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36 .Nd asynchronous task execution
44 typedef void (*task_fn_t)(void *context, int pending);
46 typedef void (*taskqueue_enqueue_fn)(void *context);
49 STAILQ_ENTRY(task) ta_link; /* link for queue */
50 u_short ta_pending; /* count times queued */
51 u_short ta_priority; /* priority of task in queue */
52 task_fn_t ta_func; /* task handler */
53 void *ta_context; /* argument for handler */
58 .Ft struct taskqueue *
59 .Fn taskqueue_create "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
60 .Ft struct taskqueue *
61 .Fn taskqueue_create_fast "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
63 .Fn taskqueue_free "struct taskqueue *queue"
65 .Fn taskqueue_enqueue "struct taskqueue *queue" "struct task *task"
67 .Fn taskqueue_enqueue_fast "struct taskqueue *queue" "struct task *task"
69 .Fn taskqueue_enqueue_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "int ticks"
71 .Fn taskqueue_cancel "struct taskqueue *queue" "struct task *task" "u_int *pendp"
73 .Fn taskqueue_cancel_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "u_int *pendp"
75 .Fn taskqueue_drain "struct taskqueue *queue" "struct task *task"
77 .Fn taskqueue_drain_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task"
79 .Fn taskqueue_member "struct taskqueue *queue" "struct thread *td"
81 .Fn taskqueue_run "struct taskqueue *queue"
82 .Fn TASK_INIT "struct task *task" "int priority" "task_fn_t func" "void *context"
83 .Fn TASK_INITIALIZER "int priority" "task_fn_t func" "void *context"
84 .Fn TASKQUEUE_DECLARE "name"
85 .Fn TASKQUEUE_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
86 .Fn TASKQUEUE_FAST_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
87 .Fn TASKQUEUE_DEFINE_THREAD "name"
88 .Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
89 .Fn TIMEOUT_TASK_INIT "struct taskqueue *queue" "struct timeout_task *timeout_task" "int priority" "task_fn_t func" "void *context"
91 These functions provide a simple interface for asynchronous execution
96 is used to create new queues.
99 include a name that should be unique,
102 flags that specify whether the call to
105 a function that is called from
106 .Fn taskqueue_enqueue
107 when a task is added to the queue,
108 and a pointer to the memory location where the identity of the
109 thread that services the queue is recorded.
110 .\" XXX The rest of the sentence gets lots in relation to the first part.
111 The function called from
112 .Fn taskqueue_enqueue
113 must arrange for the queue to be processed
114 (for instance by scheduling a software interrupt or waking a kernel
116 The memory location where the thread identity is recorded is used
117 to signal the service thread(s) to terminate--when this value is set to
118 zero and the thread is signaled it will terminate.
119 If the queue is intended for use in fast interrupt handlers
120 .Fn taskqueue_create_fast
121 should be used in place of
122 .Fn taskqueue_create .
126 should be used to free the memory used by the queue.
127 Any tasks that are on the queue will be executed at this time after
128 which the thread servicing the queue will be signaled that it should exit.
130 To add a task to the list of tasks queued on a taskqueue, call
131 .Fn taskqueue_enqueue
132 with pointers to the queue and task.
136 then it is simply incremented to reflect the number of times the task
137 was enqueued, up to a cap of USHRT_MAX.
139 the task is added to the list before the first task which has a lower
141 value or at the end of the list if no tasks have a lower priority.
142 Enqueueing a task does not perform any memory allocation which makes
143 it suitable for calling from an interrupt handler.
144 This function will return
146 if the queue is being freed.
149 .Fn taskqueue_enqueue_fast
150 should be used in place of
151 .Fn taskqueue_enqueue
152 when the enqueuing must happen from a fast interrupt handler.
153 This method uses spin locks to avoid the possibility of sleeping in the fast
156 When a task is executed,
157 first it is removed from the queue,
160 is recorded and then the field is zeroed.
163 from the task structure is called with the value of the field
165 as its first argument
168 as its second argument.
173 is called on the task pointer passed to
174 .Fn taskqueue_enqueue .
177 .Fn taskqueue_enqueue_timeout
178 is used to schedule the enqueue after the specified amount of
180 Only non-fast task queues can be used for
185 argument is negative, the already scheduled enqueueing is not re-scheduled.
