5b326a32ce
Phabric: https://reviews.freebsd.org/D1247 Reviewed by: jhb, avg Sponsored by: Spectra Logic Corporation sys/kern_subr_taskqueue.c: Modify taskqueue_drain_all() processing to use a temporary "barrier task", rather than rely on a user task that may be destroyed during taskqueue_drain_all()'s execution. The barrier task is queued behind all previously queued tasks and then has its priority elevated so that future tasks cannot pass it in the queue. Use a similar barrier scheme to drain threads processing current tasks. This requires taskqueue_run_locked() to insert and remove the taskqueue_busy object for the running thread for every task processed. share/man/man9/taskqueue.9: Remove warning about live-lock issues with taskqueue_drain_all() and indicate that it does not wait for tasks queued after it begins processing.
509 lines
15 KiB
Groff
509 lines
15 KiB
Groff
.\" -*- nroff -*-
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.\"
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.\" Copyright (c) 2000 Doug Rabson
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.\"
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.\" All rights reserved.
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.\"
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.\" This program is free software.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 4, 2015
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.Dt TASKQUEUE 9
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.Os
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.Sh NAME
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.Nm taskqueue
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.Nd asynchronous task execution
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/kernel.h
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.In sys/malloc.h
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.In sys/queue.h
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.In sys/taskqueue.h
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.Bd -literal
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typedef void (*task_fn_t)(void *context, int pending);
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typedef void (*taskqueue_enqueue_fn)(void *context);
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struct task {
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STAILQ_ENTRY(task) ta_link; /* link for queue */
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u_short ta_pending; /* count times queued */
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u_short ta_priority; /* priority of task in queue */
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task_fn_t ta_func; /* task handler */
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void *ta_context; /* argument for handler */
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};
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enum taskqueue_callback_type {
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TASKQUEUE_CALLBACK_TYPE_INIT,
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TASKQUEUE_CALLBACK_TYPE_SHUTDOWN,
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};
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typedef void (*taskqueue_callback_fn)(void *context);
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struct timeout_task;
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.Ed
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.Ft struct taskqueue *
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.Fn taskqueue_create "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
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.Ft struct taskqueue *
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.Fn taskqueue_create_fast "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context"
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.Ft int
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.Fn taskqueue_start_threads "struct taskqueue **tqp" "int count" "int pri" "const char *name" "..."
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.Ft int
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.Fo taskqueue_start_threads_pinned
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.Fa "struct taskqueue **tqp" "int count" "int pri" "int cpu_id"
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.Fa "const char *name" "..."
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.Fc
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.Ft void
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.Fn taskqueue_set_callback "struct taskqueue *queue" "enum taskqueue_callback_type cb_type" "taskqueue_callback_fn callback" "void *context"
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.Ft void
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.Fn taskqueue_free "struct taskqueue *queue"
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.Ft int
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.Fn taskqueue_enqueue "struct taskqueue *queue" "struct task *task"
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.Ft int
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.Fn taskqueue_enqueue_fast "struct taskqueue *queue" "struct task *task"
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.Ft int
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.Fn taskqueue_enqueue_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "int ticks"
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.Ft int
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.Fn taskqueue_cancel "struct taskqueue *queue" "struct task *task" "u_int *pendp"
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.Ft int
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.Fn taskqueue_cancel_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task" "u_int *pendp"
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.Ft void
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.Fn taskqueue_drain "struct taskqueue *queue" "struct task *task"
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.Ft void
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.Fn taskqueue_drain_timeout "struct taskqueue *queue" "struct timeout_task *timeout_task"
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.Ft void
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.Fn taskqueue_drain_all "struct taskqueue *queue"
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.Ft void
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.Fn taskqueue_block "struct taskqueue *queue"
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.Ft void
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.Fn taskqueue_unblock "struct taskqueue *queue"
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.Ft int
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.Fn taskqueue_member "struct taskqueue *queue" "struct thread *td"
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.Ft void
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.Fn taskqueue_run "struct taskqueue *queue"
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.Fn TASK_INIT "struct task *task" "int priority" "task_fn_t func" "void *context"
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.Fn TASK_INITIALIZER "int priority" "task_fn_t func" "void *context"
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.Fn TASKQUEUE_DECLARE "name"
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.Fn TASKQUEUE_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
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.Fn TASKQUEUE_FAST_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init"
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.Fn TASKQUEUE_DEFINE_THREAD "name"
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.Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
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.Fn TIMEOUT_TASK_INIT "struct taskqueue *queue" "struct timeout_task *timeout_task" "int priority" "task_fn_t func" "void *context"
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.Sh DESCRIPTION
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These functions provide a simple interface for asynchronous execution
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of code.
