Solaris™ Internals: Core Kernel Components

3.6. Reader/Writer Locks

Reader/writer (RW) locks provide mutual exclusion semantics on write locks. Only one thread at a time is allowed to own the write lock, but there is concurrent access for readers. These locks are designed for scenarios where it is acceptable to have multiple threads reading the data at the same time, but only one writer. While a writer is holding the lock, no readers are allowed. Also, because of the wakeup mechanism, a writer lock is a better solution for kernel code segments that require relatively long hold times, as we will see shortly.

The basic mechanics of RW locks are similar to mutexes, in that RW locks have an initialization function (rw_init()), an entry function to acquire the lock (rw_enter()), and an exit ...