Snooping can be a good choice when the number of cache controllers is relatively small and there is not too much communication overhead.

In working on my final project, I found this link to be useful when thinking about mutexes. Interestingly, it talks about them in almost exactly the same way as we did caches:

http://stackoverflow.com/questions/18037216/what-are-the-differences-among-mutex-semaphore-and-read-write-locks