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. The mutex functions are:

mutex_destroy(9F)

Releases any associated storage.

mutex_enter(9F)

Acquires a mutex.

mutex_exit(9F)

Releases a mutex.

mutex_init(9F)

Initializes a mutex.

mutex_owned(9F)

Tests to determine whether the mutex is held by the current thread. To be used in ASSERT(9F) only.

mutex_tryenter(9F)

Acquires a mutex if available, but does not block.

Setting Up Mutexes

Device drivers usually allocate a mutex for each driver data structure. The mutex is typically a field in the structure of type kmutex_t. mutex_init(9F) is called to prepare the mutex for use. This call is usually made 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, NULL, MUTEX_DRIVER, NULL);
/* ... */

For a more complete example of mutex initialization, see Chapter 6, Driver Autoconfiguration.

The driver must destroy the mutex with mutex_destroy(9F) before being unloaded. Destroying the mutex 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 tasks:

• Acquire the mutex

• Access the data

• Release the mutex

The scope of a mutex, that is, the data the mutex protects, is entirely up to the programmer. A mutex protects a data structure only if every code path that accesses the data structure does so while holding the mutex.