Writing Device Drivers

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.

Table 4-1 Mutex Routines




Initializes a mutex. 


Releases any associated storage. 


Acquires a mutex. 


Acquires a mutex if available; but does not block. 


Releases a mutex. 


Test sif the mutex is held by the current thread. To be used in ASSERT(9F) only. 

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:

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

 	xsp->busy = 0;

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.