NAME
kthread_start, kthread_shutdown, kthread_add, kthread_exit,
kthread_resume, kthread_suspend, kthread_suspend_check - kernel threads
SYNOPSIS
#include <sys/kthread.h>
void
kthread_start(const void *udata);
void
kthread_shutdown(void *arg, int howto);
void
kthread_exit(void);
int
kthread_resume(struct thread *td);
int
kthread_suspend(struct thread *td, int timo);
void
kthread_suspend_check(struct thread *td);
#include <sys/unistd.h>
int
kthread_add(void (*func)(void *), void *arg, struct proc *procp,
struct thread **newtdpp, int flags, int pages, const char *fmt,
...);
int
kproc_kthread_add(void (*func)(void *), void *arg, struct proc **procptr,
struct thread **tdptr, int flags, int pages, char * procname,
const char *fmt, ...);
DESCRIPTION
In FreeBSD 8.0, the older family of kthread_*(9) functions was renamed to
be the kproc_*(9) family of functions, as they were previously misnamed
and actually produced kernel processes. This new family of kthread_*(9)
functions was added to produce real kernel threads. See the kproc(9) man
page for more information on the renamed calls. Also note that the
kproc_kthread_add(9) function appears in both pages as its functionality
is split.
The function kthread_start() is used to start “internal” daemons such as
bufdaemon, pagedaemon, vmdaemon, and the syncer and is intended to be
called from SYSINIT(9). The udata argument is actually a pointer to a
struct kthread_desc which describes the kernel thread that should be
created:
struct kthread_desc {
char *arg0;
void (*func)(void);
struct thread **global_threadpp;
};
The structure members are used by kthread_start() as follows:
arg0 String to be used for the name of the thread.
This string will be copied into the td_name member
of the new threads’ struct thread.
func The main function for this kernel thread to run.
global_threadpp A pointer to a struct thread pointer that should
be updated to point to the newly created thread’s
thread structure. If this variable is NULL, then
it is ignored. The thread will be a subthread of
proc0 (PID 0).
The 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 procp argument, or if that is NULL, to proc0. The func argument
specifies the function that the thread should execute. The arg argument
is an arbitrary pointer that is passed in as the only argument to func
when it is called by the new thread. The newtdpp pointer points to a
struct thread pointer that is to be updated to point to the newly created
thread. If this argument is NULL, then it is ignored. The flags
argument may be set to RFSTOPPED to leave the thread in a stopped state.
The caller must call sched_add() to start the thread. The 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 printf(9) argument list that is used to build the name
of the new thread and is stored in the td_name member of the new thread’s
struct thread.
The kproc_kthread_add() function is much like the kthread_add() function
above except that if the kproc does not already exist, it is created.
This function is better documented in the kproc(9) manual page.
The 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 kthread_resume(), kthread_suspend(), and 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 kthread_suspend_check() passing in curthread
as the only argument. This function checks to see if the kernel thread
has been asked to suspend. If it has, it will 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 td
argument points to the struct thread of the kernel thread to suspend or
resume. For kthread_suspend(), the timo argument specifies a timeout to
wait for the kernel thread to acknowledge the suspend request and suspend
itself.
The 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
kthread_suspend().
RETURN VALUES
The kthread_add(), kthread_resume(), and kthread_suspend() functions
return zero on success and non-zero on failure.
EXAMPLES
This example demonstrates the use of a struct kthread_desc and the
functions kthread_start(), kthread_shutdown(), and
kthread_suspend_check() to run the bufdaemon process.
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(bufdaemonthread);
...
}
}
ERRORS
The kthread_resume() and kthread_suspend() functions will fail if:
[EINVAL] The td argument does not reference a kernel thread.
The kthread_add() function will fail if:
[ENOMEM] Memory for a thread’s stack could not be allocated.
SEE ALSO
kproc(9), SYSINIT(9), wakeup(9)
HISTORY
The kthread_start() function first appeared in FreeBSD 2.2 where it
created a whole process. It was converted to create threads in
FreeBSD 8.0. The kthread_shutdown(), kthread_exit(), kthread_resume(),
kthread_suspend(), and kthread_suspend_check() functions were introduced
in FreeBSD 4.0 and were converted to threads in FreeBSD 8.0. The
kthread_create() call was renamed to kthread_add() in FreeBSD 8.0. The
old functionality of creating a kernel process was renamed to
kproc_create(9). Prior to FreeBSD 5.0, the kthread_shutdown(),
kthread_resume(), kthread_suspend(), and kthread_suspend_check()
functions were named shutdown_kproc(), resume_kproc(), shutdown_kproc(),
and kproc_suspend_loop(), respectively.