The
callout_init() function initializes the callout handle
c for use. No operations can be performed on the callout before it is initialized. If the
flags argument is
CALLOUT_MPSAFE, the handler will be called without getting the global kernel lock. In this case it should only use functions that are multiprocessor safe.
callout_destroy() destroys the callout, preventing further use. It is provided as a diagnostic facility intended to catch bugs. To ensure future compatibility,
callout_destroy() should always be called when the callout is no longer required (for instance, when a device is being detached).
The
callout_reset() function resets and starts the timer associated with the callout handle
c. When the timer expires after
ticks/hz seconds, the function specified by
func will be called with the argument
arg. If the timer associated with the callout handle is already running, the callout will simply be rescheduled to execute at the newly specified time. Once the timer is started, the callout handle is marked as
PENDING. Once the timer expires, the handle is marked as
EXPIRED and
INVOKING, and the
PENDING status is cleared.
The
callout_setfunc() function sets the function and argument of the callout handle
c to
func and
arg respectively. The callout handle must already be initialized. If a callout will always be used with the same function and argument, then
callout_setfunc() used in conjunction with
callout_schedule() is slightly more efficient than using
callout_reset().
The
callout_stop() function requests that the timer associated with the callout handle
c be stopped. The
PENDING and
EXPIRED status for the callout handle is cleared. It is safe to call
callout_stop() on a callout handle that is not pending, so long as it is initialized.
callout_stop() will return a non-zero value if the callout was
EXPIRED. Note that
callout_stop() can return while the callout is running on a different CPU or at a different interrupt priority level on the current CPU. It can only be said to prevent the callout from firing in the future, unless explicitly re-scheduled. To stop a callout and wait for completion, use
callout_halt().
callout_halt() acts much like
callout_stop(), but waits for the callout to complete if it is currently in-flight.
callout_halt() may not be called from a hard interrupt handler as it will sleep if the callout is currently executing. If the callout can take locks (such as mutexes or RW locks), the caller of
callout_halt() must not hold any of those locks, otherwise the two could deadlock. To facilitate this,
callout_halt() can optionally release a single mutex specified by the
interlock parameter. If
interlock is not
NULL and the calling thread must wait for the callout to complete,
interlock will be released before waiting and re-acquired before returning. If no wait is required,
interlock will not be released. However, to avoid race conditions the caller should always assume that
interlock has been released and reacquired, and act accordingly.
The
callout_pending() function tests the
PENDING status of the callout handle
c. A
PENDING callout is one that has been started and whose function has not yet been called. Note that it is possible for a callout's timer to have expired without its function being called if interrupt level has not dropped low enough to let softclock interrupts through. Note that it is only safe to test
PENDING status when at softclock interrupt level or higher.
The
callout_expired() function tests to see if the callout's timer has expired and its function called.
The
callout_active() function returns true if a timer has been started but not explicitly stopped, even if it has already fired.
callout_active(
foo) is logically the same as
callout_pending(
foo) ||
callout_expired(
foo); it is implemented as a separate function for compatibility with
FreeBSD and for the special case of
TCP_TIMER_ISARMED(). Its use is not recommended.
The
callout_invoking() function tests the
INVOKING status of the callout handle
c. This flag is set just before a callout's function is being called. Since the priority level is lowered prior to invocation of the callout function, other pending higher-priority code may run before the callout function is allowed to run. This may create a race condition if this higher-priority code deallocates storage containing one or more callout structures whose callout functions are about to be run. In such cases, one technique to prevent references to deallocated storage would be to test whether any callout functions are in the
INVOKING state using
callout_invoking(), and if so, to mark the data structure and defer storage deallocation until the callout function is allowed to run. For this handshake protocol to work, the callout function will have to use the
callout_ack() function to clear this flag.
The
callout_ack() function clears the
INVOKING state in the callout handle
c. This is used in situations where it is necessary to protect against the race condition described under
callout_invoking().