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Synchronization Primitives

This chapter is not intended as an introduction to synchronization. It is assumed that you have some understanding of the basic concepts of locks and semaphores already. If you need additional background reading, synchronization is covered in most introductory operating systems texts. However, since synchronization in the kernel is somewhat different from locking in an application this chapter does provide a brief overview to help ease the transition, or for experienced kernel developers, to refresh your memory.

As a Mac OS X kernel programmer, you have many choices of synchronization mechanisms at your disposal. The kernel itself provides two such mechanisms: locks and semaphores.

A lock is used for basic protection of shared resources. Multiple threads can attempt to acquire a lock, but only one thread can actually hold it at any given time (at least for traditional locks—more on this later). While that thread holds the lock, the other threads must wait. There are several different types of locks, differing mainly in what threads do while waiting to acquire them.

A semaphore is much like a lock, except that a finite number of threads can hold it simultaneously. Semaphores can be thought of as being much like piles of tokens. Multiple threads can take these tokens, but when there are none left, a thread must wait until another thread returns one. It is important to note that semaphores can be implemented in many different ways, so Mach semaphores may not behave in the same way as semaphores on other platforms.

In addition to locks and semaphores, certain low-level synchronization primitives like test and set are also available, along with a number of other atomic operations. These additional operations are described in libkern/gen/OSAtomicOperations.c in the kernel sources. Such atomic operations may be helpful if you do not need something as robust as a full-fledged lock or semaphore. Since they are not general synchronization mechanisms, however, they are beyond the scope of this chapter.

Contents:

Semaphores
Condition Variables
Locks

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