panic函数分析

在阅读协议栈代码（内核版本2.6.38），在很多地方都看到panic调用，只是知道大体功能，从未具体分析过，也一直想把协议栈的分析过程写下来，很懒，就一有写，算今天是个开始吧。分析panic太艰难啦，涉及的东西太多啦。

/**
 *    panic - halt the system
 *    @fmt: The text string to print
 *
 *    Display a message, then perform cleanups.
 *
 *    This function never returns.
 */
NORET_TYPE void panic(const char * fmt, ...)
{
    static char buf[1024];
    va_list args;
    long i, i_next = 0;
    int state = 0;

    /*
     * 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...
     */
    /**
     * 英文注释已经很清晰啦，就是在panic执行过程CPU不能被抢占。
     */
    preempt_disable();
    /**
     * 修改console 级别，以便printk能把消息打印出来。
     */
    console_verbose();

/*
 * Unlock any spinlocks which will prevent
 * us from getting the
 message out.

* 英文注释没有看多大明白啊，大概意思好像是抑制打印。
 */
    bust_spinlocks(1);
    va_start(args, fmt);
    vsnprintf(buf, sizeof(buf), fmt, args);
    va_end(args);
    printk(KERN_EMERG "Kernel panic - not syncing: %s\n",buf);
#ifdef CONFIG_DEBUG_BUGVERBOSE
    dump_stack();
#endif

    /*
     * If we have crashed and we have a crash kernel loaded let it handle
     * everything else.
     * Do we want to call this before we try to display a message?
     */
    crash_kexec(NULL);
    /××
     × 应该主要是dump CPU寄存器的状态值已经一些堆栈的信息。
     ×/
    kmsg_dump(KMSG_DUMP_PANIC);

    /*
     * 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();
    /××
     × 调用注册到panic时需要特殊处理的函数，如kgdb调试的入口等，
     ×/
    atomic_notifier_call_chain(&panic_notifier_list, 0, buf);

    bust_spinlocks(0);

    if (!panic_blink)
        panic_blink = no_blink;

     /**

      * 如果sysctl配置了panic_timeout > 0则在panic_timeout后重       * 启系统

      */

    if (panic_timeout > 0) {
        /*
         * Delay timeout seconds before rebooting the machine.
         * We can't use the "normal" timers since we just panicked.
         */
        printk(KERN_EMERG "Rebooting in %d seconds..", 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);
        }
        /*
         * 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;
        printk(KERN_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
    local_irq_enable();
/**
 *  进行死循环，抢占被禁止，CPU一直运行panic程序中运行。
 */
    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);
    }
}