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

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.Dd October 24, 2000
.Dt KTHREAD 9
.Os
.Sh NAME
.Nm kproc_start ,
.Nm kproc_shutdown ,
.Nm kthread_create ,
.Nm kthread_exit ,
.Nm kthread_resume ,
.Nm kthread_suspend ,
.Nm kthread_suspend_check
.Nd kernel threads
.Sh SYNOPSIS
.Fd #include <sys/kthread.h>
.Ft void
.Fn kproc_start "const void *udata"
.Ft void
.Fn kproc_shutdown "void *arg" "int howto"
.Ft int
.Fn kthread_create "void (*func)(void *)" "void *arg" "struct proc **newpp" "int flags" "const char *fmt" "..."
.Ft void
.Fn kthread_exit "int ecode"
.Ft int
.Fn kthread_resume "struct proc *p"
.Ft int
.Fn kthread_suspend "struct proc *p" "int timo"
.Ft void
.Fn kthread_suspend_check "struct proc *p"
.Sh DESCRIPTION
.Pp
The function
.Fn kproc_start
is used to start
.Dq internal
daemons such as bufdaemon, pagedaemon, vmdaemon, and the syncer and is intended
to be called from
.Xr SYSINIT 9 .
The
.Fa udata
argument is actually a pointer to a
.Li struct kproc_desc
which describes the kernel thread that should be created:
.Bd -literal -offset indent
struct kproc_desc {
        char            *arg0;
        void            (*func) __P((void));
        struct proc     **global_procpp;
};
.Ed
.Pp
The structure members are used by
.Fn kproc_start
as follows:
.Bl -tag -width "global_procpp" -offset indent
.It Va arg0
String to be used for the name of the process.
This string will be copied into the
.Va p_comm
member of the new process'
.Li struct proc .
.It Va func
The main function for this kernel process to run.
.It Va global_procpp
A pointer to a
.Li struct proc
pointer that should be updated to point to the newly created process' process
structure.
If this variable is
.Dv NULL ,
then it is ignored.
.El
.Pp
The
.Fn kthread_create
function is used to create a kernel thread.
The new thread shares its address space with process 0, the swapper process,
and runs in kernel mode only.
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 process.
The
.Fa newpp
pointer points to a
.Li struct proc
pointer that is to be updated to point to the newly created process.
If this argument is
.Dv NULL ,
then it is ignored.
The
.Fa flags
argument specifies a set of flags as described in
.Xr rfork 2 .
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 p_comm
member of the new thread's
.Li struct proc .
.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.
The
.Fa ecode
argument specifies the exit status of the thread.
.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
passing in
.Va curproc
as the only argument.
This function checks to see if the kernel thread has been asked to suspend.
If it has, it will
.Xr tsleep 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 p
argument points to the
.Li struct proc
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 kproc_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_create ,
.Fn kthread_resume ,
and
.Fn kthread_suspend
functions return zero on success and non-zero on failure.
Upon failure, the global variable
.Va errno
is set to indicate the error.
.Sh EXAMPLES
This example demonstrates the use of a
.Li struct kproc_desc
and the functions
.Fn kproc_start ,
.Fn kproc_shutdown ,
and
.Fn kthread_suspend_check
to run the
.Dq bufdaemon
process.
.Bd -literal -offset indent
static struct proc *bufdaemonproc;

static struct kproc_desc buf_kp = {
	"bufdaemon",
	buf_daemon,
	&bufdaemonproc
};
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start,
    &buf_kp)

static void
buf_daemon()
{
	...
	/*
	 * This process needs to be suspended prior to shutdown sync.
	 */
	EVENTHANDLER_REGISTER(shutdown_pre_sync, kproc_shutdown,
	    bufdaemonproc, SHUTDOWN_PRI_LAST);
	...
	for (;;) {
		kthread_suspend_check(bufdaemonproc);
		...
	}
}
.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 p
argument does not reference a kernel thread.
.El
.Pp
The
.Fn kthread_create
function will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The system-imposed limit on the total
number of processes under execution would be exceeded.
The limit is given by the
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROC .
.It Bq Er EINVAL
The
.Dv RFCFDG
flag was specified in the
.Fa flags
parameter.
.El
.Sh SEE ALSO
.Xr SYSINIT 9 ,
.Xr rfork 2
.Sh HISTORY
The
.Fn kproc_start
function first appeared in
.Fx 2.2 .
The
.Fn kproc_shutdown ,
.Fn kthread_create ,
.Fn kthread_exit ,
.Fn kthread_resume ,
.Fn kthread_suspend ,
and
.Fn kthread_suspend_check
functions were introducted in
.Fx 4.0 .
Prior to
.Fx 5.0 ,
the
.Fn kproc_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.
.Sh BUGS
If a kernel thread exits its main function (specified as the
.Fa func
argument to
.Fn kthread_create
or in the
.Li struct kproc_desc
passed to
.Fn kpoc_start )
then it will attempt to return backwards through the
.Fn fork1
trampoline code with unpredictable results.
The code should instead use a trampoline function to launch
.Fa func
and call
.Fn kthread_exit
if
.Fa func
returns.