* Other locks built on top of spinlock and mutex

Atomic counters (e.g., used for refcounts):

struct atomic_counter {
	unsigned int count; // the "data" being protected by the lock below
	mutex m; // there's a variant in linux that uses a spinlock
}

A non-exclusive counter, called a read-write-semaphore (rwsem):

- protects a piece of data X
- allows multiple readers (of X) to enter the CS
- but only one writer can enter the CS and modify X
- has 4 APIs: lock_read, unlock_read, lock_write, unlock_write

- userful if you expect lots of threads reading the protected data, but only
  an occasional modification to the data.

- allowing multiple readers improves throughput (while maintaining data
  consistency).