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.Pp
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The function
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.Fn taskqueue_create
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is used to create new queues.
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The arguments to
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.Fn taskqueue_create
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include a name that should be unique,
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a set of
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.Xr malloc 9
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flags that specify whether the call to
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.Fn malloc
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is allowed to sleep,
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a function that is called from
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.Fn taskqueue_enqueue
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when a task is added to the queue,
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and a pointer to the memory location where the identity of the
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thread that services the queue is recorded.
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.\" XXX The rest of the sentence gets lots in relation to the first part.
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The function called from
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.Fn taskqueue_enqueue
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must arrange for the queue to be processed
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(for instance by scheduling a software interrupt or waking a kernel
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thread).
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The memory location where the thread identity is recorded is used
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to signal the service thread(s) to terminate--when this value is set to
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zero and the thread is signaled it will terminate.
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If the queue is intended for use in fast interrupt handlers
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.Fn taskqueue_create_fast
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should be used in place of
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.Fn taskqueue_create .
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.Pp
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The function
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.Fn taskqueue_free
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should be used to free the memory used by the queue.
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Any tasks that are on the queue will be executed at this time after
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which the thread servicing the queue will be signaled that it should exit.
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.Pp
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Once a taskqueue has been created, its threads should be started using
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.Fn taskqueue_start_threads
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or
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.Fn taskqueue_start_threads_pinned .
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.Fn taskqueue_start_threads_pinned
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takes a
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.Va cpu_id
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argument which will cause the threads which are started for the taskqueue
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to be pinned to run on the given CPU.
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Callbacks may optionally be registered using
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.Fn taskqueue_set_callback .
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Currently, callbacks may be registered for the following purposes:
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.Bl -tag -width TASKQUEUE_CALLBACK_TYPE_SHUTDOWN
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.It Dv TASKQUEUE_CALLBACK_TYPE_INIT
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This callback is called by every thread in the taskqueue, before it executes
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any tasks.
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This callback must be set before the taskqueue's threads are started.
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.It Dv TASKQUEUE_CALLBACK_TYPE_SHUTDOWN
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This callback is called by every thread in the taskqueue, after it executes
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its last task.
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This callback will always be called before the taskqueue structure is
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reclaimed.
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.El
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.Pp
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To add a task to the list of tasks queued on a taskqueue, call
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.Fn taskqueue_enqueue
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with pointers to the queue and task.
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If the task's
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.Va ta_pending
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field is non-zero,
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then it is simply incremented to reflect the number of times the task
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was enqueued, up to a cap of USHRT_MAX.
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Otherwise,
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the task is added to the list before the first task which has a lower
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.Va ta_priority
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value or at the end of the list if no tasks have a lower priority.
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Enqueueing a task does not perform any memory allocation which makes
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it suitable for calling from an interrupt handler.
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This function will return
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.Er EPIPE
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if the queue is being freed.
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.Pp
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The function
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.Fn taskqueue_enqueue_fast
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should be used in place of
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.Fn taskqueue_enqueue
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when the enqueuing must happen from a fast interrupt handler.
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This method uses spin locks to avoid the possibility of sleeping in the fast
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interrupt context.
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.Pp
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When a task is executed,
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first it is removed from the queue,
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the value of
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.Va ta_pending
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is recorded and then the field is zeroed.
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The function
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.Va ta_func
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from the task structure is called with the value of the field
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.Va ta_context
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as its first argument
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and the value of
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.Va ta_pending
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as its second argument.
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After the function
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.Va ta_func
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returns,
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.Xr wakeup 9
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is called on the task pointer passed to
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.Fn taskqueue_enqueue .
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.Pp
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The
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.Fn taskqueue_enqueue_timeout
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is used to schedule the enqueue after the specified amount of
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.Va ticks .
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Only non-fast task queues can be used for
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.Va timeout_task
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scheduling.
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If the
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.Va ticks
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argument is negative, the already scheduled enqueueing is not re-scheduled.
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Otherwise, the task is scheduled for enqueueing in the future,
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after the absolute value of
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.Va ticks
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is passed.
