在kernel代码的任意位置直接调用dump_stack方法即可。dump_stack已经包含在内核符号表中，并在include/linux/kernel.h中被声明。获取栈信息的原理是使用ebp逐层回溯。dump_stack函数在arch/x86/kernel/traps_32.c中定义(kernel 2.6.24)如下：

/*
 * The architecture-independent dump_stack generator
 */
void dump_stack(void)
{
	unsigned long stack;

	printk("Pid: %d, comm: %.20s %s %s %.*s\n",
		current->pid, current->comm, print_tainted(),
		init_utsname()->release,
		(int)strcspn(init_utsname()->version, " "),
		init_utsname()->version);
	show_trace(current, NULL, &stack);
}
EXPORT_SYMBOL(dump_stack);

在start_kernel函数开头加入dump_stack打印的栈信息如上图所示。

asmlinkage void __init start_kernel(void)
{
	char * command_line;
	extern struct kernel_param __start___param[], __stop___param[];

	smp_setup_processor_id();
        
        dump_stack();

	/* Need to run as early as possible, to initialize the lockdep hash: */
	unwind_init();
	lockdep_init();
	cgroup_init_early();

	local_irq_disable();
	early_boot_irqs_off();
	early_init_irq_lock_class();

        ......
	/*
	 * HACK ALERT! This is early. We're enabling the console before
	 * we've done PCI setups etc, and console_init() must be aware of
	 * this. But we do want output early, in case something goes wrong.
	 */
	console_init();
	if (panic_later)
		panic(panic_later, panic_param);

	lockdep_info();

	/* Need to run this when irqs are enabled, because it wants
	 * to self-test [hard/soft]-irqs on/off lock inversion bugs
	 * too: */
	locking_selftest();
        ......
}

        

---

       dump_stack借助printk将栈信息写入kernel的ring buffer中，当start_kernel调用到console_init后，会将ring buffer的信息全部输出到每个console中显示。在kernel启动完成后，使用dmesg也可将ring buffer中的启动信息输出。因此，无论dump_stack在何处调用，即使当时无法立即在屏幕上显示，都存在于ring buffer中，一旦kernel调用console_init完成，即把ring buffer的全部内容打印到屏幕，并且，后续只要缓冲区不覆盖即可从中导出栈信息。

       kernel调用console_init时，读取.con_initcall.init段的每个条目依次对各console初始化。console_init函数实现如下：

/drivers/char/tty_io.c
/*
 * Initialize the console device. This is called *early*, so
 * we can't necessarily depend on lots of kernel help here.
 * Just do some early initializations, and do the complex setup
 * later.
 */
void __init console_init(void)
{
	initcall_t *call;

	/* Setup the default TTY line discipline. */
	(void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);

	/*
	 * set up the console device so that later boot sequences can 
	 * inform about problems etc..
	 */
	call = __con_initcall_start;
	while (call < __con_initcall_end) {
		(*call)();
		call++;
	}
}

.con_initcall.init段在/arch/x86/kernel/vmlinux_32.lds.S中定义：

  .initcall.init : AT(ADDR(.initcall.init) - LOAD_OFFSET) {
  	__initcall_start = .;
	INITCALLS
  	__initcall_end = .;
  }
  .con_initcall.init : AT(ADDR(.con_initcall.init) - LOAD_OFFSET) {
  	__con_initcall_start = .;
	*(.con_initcall.init)
  	__con_initcall_end = .;
  }

.con_initcall.init段中的每个条目都是函数指针，指针使用console_initcall宏注册。console_initcall宏定义如下：

<linux/init.h>
#define console_initcall(fn) \
	static initcall_t __initcall_##fn \
	__attribute_used__ __attribute__((__section__(".con_initcall.init")))=fn

例如kernel中console设备注册要使用console_initcall宏将init函数写入.con_initcall.init段中。

/drivers/s390/char/con3215.c
/*
 * 3215 console initialization code called from console_init().
 * NOTE: This is called before kmalloc is available.
 */
static int __init
con3215_init(void)
{
	struct ccw_device *cdev;
	struct raw3215_info *raw;
	struct raw3215_req *req;
	int i;

	/* Check if 3215 is to be the console */
	if (!CONSOLE_IS_3215)
		return -ENODEV;

