Thread Synchronization

In addition to protecting shared data, drivers often need to synchronize execution among multiple threads.

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.

The condvar(9F) functions are:

cv_broadcast(9F): Signals all threads waiting on the condition variable.
cv_destroy(9F): Destroys a condition variable.
cv_init(9F): Initializes a condition variable.
cv_signal(9F): Signals one thread waiting on the condition variable.
cv_timedwait(9F): Waits for condition, time-out, or signal. See Threads Unable to Receive Signals.
cv_timedwait_sig(9F): Waits for condition or time-out.
cv_wait(9F): Waits for condition.
cv_wait_sig(9F): Waits for condition or return zero on receipt of a signal. See Threads Unable to Receive Signals.

Initializing Condition Variables

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

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

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

Waiting for the Condition

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

Acquire the mutex guarding the condition.
Test the condition.
If the test results do not allow the thread to continue, use cv_wait(9F) to block the current thread on the condition. The cv_wait(9F) function releases the mutex before blocking the thread and reacquires the mutex before returning. On return from cv_wait(9F), repeat the test.
After the test allows the thread to continue, set the condition to its new value. For example, set a device flag to busy.
Release the mutex.

Signaling the Condition

Follow these steps in the code path to signal the condition:

Acquire the mutex guarding the condition.
Set the condition.
Signal the blocked thread with cv_broadcast(9F).
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);
}

`cv_wait()` and `cv_timedwait()` Functions

If a thread is blocked on a condition with cv_wait(9F) and that condition does not occur, the thread would wait forever. To avoid that situation, use cv_timedwait(9F), which depends upon another thread to perform a wakeup. cv_timedwait() takes an absolute wait time as an argument. cv_timedwait() returns -1 if the time is reached and the event has not occurred. cv_timedwait() returns a positive value if the condition is met.

cv_timedwait(9F) requires an absolute wait time expressed in clock ticks since the system was last rebooted. The wait time can be determined by retrieving the current value with ddi_get_lbolt(9F). The driver usually has a maximum number of seconds or microseconds to wait, so this value is converted to clock ticks with drv_usectohz(9F) and added to the value from ddi_get_lbolt(9F).

The following example shows how to use cv_timedwait(9F) to wait up to five seconds to access the device before returning EIO to the caller.

Example 3-2 Using `cv_timedwait()`

clock_t            cur_ticks, to;
mutex_enter(&xsp->mu);
while (xsp->busy) {
        cur_ticks = ddi_get_lbolt();
        to = cur_ticks + drv_usectohz(5000000); /* 5 seconds from now */
        if (cv_timedwait(&xsp->cv, &xsp->mu, to) == -1) {
                /*
                 * The timeout time 'to' was reached without the
                 * condition being signaled.
                 */
                /* tidy up and exit */
                mutex_exit(&xsp->mu);
                return (EIO);
        }
}
xsp->busy = 1;
mutex_exit(&xsp->mu);

Although device driver writers generally prefer to use cv_timedwait(9F) over cv_wait(9F), sometimes cv_wait(9F) is a better choice. For example, cv_wait(9F) is better if a driver is waiting on the following conditions:

Internal driver state changes, where such a state change might require some command to be executed, or a set amount of time to pass
Something the driver needs to single-thread
Some situation that is already managing a possible timeout, as when “A” depends on “B,” and “B” is using cv_timedwait(9F)

`cv_wait_sig()` Function

A driver might be waiting for a condition that cannot occur or will not happen for a long time. In such cases, the user can send a signal to abort the thread. Depending on the driver design, the signal might not cause the driver to wake up.

cv_wait_sig(9F) allows a signal to unblock the thread. This capability enables the user to break out of potentially long waits by sending a signal to the thread with kill(1) or by typing the interrupt character. cv_wait_sig(9F) returns zero if it is returning because of a signal, or nonzero if the condition occurred. However, see Threads Unable to Receive Signals for cases in which signals might not be received.

The following example shows how to use cv_wait_sig(9F) to allow a signal to unblock the thread.

Example 3-3 Using `cv_wait_sig()`

mutex_enter(&xsp->mu);
while (xsp->busy) {
        if (cv_wait_sig(&xsp->cv, &xsp->mu) == 0) {
        /* Signaled while waiting for the condition */
                /* tidy up and exit */
                mutex_exit(&xsp->mu);
                return (EINTR);
        }
}
xsp->busy = 1;
mutex_exit(&xsp->mu);

`cv_timedwait_sig()` Function

cv_timedwait_sig(9F) is similar to cv_timedwait(9F) and cv_wait_sig(9F), except that cv_timedwait_sig() returns -1 without the condition being signaled after a timeout has been reached, or 0 if a signal (for example, kill(2)) is sent to the thread.

For both cv_timedwait(9F) and cv_timedwait_sig(9F), time is measured in absolute clock ticks since the last system reboot.