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.Pp
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The
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.Fn taskqueue_cancel
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function is used to cancel a task.
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The
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.Va ta_pending
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count is cleared, and the old value returned in the reference
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parameter
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.Fa pendp ,
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if it is
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.Pf non- Dv NULL .
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If the task is currently running,
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.Dv EBUSY
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is returned, otherwise 0.
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To implement a blocking
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.Fn taskqueue_cancel
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that waits for a running task to finish, it could look like:
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.Bd -literal -offset indent
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while (taskqueue_cancel(tq, task, NULL) != 0)
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taskqueue_drain(tq, task);
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.Ed
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.Pp
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Note that, as with
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.Fn taskqueue_drain ,
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the caller is responsible for ensuring that the task is not re-enqueued
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after being canceled.
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.Pp
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Similarly, the
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.Fn taskqueue_cancel_timeout
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function is used to cancel the scheduled task execution.
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.Pp
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The
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.Fn taskqueue_drain
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function is used to wait for the task to finish, and
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the
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.Fn taskqueue_drain_timeout
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function is used to wait for the scheduled task to finish.
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There is no guarantee that the task will not be
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enqueued after call to
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.Fn taskqueue_drain .
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If the caller wants to put the task into a known state,
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then before calling
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.Fn taskqueue_drain
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the caller should use out-of-band means to ensure that the task
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would not be enqueued.
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For example, if the task is enqueued by an interrupt filter, then
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the interrupt could be disabled.
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.Pp
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The
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.Fn taskqueue_drain_all
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function is used to wait for all pending and running tasks that
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are enqueued on the taskqueue to finish.
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Tasks posted to the taskqueue after
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.Fn taskqueue_drain_all
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begins processing,
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including pending enqueues scheduled by a previous call to
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.Fn taskqueue_enqueue_timeout ,
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do not extend the wait time of
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.Fn taskqueue_drain_all
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and may complete after
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.Fn taskqueue_drain_all
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returns.
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.Pp
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The
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.Fn taskqueue_block
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function blocks the taskqueue.
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It prevents any enqueued but not running tasks from being executed.
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Future calls to
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.Fn taskqueue_enqueue
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will enqueue tasks, but the tasks will not be run until
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.Fn taskqueue_unblock
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is called.
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Please note that
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.Fn taskqueue_block
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does not wait for any currently running tasks to finish.
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Thus, the
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.Fn taskqueue_block
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does not provide a guarantee that
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.Fn taskqueue_run
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is not running after
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.Fn taskqueue_block
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returns, but it does provide a guarantee that
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.Fn taskqueue_run
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will not be called again
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until
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.Fn taskqueue_unblock
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is called.
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If the caller requires a guarantee that
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.Fn taskqueue_run
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is not running, then this must be arranged by the caller.
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Note that if
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.Fn taskqueue_drain
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is called on a task that is enqueued on a taskqueue that is blocked by
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.Fn taskqueue_block ,
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then
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.Fn taskqueue_drain
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can not return until the taskqueue is unblocked.
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This can result in a deadlock if the thread blocked in
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.Fn taskqueue_drain
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is the thread that is supposed to call
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.Fn taskqueue_unblock .
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Thus, use of
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.Fn taskqueue_drain
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after
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.Fn taskqueue_block
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is discouraged, because the state of the task can not be known in advance.
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The same caveat applies to
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.Fn taskqueue_drain_all .
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.Pp
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The
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.Fn taskqueue_unblock
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function unblocks the previously blocked taskqueue.
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All enqueued tasks can be run after this call.
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.Pp
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The
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.Fn taskqueue_member
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function returns
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.No 1
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if the given thread
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.Fa td
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is part of the given taskqueue
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.Fa queue
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and
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.No 0
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otherwise.
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.Pp
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The
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.Fn taskqueue_run
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function will run all pending tasks in the specified
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.Fa queue .
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Normally this function is only used internally.
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.Pp
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A convenience macro,
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.Fn TASK_INIT "task" "priority" "func" "context"
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is provided to initialise a
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.Va task
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structure.
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The
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.Fn TASK_INITIALIZER
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macro generates an initializer for a task structure.
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A macro
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.Fn TIMEOUT_TASK_INIT "queue" "timeout_task" "priority" "func" "context"
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initializes the
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.Va timeout_task
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structure.
