Condition Variables in Thread Synchronization

Condition variables are a standard form of thread synchronization. They are designed to be used with mutexes. The associated mutex is used to ensure that a condition can be checked atomically, and that the thread can block on the associated condition variable without missing either a change to the condition or a signal that the condition has changed.

Table 3–4 lists the condvar(9F) interfaces.

Initializing Condition Variables

Declare a condition variable (type kcondvar_t) for each condition. Usually, this is done in the driver's soft-state structure. Use cv_init(9F) to initialize each one. Similar to mutexes, condition variables are usually initialized at attach(9E) time. For example:

cv_init(&xsp->cv, NULL, CV_DRIVER, NULL);

For a more complete example of condition variable initialization see Chapter 5, Driver Autoconfiguration.

Waiting for the Condition

To use condition variables, follow these steps in the code path waiting for the condition:

1. Acquire the mutex guarding the condition.

2. Test the condition.

3. If the test results do not allow the thread to continue, use cv_wait(9F) to block the current thread on the condition. cv_wait(9F) releases the mutex before blocking. Upon return from cv_wait(9F) (which will reacquire the mutex before returning), repeat the test.

4. Once the test allows the thread to continue, set the condition to its new value. For example, set a device flag to busy.

5. Release the mutex.

Signaling the Condition

Follow these steps in the code path signaling the condition:

1. Acquire the mutex guarding the condition.

2. Set the condition.

3. Signal the blocked thread with cv_broadcast(9F).

4. Release the mutex.

Example 3–1 uses a busy flag along with mutex and condition variables to force the read(9E) routine to wait until the device is no longer busy before starting a transfer.

Example 3–1 Using Mutexes and Condition Variables

static int
xxread(dev_t dev, struct uio *uiop, cred_t *credp)
{
        struct xxstate *xsp;
        ...
        mutex_enter(&xsp->mu);
        while (xsp->busy)
                cv_wait(&xsp->cv, &xsp->mu);
        xsp->busy = 1;
        mutex_exit(&xsp->mu);
        /* perform the data access */
}

static uint_t
xxintr(caddr_t arg)
{
        struct xxstate *xsp = (struct xxstate *)arg;
        mutex_enter(&xsp->mu);
        xsp->busy = 0;
        cv_broadcast(&xsp->cv);
        mutex_exit(&xsp->mu);
}