freebsd-dev/share/man/man9/kthread.9

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.Dd July 15, 2014
.Dt KTHREAD 9
.Os
.Sh NAME
.Nm kthread_start ,
.Nm kthread_shutdown ,
.Nm kthread_add ,
.Nm kthread_exit ,
.Nm kthread_resume ,
.Nm kthread_suspend ,
.Nm kthread_suspend_check
.Nd "kernel threads"
.Sh SYNOPSIS
.In sys/kthread.h
.Ft void
.Fn kthread_start "const void *udata"
.Ft void
.Fn kthread_shutdown "void *arg" "int howto"
.Ft void
.Fn kthread_exit "void"
.Ft int
.Fn kthread_resume "struct thread *td"
.Ft int
.Fn kthread_suspend "struct thread *td" "int timo"
.Ft void
.Fn kthread_suspend_check "void"
.In sys/unistd.h
.Ft int
.Fo kthread_add
.Fa "void (*func)(void *)" "void *arg" "struct proc *procp"
.Fa "struct thread **newtdpp" "int flags" "int pages"
.Fa "const char *fmt" ...
.Fc
.Ft int
.Fo kproc_kthread_add
.Fa "void (*func)(void *)" "void *arg"
.Fa "struct proc **procptr" "struct thread **tdptr"
.Fa "int flags" "int pages" "char * procname" "const char *fmt" "..."
.Fc
.Sh DESCRIPTION
In
.Fx 8.0 ,
the older family of
.Fn kthread_* 9
functions was renamed to be the
.Fn  kproc_* 9
family of functions,
as they were previously misnamed
and actually produced kernel processes.
This new family of
.Fn kthread_* 9
functions was added to produce
.Em real
kernel threads.
See the
.Xr kproc 9
man page for more information on the renamed calls.
Also note that the
.Fn kproc_kthread_add 9
function appears in both pages as its functionality is split.
.Pp
The function
.Fn kthread_start
is used to start
.Dq internal
daemons such as
.Nm bufdaemon , pagedaemon , vmdaemon ,
and the
.Nm syncer
and is intended
to be called from
.Xr SYSINIT 9 .
The
.Fa udata
argument is actually a pointer to a
.Vt "struct kthread_desc"
which describes the kernel thread that should be created:
.Bd -literal -offset indent
struct kthread_desc {
	char		*arg0;
	void		(*func)(void);
	struct thread	**global_threadpp;
};
.Ed
.Pp
The structure members are used by
.Fn kthread_start
as follows:
.Bl -tag -width ".Va global_threadpp" -offset indent
.It Va arg0
String to be used for the name of the thread.
This string will be copied into the
.Va td_name
member of the new threads'
.Vt "struct thread" .
.It Va func
The main function for this kernel thread to run.
.It Va global_threadpp
A pointer to a
.Vt "struct thread"
pointer that should be updated to point to the newly created thread's
.Vt thread
structure.
If this variable is
.Dv NULL ,
then it is ignored.
The thread will be a subthread of
.Va proc0
(PID 0).
.El
.Pp
The
.Fn kthread_add
function is used to create a kernel thread.
The new thread runs in kernel mode only.
It is added to the process specified by the
.Fa procp
argument, or if that is
.Dv NULL ,
to
.Va proc0 .
The
.Fa func
argument specifies the function that the thread should execute.
The
.Fa arg
argument is an arbitrary pointer that is passed in as the only argument to
.Fa func
when it is called by the new thread.
The
.Fa newtdpp
pointer points to a
.Vt "struct thread"
pointer that is to be updated to point to the newly created thread.
If this argument is
.Dv NULL ,
then it is ignored.
The
.Fa flags
argument may be set to
.Dv RFSTOPPED
to leave the thread in a stopped state.
The caller must call
.Fn sched_add
to start the thread.
The
.Fa pages
argument specifies the size of the new kernel thread's stack in pages.
If 0 is used, the default kernel stack size is allocated.
The rest of the arguments form a
.Xr printf 9
argument list that is used to build the name of the new thread and is stored
in the
.Va td_name
member of the new thread's
.Vt "struct thread" .
.Pp
The
.Fn kproc_kthread_add
function is much like the
.Fn kthread_add
function above except that if the kproc does not already
exist, it is created.
This function is better documented in the
.Xr kproc 9
manual page.
.Pp
The
.Fn kthread_exit
function is used to terminate kernel threads.
It should be called by the main function of the kernel thread rather than
letting the main function return to its caller.
.\" XXX "int ecode" argument isn't documented.
.Pp
The
.Fn kthread_resume ,
.Fn kthread_suspend ,
and
.Fn kthread_suspend_check
functions are used to suspend and resume a kernel thread.
During the main loop of its execution, a kernel thread that wishes to allow
itself to be suspended should call
.Fn kthread_suspend_check
in order to check if the it has been asked to suspend.
If it has, it will
.Xr msleep 9
until it is told to resume.
Once it has been told to resume it will return allowing execution of the
kernel thread to continue.
The other two functions are used to notify a kernel thread of a suspend or
resume request.
The
.Fa td
argument points to the
.Vt "struct thread"
of the kernel thread to suspend or resume.
For
.Fn kthread_suspend ,
the
.Fa timo
argument specifies a timeout to wait for the kernel thread to acknowledge the
suspend request and suspend itself.
.Pp
The
.Fn kthread_shutdown
function is meant to be registered as a shutdown event for kernel threads that
need to be suspended voluntarily during system shutdown so as not to interfere
with system shutdown activities.
The actual suspension of the kernel thread is done with
.Fn kthread_suspend .
.Sh RETURN VALUES
The
.Fn kthread_add ,
.Fn kthread_resume ,
and
.Fn kthread_suspend
functions return zero on success and non-zero on failure.
.Sh EXAMPLES
This example demonstrates the use of a
.Vt "struct kthread_desc"
and the functions
.Fn kthread_start ,
.Fn kthread_shutdown ,
and
.Fn kthread_suspend_check
to run the
.Nm bufdaemon
process.
.Bd -literal -offset indent
static struct thread *bufdaemonthread;

