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DESCRIPTION

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:

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().

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();
		...
	}
}