原地址：http://blog.csdn.net/lights_joy/archive/2009/04/23/4102575.aspx
創(chuàng)建內(nèi)核線程可以使用kernel_thread函數(shù)：

/*

?* Create a kernel thread.

?*/

pid_t kernel_thread(int?(*fn) (void?*),?void?*arg,?unsigned?long?flags)

{

?????struct?pt_regs regs;

?

?????memset(&regs, 0,?sizeof(regs));

?

?????regs.r1 = (unsigned?long)arg;

?????regs.p1 = (unsigned?long)fn;

?????regs.pc = (unsigned?long)kernel_thread_helper;

?????regs.orig_p0 = -1;

?????/* Set bit 2 to tell ret_from_fork we should be returning to kernel

????????mode.??*/

?????regs.ipend = 0x8002;

?????__asm__ __volatile__("%0 = syscfg;":"=d"(regs.syscfg):);

?????return?do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL,

????????????????NULL);

}

注意這里的pc值的設(shè)置，它指向了kernel_thread_help，這將是這個(gè)內(nèi)核線程要執(zhí)行的第一行語(yǔ)句：

/*

?* This gets run with P1 containing the

?* function to call, and R1 containing

?* the "args".??Note P0 is clobbered on the way here.

?*/

void?kernel_thread_helper(void);

__asm__(".section .text\n"

?????".align 4\n"

?????"_kernel_thread_helper:\n\t"

?????"\tsp += -12;\n\t"

?????"\tr0 = r1;\n\t"?"\tcall (p1);\n\t"?"\tcall _do_exit;\n"?".previous;");

在這段代碼中，將跳轉(zhuǎn)到用戶指定的函數(shù)，然后調(diào)用do_exit進(jìn)行一些清理工作。

具體的創(chuàng)建工作由do_fork完成，此時(shí)傳遞進(jìn)去的stack_start和stack_size的值都為0。

1.1.1???do_fork

這個(gè)函數(shù)完成線程的創(chuàng)建，它的關(guān)鍵代碼如下：

/*

?*??Ok, this is the main fork-routine.

?*

?* It copies the process, and if successful kick-starts

?* it and waits for it to finish using the VM if required.

?*/

long?do_fork(unsigned?long?clone_flags,

???????????unsigned?long?stack_start,

???????????struct?pt_regs *regs,

???????????unsigned?long?stack_size,

???????????int?__user *parent_tidptr,

???????????int?__user *child_tidptr)

{

?????struct?task_struct *p;

?????int?trace = 0;

?????struct?pid *pid = alloc_pid();

?????long?nr;

……………………

?

?????p = copy_process(clone_flags, stack_start, regs, stack_size, parent_tidptr, child_tidptr, pid);

?

?????/*

??????* Do this prior waking up the new thread - the thread pointer

??????* might get invalid after that point, if the thread exits quickly.

??????*/

?????if?(!IS_ERR(p)) {

?????????struct?completion vfork;

…………………….

?

?????????if?(!(clone_flags & CLONE_STOPPED))

??????????????wake_up_new_task(p, clone_flags);

?????????else

??????????????p->state = TASK_STOPPED;

……………………….

?????}?else?{

?????????free_pid(pid);

?????????nr = PTR_ERR(p);

?????}

?????return?nr;

}

它首先為此線程分配一個(gè)pid號(hào)，然后復(fù)制出一個(gè)新的task_struct，最后喚醒此線程，當(dāng)然此時(shí)還不會(huì)進(jìn)入執(zhí)行狀態(tài)。

1.1.2???copy_process

這個(gè)函數(shù)用于從當(dāng)前線程復(fù)制一個(gè)task_struct出來。

/*

?* This creates a new process as a copy of the old one,

?* but does not actually start it yet.

?*

?* It copies the registers, and all the appropriate

?* parts of the process environment (as per the clone

?* flags). The actual kick-off is left to the caller.

?*/

static?struct?task_struct *copy_process(unsigned?long?clone_flags,

???????????????????????unsigned?long?stack_start,

???????????????????????struct?pt_regs *regs,

???????????????????????unsigned?long?stack_size,

???????????????????????int?__user *parent_tidptr,

???????????????????????int?__user *child_tidptr,

???????????????????????struct?pid *pid)

{

?????int?retval;

?????struct?task_struct *p = NULL;

……………………….

?????retval = -ENOMEM;

?????p = dup_task_struct(current);

?????if?(!p)

?????????goto?fork_out;

………………………..

?????retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);

?????if?(retval)

?????????goto?bad_fork_cleanup_namespaces;

…………………………

?????return?p;

}

它首先調(diào)用dup_task_struct得到一個(gè)task_struct，同時(shí)也給這個(gè)新的線程分配了一個(gè)thread_info的結(jié)構(gòu)體，這也是這個(gè)新線程的棧，使用BUDDY算法分配，保證以8K對(duì)齊。

接著調(diào)用copy_thread進(jìn)行線程的復(fù)制。

int

copy_thread(int?nr,?unsigned?long?clone_flags,

?????????unsigned?long?usp,?unsigned?long?topstk,

?????????struct?task_struct *p,?struct?pt_regs *regs)

{

?????struct?pt_regs *childregs;

?

