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typedef void (*task_fn_t)(void *context, int pending);

typedef void (*taskqueue_enqueue_fn)(void *context);

struct task {
	STAILQ_ENTRY(task)	ta_link;	/* link for queue */
	int			ta_pending;	/* count times queued */
	int			ta_priority;	/* priority of task in queue */
	task_fn_t		ta_func;	/* task handler */
	void			*ta_context;	/* argument for handler */
};

DESCRIPTION

The function taskqueue_create() is used to create new queues. The arguments to taskqueue_create() include a name that should be unique, a set of kmalloc(9) flags that specify whether the call to malloc() is allowed to sleep, and a function which is called from taskqueue_enqueue() when a task is added to the queue to allow the queue to arrange to be run later (for instance by scheduling a software interrupt or waking a kernel thread).

The function taskqueue_free() should be used to remove the queue from the global list of queues and free the memory used by the queue. Any tasks that are on the queue will be executed at this time.

The system maintains a list of all queues which can be searched using taskqueue_find(). The first queue whose name matches is returned, otherwise NULL.

To add a task to the list of tasks queued on a taskqueue, call taskqueue_enqueue() with pointers to the queue and task. If the task's ta_pending field is non-zero, then it is simply incremented to reflect the number of times the task was enqueued. Otherwise, the task is added to the list before the first task which has a lower ta_priority value or at the end of the list if no tasks have a lower priority. Enqueueing a task does not perform any memory allocation which makes it suitable for calling from an interrupt handler. This function will return EPIPE if the queue is being freed.

To execute all the tasks on a queue, call taskqueue_run(). When a task is executed, first it is removed from the queue, the value of ta_pending is recorded and then the field is zeroed. The function ta_func from the task structure is called with the value of the field ta_context as its first argument and the value of ta_pending as its second argument.

The taskqueue_enqueue_timeout() is used to schedule the enqueue after the specified amount of ticks. If the ticks argument is negative, the already scheduled enqueueing is not re-scheduled. Otherwise, the task is scheduled for enqueueing in the future, after the absolute value of ticks is passed.

The taskqueue_cancel() function is used to cancel a task. The ta_pending count is cleared, and the old value returned in the reference parameter pendp, if it is non-NULL. If the task is currently running, EBUSY is returned, otherwise 0. To implement a blocking taskqueue_cancel() that waits for a running task to finish, it could look like:

while (taskqueue_cancel(tq, task, NULL) != 0)
	taskqueue_drain(tq, task);

Note that, as with taskqueue_drain(), the caller is responsible for ensuring that the task is not re-enqueued after being canceled.

Similarly, the taskqueue_cancel_timeout() function is used to cancel the scheduled task execution.

The taskqueue_drain() function is used to wait for the task to finish, and the taskqueue_drain_timeout() function is used to wait for the scheduled task to finish. There is no guarantee that the task will not be enqueued after call to taskqueue_drain().

The taskqueue_block() function is used to block a taskqueue. When a taskqueue is blocked, calls to taskqueue_enqueue() will still enqueue tasks but they will not be run until the taskqueue is unblocked by calling taskqueue_unblock().

The taskqueue_start_threads() function is used to create and start count dedicated threads for the taskqueue specified by tqp. These threads will be created with the priority specified by pri and the name given by fmt with _N appended to it, where N is the number of the thread. If count > 1 and ncpu is -1, each of the count threads will be allocated to a different CPU among all available CPUs in a round robin fashion. The taskqueue specified by tqp must be created previously by calling taskqueue_create() with the argument enqueue set to taskqueue_thread_enqueue.

A convenience macro, TASK_INIT() is provided to initialise a task structure. The values of priority, func, and context are simply copied into the task structure fields and the ta_pending field is cleared.

Two macros, TASKQUEUE_DECLARE() and TASKQUEUE_DEFINE() are used to declare a reference to a global queue, and to define the implementation of the queue. The TASKQUEUE_DEFINE() macro arranges to call taskqueue_create() with the values of its name, enqueue and context arguments during system initialisation. After calling taskqueue_create(), the init argument to the macro is executed as a C statement, allowing any further initialisation to be performed (such as registering an interrupt handler etc.)

The system provides two global taskqueues, taskqueue_swi and taskqueue_swi_mp, which are run via a software interrupt mechanism. To use these queues, call taskqueue_enqueue() with the value of the global variable taskqueue_swi or taskqueue_swi_mp.

While taskqueue_swi acquires the mplock for its tasks, taskqueue_swi_mp is intended for mpsafe tasks and no mplock will be acquired for them. These queues can be used, for instance, for implementing interrupt handlers which must perform a significant amount of processing in the handler. The hardware interrupt handler would perform minimal processing of the interrupt and then enqueue a task to finish the work. This reduces to a minimum the amount of time spent with interrupts disabled.