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(void);

     #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() in order to
     check if the it has been asked to suspend.  If it has, it will 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 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.