导出kernel panic信息

jathefo

在kernel panic的时候总会输出类似于：
    Kernel panic - not syncing: print to file test! XXX
XXX部分一般是具体哪个模块crash了或者指示crash的类型。所以这句信息对判断panic的定位就很关键。
那么怎么导出这行信息？
有一种做法是导出到文件，因为现在kernel中也可以发生文件操作，就是通过filp_open(),get_fs(),set_fs()等函数来完成。关于这个的测试程序网上有很多，可以参考。

于是我在panic()函数加入了对文件读写的操作，代码如下：
void panic(const char *fmt, ...)
{
        static DEFINE_SPINLOCK(panic_lock);
        static char buf[1024];
        va_list args;
        long i, i_next = 0;
        int state = 0;
    //for printing panic to file
    char sbuf[512];
    struct file *fp = NULL;
    mm_segment_t old_fs;

    //print panic to file
    sprintf(sbuf,"Kernel panic - not syncing: %s\n",buf);
    printk("kern panic - filp_open\n");
    if(fp == NULL)
        fp = filp_open(INFO_FILE, O_RDWR | O_CREAT, 0644);
    if (IS_ERR(fp)) {
        printk(KERN_EMERG "error occured while opening file %s, exiting...\n", INFO_FILE);
        fp = NULL;
        goto next;
    }
    printk("kern panic - open file success, fp: %p\n",fp);
    printk("kern panic - write info to file\n");
    old_fs = get_fs();
    set_fs(KERNEL_DS);
    fp->f_op->write(fp, (char *)sbuf, sizeof(sbuf), &fp->f_pos);
    set_fs(old_fs);
    filp_close(fp, NULL);
    printk("kern panic - filp_close\n");

next:
        trace_kernel_panic(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();

        /*
         * 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);
...
}
上述红色部分是我添加的，其中INFO_FILE是"/data/log-panic"。经过测试发现整个调试信息打印都正常，但是在/data目录下就是没有生成log-panic文件。
有一种可能就是在读写文件之前，data分区没有挂载好。于是我写了一个驱动模块，在驱动中panic，手动加载模块，这样系统就能完全启动好。但是在/data目录下还是没能生成log-panic文件。
测试的log如下：
    [   69.383373] kern panic - filp_open
    [   69.386312] kern panic - open file success, fp: ffffffc0b8354480
    [   69.386325] kern panic - write info to file
    [   69.390874] kern panic - filp_close
    [   69.393521] Kernel panic - not syncing: print to file test!

问题： 如果分区加载都正常，文件的打开、读写、关闭都顺利，为什么分区中没有生成相应的文件呢？