Signaling the Condition

Follow these steps in the code path to signal 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.

The following example 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);
}