?????childregs = (struct?pt_regs *) (task_stack_page(p) + THREAD_SIZE) - 1;

?????*childregs = *regs;

?????childregs->r0 = 0;

?

?????p->thread.usp = usp;

?????p->thread.ksp = (unsigned?long)childregs;

?????p->thread.pc = (unsigned?long)ret_from_fork;

?

?????return?0;

}

注意這里在新線程的棧的底端復(fù)制了一份pt_regs，而這份pt_regs的PC指針是指向kernel_thread_helper的。且新線程的PC指針是指向ret_from_fork函數(shù)。

1.1.3???wake_up_new_task

這個(gè)函數(shù)用于把線程放到一個(gè)CPU核的任務(wù)隊(duì)列中。

/*

?* wake_up_new_task - wake up a newly created task for the first time.

?*

?* This function will do some initial scheduler statistics housekeeping

?* that must be done for every newly created context, then puts the task

?* on the runqueue and wakes it.

?*/

void?fastcall wake_up_new_task(struct?task_struct *p,?unsigned?long?clone_flags)

{

?????struct?rq *rq, *this_rq;

?????unsigned?long?flags;

?????int?this_cpu, cpu;

?

?????rq = task_rq_lock(p, &flags);

?????BUG_ON(p->state != TASK_RUNNING);

?????this_cpu = smp_processor_id();

?????cpu = task_cpu(p);

?

?????/*

??????* We decrease the sleep average of forking parents

??????* and children as well, to keep max-interactive tasks

??????* from forking tasks that are max-interactive. The parent

??????* (current) is done further down, under its lock.

??????*/

?????p->sleep_avg = JIFFIES_TO_NS(CURRENT_BONUS(p) *

?????????CHILD_PENALTY / 100 * MAX_SLEEP_AVG / MAX_BONUS);

?

?????p->prio = effective_prio(p);

?

?????if?(likely(cpu == this_cpu)) {

?????????if?(!(clone_flags & CLONE_VM)) {

??????????????/*

???????????????* The VM isn't cloned, so we're in a good position to

???????????????* do child-runs-first in anticipation of an exec. This

???????????????* usually avoids a lot of COW overhead.

???????????????*/

??????????????if?(unlikely(!current->array))

???????????????????__activate_task(p, rq);

??????????????else?{

???????????????????p->prio = current->prio;

???????????????????p->normal_prio = current->normal_prio;

???????????????????list_add_tail(&p->run_list, &current->run_list);

???????????????????p->array?= current->array;

???????????????????p->array->nr_active++;

???????????????????inc_nr_running(p, rq);

??????????????}

??????????????set_need_resched();

?????????}?else

??????????????/* Run child last */

??????????????__activate_task(p, rq);

?????????/*

??????????* We skip the following code due to cpu == this_cpu

???????????*

??????????*???task_rq_unlock(rq, &flags);

??????????*???this_rq = task_rq_lock(current, &flags);

??????????*/

?????????this_rq = rq;

?????}?else?{

?????????this_rq = (struct?rq *)cpu_rq(this_cpu);

?

?????????/*

??????????* Not the local CPU - must adjust timestamp. This should

??????????* get optimised away in the !CONFIG_SMP case.

??????????*/

?????????p->timestamp = (p->timestamp - this_rq->most_recent_timestamp)

???????????????????????+ rq->most_recent_timestamp;

?????????__activate_task(p, rq);

?????????if?(TASK_PREEMPTS_CURR(p, rq))

??????????????resched_task(rq->curr);

?

?????????/*

??????????* Parent and child are on different CPUs, now get the

??????????* parent runqueue to update the parent's ->sleep_avg:

??????????*/

?????????task_rq_unlock(rq, &flags);

?????????this_rq = task_rq_lock(current, &flags);

?????}

?????current->sleep_avg = JIFFIES_TO_NS(CURRENT_BONUS(current) *

?????????PARENT_PENALTY / 100 * MAX_SLEEP_AVG / MAX_BONUS);

?????task_rq_unlock(this_rq, &flags);

}

這個(gè)函數(shù)挺長(zhǎng)的，但實(shí)際上將新線程加入隊(duì)列的工作是由__activate_task這個(gè)函數(shù)完成的：

/*

?* __activate_task - move a task to the runqueue.

?*/

static?void?__activate_task(struct?task_struct *p,?struct?rq *rq)

{

?????struct?prio_array *target = rq->active;

?

?????if?(batch_task(p))

?????????target = rq->expired;

?????enqueue_task(p, target);

?????inc_nr_running(p, rq);

}

再看enqueue_task：

static?void?enqueue_task(struct?task_struct *p,?struct?prio_array *array)

{

?????sched_info_queued(p);

?????list_add_tail(&p->run_list,?array->queue + p->prio);

?????__set_bit(p->prio,?array->bitmap);

?????array->nr_active++;

?????p->array?=?array;

}

快樂蝦

http://blog.csdn.net/lights_joy/

lights@hb165.com

??

本文適用于

ADSP-BF561

uclinux-2008r1.5-RC3(移植到vdsp5)

Visual DSP++ 5.0(update 5)

??

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