinator.com