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TASKQUEUE(9)		 BSD Kernel Developer's Manual		  TASKQUEUE(9)

     taskqueue — asynchronous task execution

     #include <sys/param.h>
     #include <sys/kernel.h>
     #include <sys/malloc.h>
     #include <sys/queue.h>
     #include <sys/taskqueue.h>

     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 */
	     u_short		     ta_pending;     /* count times queued */
	     u_short		     ta_priority;    /* priority of task in queue */
	     task_fn_t		     ta_func;	     /* task handler */
	     void		     *ta_context;    /* argument for handler */

     struct taskqueue *
     taskqueue_create(const char *name, int mflags,
	 taskqueue_enqueue_fn enqueue, void *context);

     struct taskqueue *
     taskqueue_create_fast(const char *name, int mflags,
	 taskqueue_enqueue_fn enqueue, void *context);

     taskqueue_free(struct taskqueue *queue);

     taskqueue_enqueue(struct taskqueue *queue, struct task *task);

     taskqueue_enqueue_fast(struct taskqueue *queue, struct task *task);

     taskqueue_run(struct taskqueue *queue);

     taskqueue_run_fast(struct taskqueue *queue);

     taskqueue_drain(struct taskqueue *queue, struct task *task);

     taskqueue_member(struct taskqueue *queue, struct thread *td);

     TASK_INIT(struct task *task, int priority, task_fn_t *func,
	 void *context);


     TASKQUEUE_DEFINE(name, taskqueue_enqueue_fn enqueue, void *context,

     TASKQUEUE_FAST_DEFINE(name, taskqueue_enqueue_fn enqueue, void *context,



     These functions provide a simple interface for asynchronous execution of

     The function taskqueue_create() is used to create new queues.  The argu‐
     ments to taskqueue_create() include a name that should be unique, a set
     of malloc(9) flags that specify whether the call to malloc() is allowed
     to sleep, a function that is called from taskqueue_enqueue() when a task
     is added to the queue, and a pointer to the memory location where the
     identity of the thread that services the queue is recorded.  The function
     called from taskqueue_enqueue() must arrange for the queue to be pro‐
     cessed (for instance by scheduling a software interrupt or waking a ker‐
     nel thread).  The memory location where the thread identity is recorded
     is used to signal the service thread(s) to terminate--when this value is
     set to zero and the thread is signaled it will terminate.	If the queue
     is intended for use in fast interrupt handlers taskqueue_create_fast()
     should be used in place of taskqueue_create().

     The function taskqueue_free() should be used to free the memory used by
     the queue.	 Any tasks that are on the queue will be executed at this time
     after which the thread servicing the queue will be signaled that it
     should exit.

     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.

     The function taskqueue_enqueue_fast() should be used in place of
     taskqueue_enqueue() when the enqueuing must happen from a fast interrupt
     handler.  This method uses spin locks to avoid the possibility of sleep‐
     ing in the fast interrupt context.

     To execute all the tasks on a queue, call taskqueue_run() or
     taskqueue_run_fast() depending on the flavour of the queue.  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.	 After
     the function ta_func returns, wakeup(9) is called on the task pointer
     passed to taskqueue_enqueue().

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

     The taskqueue_member() function returns 1 if the given thread td is part
     of the given taskqeueue queue and 0 otherwise.

     A convenience macro, TASK_INIT(task, priority, func, context) 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.

     Five macros TASKQUEUE_DECLARE(name), TASKQUEUE_DEFINE(name, enqueue,
     context, init), TASKQUEUE_FAST_DEFINE(name, enqueue, context, init), and
     to declare a reference to a global queue, to define the implementation of
     the queue, and declare a queue that uses its own thread.  The
     TASKQUEUE_DEFINE() macro arranges to call taskqueue_create() with the
     values of its name, enqueue and context arguments during system initiali‐
     sation.  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 TASKQUEUE_DEFINE_THREAD() macro defines a new taskqueue with its own
     kernel thread to serve tasks.  The variable struct taskqueue
     *taskqueue_name is used to enqueue tasks onto the queue.

     taskqueue is created with taskqueue_create_fast().

   Predefined Task Queues
     The system provides four global taskqueues, taskqueue_fast,
     taskqueue_swi, taskqueue_swi_giant, and taskqueue_thread.	The
     taskqueue_fast queue is for swi handlers dispatched from fast interrupt
     handlers, where sleep mutexes cannot be used.  The swi taskqueues are run
     via a software interrupt mechanism.  The taskqueue_swi queue runs without
     the protection of the Giant kernel lock, and the taskqueue_swi_giant
     queue runs with the protection of the Giant kernel lock.  The thread
     taskqueue taskqueue_thread runs in a kernel thread context, and tasks run
     from this thread do not run under the Giant kernel lock.  If the caller
     wants to run under Giant, he should explicitly acquire and release Giant
     in his taskqueue handler routine.

     To use these queues, call taskqueue_enqueue() with the value of the
     global taskqueue variable for the queue you wish to use (taskqueue_swi,
     taskqueue_swi_giant, or taskqueue_thread).	 Use taskqueue_enqueue_fast()
     for the global taskqueue variable taskqueue_fast.

     The software interrupt 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 dis‐

     The thread queue can be used, for instance, by interrupt level routines
     that need to call kernel functions that do things that can only be done
     from a thread context.  (e.g., call malloc with the M_WAITOK flag.)

     Note that tasks queued on shared taskqueues such as taskqueue_swi may be
     delayed an indeterminate amount of time before execution.	If queueing
     delays cannot be tolerated then a private taskqueue should be created
     with a dedicated processing thread.

     ithread(9), kthread(9), swi(9)

     This interface first appeared in FreeBSD 5.0.  There is a similar facil‐
     ity called tqueue in the Linux kernel.

     This manual page was written by Doug Rabson.

BSD				August 18, 2009				   BSD

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