186 Otherwise, the task is scheduled for enqueueing in the future,
187 after the absolute value of
193 function is used to cancel a task.
196 count is cleared, and the old value returned in the reference
201 If the task is currently running,
203 is returned, otherwise 0.
204 To implement a blocking
206 that waits for a running task to finish, it could look like:
207 .Bd -literal -offset indent
208 while (taskqueue_cancel(tq, task, NULL) != 0)
209 taskqueue_drain(tq, task);
213 .Fn taskqueue_drain ,
214 the caller is responsible for ensuring that the task is not re-enqueued
215 after being canceled.
218 .Fn taskqueue_cancel_timeout
219 function is used to cancel the scheduled task execution.
223 function is used to wait for the task to finish, and
225 .Fn taskqueue_drain_timeout
226 function is used to wait for the scheduled task to finish.
227 There is no guarantee that the task will not be
228 enqueued after call to
229 .Fn taskqueue_drain .
237 is part of the given taskqueue
245 function will run all pending tasks in the specified
247 Normally this function is only used internally.
250 .Fn TASK_INIT "task" "priority" "func" "context"
251 is provided to initialise a
256 macro generates an initializer for a task structure.
258 .Fn TIMEOUT_TASK_INIT "queue" "timeout_task" "priority" "func" "context"
267 are simply copied into the task structure fields and the
272 .Fn TASKQUEUE_DECLARE "name" ,
273 .Fn TASKQUEUE_DEFINE "name" "enqueue" "context" "init" ,
274 .Fn TASKQUEUE_FAST_DEFINE "name" "enqueue" "context" "init" ,
276 .Fn TASKQUEUE_DEFINE_THREAD "name"
277 .Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
278 are used to declare a reference to a global queue, to define the
279 implementation of the queue, and declare a queue that uses its own thread.
282 macro arranges to call
284 with the values of its
289 arguments during system initialisation.
291 .Fn taskqueue_create ,
294 argument to the macro is executed as a C statement,
295 allowing any further initialisation to be performed
296 (such as registering an interrupt handler etc.)
299 .Fn TASKQUEUE_DEFINE_THREAD
300 macro defines a new taskqueue with its own kernel thread to serve tasks.
302 .Vt struct taskqueue *taskqueue_name
303 is used to enqueue tasks onto the queue.
305 .Fn TASKQUEUE_FAST_DEFINE
307 .Fn TASKQUEUE_FAST_DEFINE_THREAD
311 .Fn TASKQUEUE_DEFINE_THREAD
312 respectively but taskqueue is created with
313 .Fn taskqueue_create_fast .
314 .Ss Predefined Task Queues
315 The system provides four global taskqueues,
318 .Va taskqueue_swi_giant ,
320 .Va taskqueue_thread .
323 queue is for swi handlers dispatched from fast interrupt handlers,
324 where sleep mutexes cannot be used.
325 The swi taskqueues are run via a software interrupt mechanism.
328 queue runs without the protection of the
331 .Va taskqueue_swi_giant
332 queue runs with the protection of the
337 runs in a kernel thread context, and tasks run from this thread do
341 If the caller wants to run under
343 he should explicitly acquire and release
345 in his taskqueue handler routine.
349 .Fn taskqueue_enqueue
350 with the value of the global taskqueue variable for the queue you wish to
352 .Va ( taskqueue_swi ,
353 .Va taskqueue_swi_giant ,
355 .Va taskqueue_thread ) .
357 .Fn taskqueue_enqueue_fast
358 for the global taskqueue variable
361 The software interrupt queues can be used,
362 for instance, for implementing interrupt handlers which must perform a
363 significant amount of processing in the handler.
364 The hardware interrupt handler would perform minimal processing of the
365 interrupt and then enqueue a task to finish the work.
366 This reduces to a minimum
367 the amount of time spent with interrupts disabled.
369 The thread queue can be used, for instance, by interrupt level routines
370 that need to call kernel functions that do things that can only be done
371 from a thread context.
372 (e.g., call malloc with the M_WAITOK flag.)
374 Note that tasks queued on shared taskqueues such as
376 may be delayed an indeterminate amount of time before execution.
377 If queueing delays cannot be tolerated then a private taskqueue should
378 be created with a dedicated processing thread.
384 This interface first appeared in
386 There is a similar facility called work_queue in the Linux kernel.
388 This manual page was written by