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The values of
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.Va priority ,
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.Va func ,
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and
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.Va context
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are simply copied into the task structure fields and the
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.Va ta_pending
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field is cleared.
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.Pp
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Five macros
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.Fn TASKQUEUE_DECLARE "name" ,
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.Fn TASKQUEUE_DEFINE "name" "enqueue" "context" "init" ,
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.Fn TASKQUEUE_FAST_DEFINE "name" "enqueue" "context" "init" ,
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and
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.Fn TASKQUEUE_DEFINE_THREAD "name"
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.Fn TASKQUEUE_FAST_DEFINE_THREAD "name"
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are used to declare a reference to a global queue, to define the
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implementation of the queue, and declare a queue that uses its own thread.
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The
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.Fn TASKQUEUE_DEFINE
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macro arranges to call
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.Fn taskqueue_create
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with the values of its
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.Va name ,
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.Va enqueue
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and
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.Va context
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arguments during system initialisation.
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After calling
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.Fn taskqueue_create ,
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the
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.Va init
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argument to the macro is executed as a C statement,
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allowing any further initialisation to be performed
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(such as registering an interrupt handler etc.)
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.Pp
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The
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.Fn TASKQUEUE_DEFINE_THREAD
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macro defines a new taskqueue with its own kernel thread to serve tasks.
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The variable
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.Vt struct taskqueue *taskqueue_name
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is used to enqueue tasks onto the queue.
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.Pp
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.Fn TASKQUEUE_FAST_DEFINE
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|
and
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.Fn TASKQUEUE_FAST_DEFINE_THREAD
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|
act just like
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.Fn TASKQUEUE_DEFINE
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|
and
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.Fn TASKQUEUE_DEFINE_THREAD
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|
respectively but taskqueue is created with
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.Fn taskqueue_create_fast .
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|
.Ss Predefined Task Queues
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|
The system provides four global taskqueues,
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|
.Va taskqueue_fast ,
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|
.Va taskqueue_swi ,
|
|
.Va taskqueue_swi_giant ,
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|
and
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.Va taskqueue_thread .
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|
The
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.Va taskqueue_fast
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|
queue is for swi handlers dispatched from fast interrupt handlers,
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where sleep mutexes cannot be used.
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The swi taskqueues are run via a software interrupt mechanism.
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|
The
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.Va taskqueue_swi
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|
queue runs without the protection of the
|
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.Va Giant
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|
kernel lock, and the
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.Va taskqueue_swi_giant
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|
queue runs with the protection of the
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.Va Giant
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|
kernel lock.
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|
The thread taskqueue
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.Va taskqueue_thread
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runs in a kernel thread context, and tasks run from this thread do
|
|
not run under the
|
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.Va Giant
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|
kernel lock.
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|
If the caller wants to run under
|
|
.Va Giant ,
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|
he should explicitly acquire and release
|
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.Va Giant
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|
in his taskqueue handler routine.
|
|
.Pp
|
|
To use these queues,
|
|
call
|
|
.Fn taskqueue_enqueue
|
|
with the value of the global taskqueue variable for the queue you wish to
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|
use
|
|
.Va ( taskqueue_swi ,
|
|
.Va taskqueue_swi_giant ,
|
|
or
|
|
.Va taskqueue_thread ) .
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|
Use
|
|
.Fn taskqueue_enqueue_fast
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|
for the global taskqueue variable
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|
.Va taskqueue_fast .
|
|
.Pp
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|
The software interrupt queues can be used,
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|
for instance, for implementing interrupt handlers which must perform a
|
|
significant amount of processing in the handler.
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|
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.
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|
.Pp
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|
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.
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|
(e.g., call malloc with the M_WAITOK flag.)
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.Pp
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|
Note that tasks queued on shared taskqueues such as
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.Va taskqueue_swi
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|
may be delayed an indeterminate amount of time before execution.
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If queueing delays cannot be tolerated then a private taskqueue should
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be created with a dedicated processing thread.
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.Sh SEE ALSO
|
|
.Xr ithread 9 ,
|
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.Xr kthread 9 ,
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.Xr swi 9
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.Sh HISTORY
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This interface first appeared in
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.Fx 5.0 .
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There is a similar facility called work_queue in the Linux kernel.
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.Sh AUTHORS
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This manual page was written by
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.An Doug Rabson .
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