最近发现在内核创建线程的时候经常会用到kthread_run(）这样的一个调用。于是准备拿出来学习一下。首先看看它的定义之处才发现它是一个宏函数，而不是一个真正意义上的函数。在include/linux/Kthread.h里有/**
* kthread_run - create and wake a thread.
* @threadfn: the function to run until signal_pending(current).
* @data: data ptr for @threadfn.
* @namefmt: printf-style name for the thread.
*
* Description: Convenient wrapper for kthread_create() followed by
* wake_up_process(). Returns the kthread or ERR_PTR(-ENOMEM).
*/
#define kthread_run(threadfn, data, namefmt, ...) \
({ \
struct task_struct *__k \
= kthread_create(threadfn, data, namefmt, ## __VA_ARGS__); \
if (!IS_ERR(__k)) \
wake_up_process(__k); \
__k; \
})
这个函数的英文注释里很明确的说明： 创建并启动一个内核线程。可见这里的函数kthread_create（）只是创建了内核线程，而最后启动是怎么启动的呢，我们看到了后面的wake_up_process（）函数，没错就是这个函数启动了这个线程，让它在一开始就一直运行下去。知道遇见kthread_should_stop函数或者kthread_stop()函数。那我们具体看看前一个函数到底做了什么吧。
在这个宏里面主要是调用了函数：kthread_create（）
这个函数是干什么的呢？在Kernel/Kthread.c里面我们可以看到：
/**
* kthread_create - create a kthread.
* @threadfn: the function to run until signal_pending(current).
* @data: data ptr for @threadfn.
* @namefmt: printf-style name for the thread.
*
* Description: This helper function creates and names a kernel
* thread. The thread will be stopped: use wake_up_process() to start
* it. See also kthread_run(), kthread_create_on_cpu().
*
* When woken, the thread will run @threadfn() with @data as its
* argument. @threadfn can either call do_exit() directly if it is a
* standalone thread for which noone will call kthread_stop(), or
* return when 'kthread_should_stop()' is true (which means
* kthread_stop() has been called). The return value should be zero
* or a negative error number; it will be passed to kthread_stop().
*
* Returns a task_struct or ERR_PTR(-ENOMEM).
*/
struct task_struct *kthread_create(int (*threadfn)(void *data),
void *data,
const char namefmt[],
...)
{
struct kthread_create_info create;
DECLARE_WORK(work, keventd_create_kthread, &create);
create.threadfn = threadfn;
create.data = data;
init_completion(&create.started);
init_completion(&create.done);
/*
* The workqueue needs to start up first:
*/
if (!helper_wq)
work.func(work.data);
else {
queue_work(helper_wq, &work);
wait_for_completion(&create.done);
}
if (!IS_ERR(create.result)) {
va_list args;
va_start(args, namefmt);
vsnprintf(create.result->comm, sizeof(create.result->comm),
namefmt, args);
va_end(args);
}
return create.result;
}
EXPORT_SYMBOL(kthread_create);
注意到上面的这段英文解释：说这个函数会创建一个名为namefmt的内核线程，这个线程刚创建时不会马上执行，要等到它将kthread_create() 返回的task_struct指针传给wake_up_process()，然后通过此函数运行线程。我们看到creat结构体，我们将传入的参数付给了它，而threadfn这个函数就是创建的运行函数。在使用中我们可以在此函数中调用kthread_should_stop（）或者kthread_stop()函数来结束线程。这里我们看到创建线程函数中使用工作队列DECLARE_WORK，我们跟踪一下发现这只是将函数#define DECLARE_WORK(n,
f, d) \
struct work_struct n = __WORK_INITIALIZER(n, f, d)
然后再跟进：
#define __WORK_INITIALIZER(n, f, d) { \
.entry = { &(n).entry, &(n).entry }, \
.func = (f), \
.data = (d), \
.timer = TIMER_INITIALIZER(NULL, 0, 0), \
}
反正目的是创建一个工作组队列，而其中keventd_create_kthread（）函数主要是起到创建线程的功能
/* We are keventd: create a thread. */
static void keventd_create_kthread(void *_create)
{
struct kthread_create_info *create = _create;
int pid;
/* We want our own signal handler (we take no signals by default). */
pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
if (pid < 0) {
create->result = ERR_PTR(pid);
} else {
wait_for_completion(&create->started);
read_lock(&tasklist_lock);
create->result = find_task_by_pid(pid);
read_unlock(&tasklist_lock);
}
complete(&create->done);
}
再看看kernel_thread（）函数最后调用到了哪里：
/*
* Create a kernel thread.
*/
pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
{
struct pt_regs regs;
long pid;
memset(®s, 0, sizeof(regs));
regs.ARM_r1 = (unsigned long)arg;
regs.ARM_r2 = (unsigned long)fn;
regs.ARM_r3 = (unsigned long)do_exit;
regs.ARM_pc = (unsigned long)kernel_thread_helper;
regs.ARM_cpsr = SVC_MODE;
pid = do_fork(flags|CLONE_VM|CLONE_UNTRACED, 0, ®s, 0, NULL, NULL);
MARK(kernel_thread_create, "%ld %p", pid, fn);
return pid;
}
EXPORT_SYMBOL(kernel_thread);
好，最后我们看到了线程通过申请进程的pid号来被创建，关键是我们要知道如何使用这个宏函数，也就是如何应用它。要注意的是它调用了创建线程函数，同时也激活了线程。所以代码中调用了它的话就隐含着已经启动一个线程。