TASK_ADD(9) | Kernel Developer's Manual | TASK_ADD(9)

# NAME

`taskq_create`, `taskq_destroy`, `taskq_barrier`, `taskq_del_barrier`, `task_set`, `task_add`, `task_del`, `task_pending`, `TASK_INITIALIZER` — task queues

# SYNOPSIS

`#include <sys/task.h>`

struct taskq *
`taskq_create`(const char *name, unsigned int nthreads, int ipl, unsigned int flags);

void
`taskq_destroy`(struct taskq *tq);

void
`taskq_barrier`(struct taskq *tq);

void
`taskq_del_barrier`(struct taskq *tq, struct task *t);

void
`task_set`(struct task *t, void (*fn)(void *), void *arg);

int
`task_add`(struct taskq *tq, struct task *t);

int
`task_del`(struct taskq *tq, struct task *t);

int
`task_pending`(struct task *t);

extern struct taskq *const systq;
extern struct taskq *const systqmp;

`TASK_INITIALIZER`(void (*fn)(void *), void *arg);

# DESCRIPTION

The taskq API provides a mechanism to defer work to a process context.

`taskq_create`() allocates a taskq and a set of threads to be used to complete work that would be inappropriate for the shared system taskq. The name argument specifies the name of the kernel threads that are created to service the work on the taskq. nthreads specifies the number of threads that will be created to handle the work. ipl specifies the highest interrupt protection level at which `task_add`() and `task_del`() will be called against the created taskq. See spl(9) for a list of the IPLs. The operational characteristics of the taskq can be altered by OR'ing the following defines into the flags argument:

`TASKQ_MPSAFE`
  The threads servicing the taskq will be run without the kernel big lock.

`taskq_destroy`() causes the resources associated with a previously created taskq to be freed. It will wait till all the tasks in the work queue are completed before returning. Calling `taskq_destroy`() against the system taskq is an error and will lead to undefined behaviour or a system fault.

`taskq_barrier`() guarantees that any task that was running on the tq taskq when the barrier was called has finished by the time the barrier returns.

`taskq_del_barrier`() either removes t from the list of pending tasks on the tq taskq, or waits until any running task has completed.

The caller of `taskq_barrier`() or `taskq_del_barrier`() must not hold locks that can block the taskq. Otherwise, the system will deadlock.

It is the responsibility of the caller to provide the `task_set`(), `task_add`(), and `task_del`() functions with pre-allocated task structures.

`task_set`() prepares the task structure t to be used in future calls to `task_add`() and `task_del`(). t will be prepared to call the function fn with the argument specified by arg. Once initialised, the t structure can be used repeatedly in calls to `task_add`() and `task_del`() and does not need to be reinitialised unless the function called and/or its argument must change.

`task_add`() schedules the execution of the work specified by the task structure t on the tq taskq. The task structure must already be initialised by `task_set`().

`task_del`() will remove the task structure t from the taskq tq. If the work was already executed or has not been added to the taskq, the call will have no effect. Calling `task_del`() against a different taskq than the one given in a previous call to `task_add`() is an error and will lead to undefined behaviour.

The kernel provides two system taskqs: systq, which executes while holding the kernel lock, and systqmp, which does not hold the kernel lock during execution. They can both be used by any subsystem for short lived tasks. They are serviced by a single thread and can therefore provide predictable ordering of work. Work can be scheduled on the system taskqs from callers at or below IPL_HIGH.

The `task_pending`() macro can be used to check if a task is scheduled to run.

A task declaration can be initialised with the `TASK_INITIALIZER`() macro. The task will be prepared to call the function specified by the fn argument with the void * argument given in arg.

# CONTEXT

`taskq_create`() and `taskq_destroy`() can be called during autoconf, or from process context. `taskq_barrier`() and `taskq_del_barrier`() can be called from process context. `task_set`(), `task_add`(), `task_del`(), and `task_pending`() can be called during autoconf, from process context, or from interrupt context.

# RETURN VALUES

`taskq_create`() returns a pointer to a taskq structure on success or `NULL` on failure.

`task_add`() will return 1 if the task t was added to the taskq tq or 0 if the task was already queued.

`task_del`() will return 1 if the task t was removed from the taskq tq or 0 if the task was not already on the queue.

`task_pending`() will return non-zero if the task is queued to run, or 0 if the task is not queued.

# SEE ALSO

autoconf(9), spl(9)

# HISTORY

The task API was originally written by David Gwynne <dlg@openbsd.org>. The task API first appeared in OpenBSD 5.5.

June 22, 2022 | Debian