Mutual Exclusion Compared to Waiting

Mutual Exclusion Compared to Waiting

Mutual exclusion allows one entity to have exclusive use of a global resource, temporarily denying use of the resource to other entities. When software is well-designed, mutual exclusion normally does not require waiting--the resource is normally free when it is requested. A driver that calls a mutual exclusion function expects to proceed without delay--although there is a chance that the resource is in use, and the driver will have to wait.

The kernel offers an array of functions for mutual exclusion, and the choice among them can be critical to performance. The functions are reviewed in the following topics:

"Basic Locks" covers basic locks and mutex locks, the best locks for multiprocessor use.
"Long-Term Locks" covers sleep locks, which can be held for longer periods.
"Reader/Writer Locks" covers a class of locks that allow multiple, concurrent, read-only access to resources that are infrequently changed.
"Priority Level Functions" covers the traditional UNIX method of mutual exclusion, the splhi() and splx() functions, which have many disadvantages.

Waiting allows a driver to coordinate its actions with a specific event or action that occurs asynchronously. A driver can wait for a specified amount of time to pass, wait for an I/O action to complete, and so on. Therefore, a driver that calls a waiting function expects to wait for something to happen--although there is a chance that the expected event has already happened, and the driver will be able to continue at once.

The kernel offers several functions that allow you to wait for specific events; and also offers functions for general synchronization. These are covered in the following topics:

"Waiting for Time to Pass" covers timer-related functions.
"Waiting for Memory to Become Available" covers memory allocation waits.
"Waiting for Block I/O to Complete" covers waits used in the pfxstrategy() entry point.
"Waiting for a General Event" covers the general-purpose functions that you can adapt to any synchronization problem.

The most general facility, the semaphore, can be used for synchronization and for locking. This topic is covered under "Semaphores".