	/* Set the console mode for VM */
	if (MACHINE_IS_VM) {
		cpcmd("TERM CONMODE 3215", NULL, 0, NULL);
		cpcmd("TERM AUTOCR OFF", NULL, 0, NULL);
	}

	/* allocate 3215 request structures */
	raw3215_freelist = NULL;
	spin_lock_init(&raw3215_freelist_lock);
	for (i = 0; i < NR_3215_REQ; i++) {
		req = (struct raw3215_req *) alloc_bootmem_low(sizeof(struct raw3215_req));
		req->next = raw3215_freelist;
		raw3215_freelist = req;
	}

	cdev = ccw_device_probe_console();
	if (IS_ERR(cdev))
		return -ENODEV;

	raw3215[0] = raw = (struct raw3215_info *)
		alloc_bootmem_low(sizeof(struct raw3215_info));
	memset(raw, 0, sizeof(struct raw3215_info));
	raw->buffer = (char *) alloc_bootmem_low(RAW3215_BUFFER_SIZE);
	raw->inbuf = (char *) alloc_bootmem_low(RAW3215_INBUF_SIZE);
	raw->cdev = cdev;
	cdev->dev.driver_data = raw;
	cdev->handler = raw3215_irq;

	raw->flags |= RAW3215_FIXED;
	tasklet_init(&raw->tasklet,
		     (void (*)(unsigned long)) raw3215_tasklet,
		     (unsigned long) raw);
	init_waitqueue_head(&raw->empty_wait);

	/* Request the console irq */
	if (raw3215_startup(raw) != 0) {
		free_bootmem((unsigned long) raw->inbuf, RAW3215_INBUF_SIZE);
		free_bootmem((unsigned long) raw->buffer, RAW3215_BUFFER_SIZE);
		free_bootmem((unsigned long) raw, sizeof(struct raw3215_info));
		raw3215[0] = NULL;
		printk("Couldn't find a 3215 console device\n");
		return -ENODEV;
	}
	register_console(&con3215);
	return 0;
}
console_initcall(con3215_init);

con3215_init函数中调用register_console函数，该函数定义如下：

/kernel/printk.c
/*
 * The console driver calls this routine during kernel initialization
 * to register the console printing procedure with printk() and to
 * print any messages that were printed by the kernel before the
 * console driver was initialized.
 */
void register_console(struct console *console)
{
	int i;
	unsigned long flags;
	struct console *bootconsole = NULL;

	if (console_drivers) {
		if (console->flags & CON_BOOT)
			return;
		if (console_drivers->flags & CON_BOOT)
			bootconsole = console_drivers;
	}

	if (!(console->flags & CON_ENABLED))
		return;

	if (bootconsole && (console->flags & CON_CONSDEV)) {
		printk(KERN_INFO "console handover: boot [%s%d] -> real [%s%d]\n",
		       bootconsole->name, bootconsole->index,
		       console->name, console->index);
		unregister_console(bootconsole);
		console->flags &= ~CON_PRINTBUFFER;
	} else {
		printk(KERN_INFO "console [%s%d] enabled\n",
		       console->name, console->index);
	}

	/*
	 *	Put this console in the list - keep the
	 *	preferred driver at the head of the list.
	 */
	acquire_console_sem();
	......
	release_console_sem();
}

其中调用了release_console_sem函数：

/kernel/printk.c
/**
 * release_console_sem - unlock the console system
 * Releases the semaphore which the caller holds on the console system
 * and the console driver list.
 * While the semaphore was held, console output may have been buffered
 * by printk().  If this is the case, release_console_sem() emits
 * the output prior to releasing the semaphore.
 * If there is output waiting for klogd, we wake it up.
 * release_console_sem() may be called from any context.
 */
void release_console_sem(void)
{
	unsigned long flags;
	unsigned long _con_start, _log_end;
	unsigned long wake_klogd = 0;

	if (console_suspended) {
		up(&secondary_console_sem);
		return;
	}

	console_may_schedule = 0;

	for ( ; ; ) {
		spin_lock_irqsave(&logbuf_lock, flags);
		wake_klogd |= log_start - log_end;
		if (con_start == log_end)
			break;			/* Nothing to print */
		_con_start = con_start;
		_log_end = log_end;
		con_start = log_end;		/* Flush */
		spin_unlock(&logbuf_lock);
		call_console_drivers(_con_start, _log_end);
		local_irq_restore(flags);
	}
	console_locked = 0;
	up(&console_sem);
	spin_unlock_irqrestore(&logbuf_lock, flags);
	if (wake_klogd)
		wake_up_klogd();
}

