kernel exception 流程

Kernel Exception 根本原因有两类，

oops

• oops是美国人比较常有的口语。就是有点意外，吃惊，或突然的意思。内核行为表现为通知感兴趣模块，打印各种信息，如寄存器值，堆栈信息…
• 当出现oops时，我们就可以根据寄存器等信息调试并解决问题。
• /proc/sys/kernel/panic_on_oops为1时导致panic。我们默认设置为1，即oops会发生panic。

panic

• Panic – 困惑，恐慌，它表示Linux kernel遇到了一个不知道该怎么继续的情况。内核行为表现为通知感兴趣模块，死机或者重启。
• 在kernel代码里，有些代码加了错误检查，发现错误可能直接调用了panic()，并输出信息提供调试。

在kernel代码里，一般会通过BUG(),BUG_ON(),panic()来拦截超出预期的行为，这是软件主动回报异常的功能。

例如：

	if (IS_ERR(clk_cs)) {
		pr_err("ERROR: timer input clock not found\n");
		BUG();
	}

由于BUG_ON调用BUG，原生得BUG直接调用panic

#ifndef HAVE_ARCH_BUG_ON
#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
#endif

下面来只看panic得流程

{
	static DEFINE_SPINLOCK(panic_lock);
	static char buf[1024];
	va_list args;
	long i, i_next = 0;
	int state = 0;

	/*
	 * Disable local interrupts. This will prevent panic_smp_self_stop
	 * from deadlocking the first cpu that invokes the panic, since
	 * there is nothing to prevent an interrupt handler (that runs
	 * after the panic_lock is acquired) from invoking panic again.
	 */
	local_irq_disable();  //1. 关中断

	/*
	 * It's possible to come here directly from a panic-assertion and
	 * not have preempt disabled. Some functions called from here want
	 * preempt to be disabled. No point enabling it later though...
	 *
	 * Only one CPU is allowed to execute the panic code from here. For
	 * multiple parallel invocations of panic, all other CPUs either
	 * stop themself or will wait until they are stopped by the 1st CPU
	 * with smp_send_stop().
	 */
	if (!spin_trylock(&panic_lock))
		panic_smp_self_stop();

	console_verbose();
	bust_spinlocks(1);
	va_start(args, fmt);
	vsnprintf(buf, sizeof(buf), fmt, args);
	va_end(args);
	pr_emerg("Kernel panic - not syncing: %s\n", buf);    //打印panic得参数
#ifdef CONFIG_DEBUG_BUGVERBOSE
	/*
	 * Avoid nested stack-dumping if a panic occurs during oops processing
	 */
	if (!test_taint(TAINT_DIE) && oops_in_progress <= 1)
		dump_stack();    //打印堆栈，避免nested crash。
#endif

	/*
	 * If we have crashed and we have a crash kernel loaded let it handle
	 * everything else.
	 * If we want to run this after calling panic_notifiers, pass
	 * the "crash_kexec_post_notifiers" option to the kernel.
	 */
	if (!crash_kexec_post_notifiers)
		crash_kexec(NULL);

	/*
	 * Note smp_send_stop is the usual smp shutdown function, which
	 * unfortunately means it may not be hardened to work in a panic
	 * situation.
	 */
	smp_send_stop();

	/*
	 * Run any panic handlers, including those that might need to
	 * add information to the kmsg dump output.
	 */
	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);    //内核通知链，通知关注panic得进程，并把panic得参数传递

	kmsg_dump(KMSG_DUMP_PANIC);    //KMSG dump

	/*
	 * If you doubt kdump always works fine in any situation,
	 * "crash_kexec_post_notifiers" offers you a chance to run
	 * panic_notifiers and dumping kmsg before kdump.
	 * Note: since some panic_notifiers can make crashed kernel
	 * more unstable, it can increase risks of the kdump failure too.
	 */
	if (crash_kexec_post_notifiers)
		crash_kexec(NULL);

	bust_spinlocks(0);

	/*
	 * We may have ended up stopping the CPU holding the lock (in
	 * smp_send_stop()) while still having some valuable data in the console
	 * buffer.  Try to acquire the lock then release it regardless of the
	 * result.  The release will also print the buffers out.  Locks debug
	 * should be disabled to avoid reporting bad unlock balance when
	 * panic() is not being callled from OOPS.
	 */
	debug_locks_off();
	console_flush_on_panic();

	if (!panic_blink)
		panic_blink = no_blink;

	if (panic_timeout > 0) {
		/*
		 * Delay timeout seconds before rebooting the machine.
		 * We can't use the "normal" timers since we just panicked.
		 */
		pr_emerg("Rebooting in %d seconds..\n", panic_timeout);

		for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP) {
			touch_nmi_watchdog();
			if (i >= i_next) {
				i += panic_blink(state ^= 1);
				i_next = i + 3600 / PANIC_BLINK_SPD;
			}
			mdelay(PANIC_TIMER_STEP);
		}
	}
	if (panic_timeout != 0) {
		/*
		 * This will not be a clean reboot, with everything
		 * shutting down.  But if there is a chance of
		 * rebooting the system it will be rebooted.
		 */
		emergency_restart();
	}
#ifdef __sparc__
	{
		extern int stop_a_enabled;
		/* Make sure the user can actually press Stop-A (L1-A) */
		stop_a_enabled = 1;
		pr_emerg("Press Stop-A (L1-A) to return to the boot prom\n");
	}
#endif
#if defined(CONFIG_S390)
	{
		unsigned long caller;

		caller = (unsigned long)__builtin_return_address(0);
		disabled_wait(caller);
	}
#endif
	pr_emerg("---[ end Kernel panic - not syncing: %s\n", buf);
	local_irq_enable();
	for (i = 0; ; i += PANIC_TIMER_STEP) {
		touch_softlockup_watchdog();
		if (i >= i_next) {
			i += panic_blink(state ^= 1);
			i_next = i + 3600 / PANIC_BLINK_SPD;
		}
		mdelay(PANIC_TIMER_STEP);
	}
}

 

流程图：