static struct kthread_desc buf_kp = {
	"bufdaemon",
	buf_daemon,
	&bufdaemonthread
};
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kthread_start,
    &buf_kp)

static void
buf_daemon()
{
	...
	/*
	 * This process needs to be suspended prior to shutdown sync.
	 */
	EVENTHANDLER_REGISTER(shutdown_pre_sync, kthread_shutdown,
	    bufdaemonthread, SHUTDOWN_PRI_LAST);
	...
	for (;;) {
		kthread_suspend_check();
		...
	}
}
.Ed
.Sh ERRORS
The
.Fn kthread_resume
and
.Fn kthread_suspend
functions will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa td
argument does not reference a kernel thread.
.El
.Pp
The
.Fn kthread_add
function will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
Memory for a thread's stack could not be allocated.
.El
.Sh SEE ALSO
.Xr kproc 9 ,
.Xr SYSINIT 9 ,
.Xr wakeup 9
.Sh HISTORY
The
.Fn kthread_start
function first appeared in
.Fx 2.2
where it created a whole process.
It was converted to create threads in
.Fx 8.0 .
The
.Fn kthread_shutdown ,
.Fn kthread_exit ,
.Fn kthread_resume ,
.Fn kthread_suspend ,
and
.Fn kthread_suspend_check
functions were introduced in
.Fx 4.0
and were converted to threads in
.Fx 8.0 .
The
.Fn kthread_create
call was renamed to
.Fn kthread_add
in
.Fx 8.0 .
The old functionality of creating a kernel process was renamed
to
.Xr kproc_create 9 .
Prior to
.Fx 5.0 ,
the
.Fn kthread_shutdown ,
.Fn kthread_resume ,
.Fn kthread_suspend ,
and
.Fn kthread_suspend_check
functions were named
.Fn shutdown_kproc ,
.Fn resume_kproc ,
.Fn shutdown_kproc ,
and
.Fn kproc_suspend_loop ,
respectively.