该函数内调用call_console_drivers函数将ring buffer中的信息写入设备中完成显示。

/kernel/printk.c
/*
 * Call the console drivers, asking them to write out
 * log_buf[start] to log_buf[end - 1].
 * The console_sem must be held.
 */
static void call_console_drivers(unsigned long start, unsigned long end)
{
	unsigned long cur_index, start_print;
	static int msg_level = -1;

	BUG_ON(((long)(start - end)) > 0);

	cur_index = start;
	start_print = start;
	while (cur_index != end) {
		if (msg_level < 0 && ((end - cur_index) > 2) &&
				LOG_BUF(cur_index + 0) == '<' &&
				LOG_BUF(cur_index + 1) >= '0' &&
				LOG_BUF(cur_index + 1) <= '7' &&
				LOG_BUF(cur_index + 2) == '>') {
			msg_level = LOG_BUF(cur_index + 1) - '0';
			cur_index += 3;
			start_print = cur_index;
		}
		while (cur_index != end) {
			char c = LOG_BUF(cur_index);

			cur_index++;
			if (c == '\n') {
				if (msg_level < 0) {
					/* printk() has already given us loglevel tags in
					 * the buffer.  This code is here in case the
					 * log buffer has wrapped right round and scribbled
					 * on those tags
					 */
					msg_level = default_message_loglevel;
				}
				_call_console_drivers(start_print, cur_index, msg_level);
				msg_level = -1;
				start_print = cur_index;
				break;
			}
		}
	}
	_call_console_drivers(start_print, end, msg_level);
}

/*
 * Write out chars from start to end - 1 inclusive
 */
static void _call_console_drivers(unsigned long start,
				unsigned long end, int msg_log_level)
{
	if ((msg_log_level < console_loglevel || ignore_loglevel) &&
			console_drivers && start != end) {
		if ((start & LOG_BUF_MASK) > (end & LOG_BUF_MASK)) {
			/* wrapped write */
			__call_console_drivers(start & LOG_BUF_MASK,
						log_buf_len);
			__call_console_drivers(0, end & LOG_BUF_MASK);
		} else {
			__call_console_drivers(start, end);
		}
	}
}

/*
 * Call the console drivers on a range of log_buf
 */
static void __call_console_drivers(unsigned long start, unsigned long end)
{
	struct console *con;

	for (con = console_drivers; con; con = con->next) {
		if ((con->flags & CON_ENABLED) && con->write &&
				(cpu_online(smp_processor_id()) ||
				(con->flags & CON_ANYTIME)))
			con->write(con, &LOG_BUF(start), end - start);
	}
}

可见其中遍历了console_drivers链表，对每个console驱动调用write接口写入字符串。在console_init函数调用前，各驱动已经完成加载，并将console设备信息存储在对应结构体内。当write调用时直接从这些结构体内取出设备信息。

上述函数调用关系图如下：

tty驱动类和cdev的关系图如下，该关系方便分析serial console、console的驱动组织结构：

---

同样在module内的任何位置也可以使用dump_stack函数，在函数运行到dump_stack后打印栈信息到ring buffer。

 

arm架构下，在include/asm-arm/system.h下声明了extern asmlinkage void __backtrace(void)函数，其采用汇编实现，同样可以获取栈信息(x86下没有这个同名函数)。汇编实现位于arch/arm/lib/backtrace.S中：

@ fp is 0 or stack frame

#define frame	r4
#define sv_fp	r5
#define sv_pc	r6
#define mask	r7
#define offset	r8

ENTRY(__backtrace)
		mov	r1, #0x10
		mov	r0, fp

ENTRY(c_backtrace)

#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
		mov	pc, lr
#else
		stmfd	sp!, {r4 - r8, lr}	@ Save an extra register so we have a location...
		movs	frame, r0		@ if frame pointer is zero
		beq	no_frame		@ we have no stack frames

                ......