Mutual-Exclusion Locks

A mutual-exclusion lock, or mutex, is usually associated with a set of data and regulates access to that data. Mutexes provide a way to allow only one thread at a time access to that data.

Setting Up Mutexes

Device drivers usually allocate a mutex for each driver data structure. The mutex is typically a field in the structure and is of type kmutex_t. mutex_init(9F) is called to prepare the mutex for use. This is usually done at attach(9E) time for per-device mutexes and _init(9E) time for global driver mutexes.

For example,

	struct xxstate *xsp;
	...
 	mutex_init(&xsp->mu, "xx mutex", MUTEX_DRIVER, NULL);
 	...

For a more complete example of mutex initialization see Chapter 5, Autoconfiguration.

The driver must destroy the mutex with mutex_destroy(9F) before being unloaded. This is usually done at detach(9E) time for per-device mutexes and _fini(9E) time for global driver mutexes.

Using Mutexes

Every section of the driver code that needs to read or write the shared data structure must do the following:

• Acquire the mutex.

• Access the data.

• Release the mutex.

For example, to protect access to the busy flag in the state structure:

	...
 	mutex_enter(&xsp->mu);
 	xsp->busy = 0;
 	mutex_exit(&xsp->mu);
 	....

The scope of a mutex--the data it protects--is entirely up to the programmer. A mutex protects some particular data structure because the programmer chooses to do so and uses it accordingly. A mutex protects a data structure only if every code path that accesses the data structure does so while holding the mutex. For additional guidelines on using mutexes see Appendix G, Advanced Topics.