A readers/writer lock regulates access to a set of data. The readers/writer lock is so called because many threads can hold the lock simultaneously for reading, but only one thread can hold it for writing.
Most device drivers do not use readers/writer locks. These locks are slower than mutexes and provide a performance gain only when protecting data that is not frequently written but is commonly read by many concurrent threads. In this case, contention for a mutex could become a bottleneck, so using a readers/writer lock might be more efficient. See rwlock(9F) for more information.