arm - How does linux (reboot) notifier list work?

Question

I am testing the reboot notifier hook on Linux kernel 5.8 running on QEMU ARM64 with busybox rootfs. This is my first time using a notifier list, so I might be wrong. I have add one reboot notifier which is executed well, but I have also added a second item to the list using .next and the handler of this notifier does not execute. Here's the snippet from my dummy driver:

static int reboot_handler_first(struct notifier_block *this, unsigned long action,
        void *data)
{
    printk(KERN_INFO "first reboot notifier called
");
    return 0;
}

static int reboot_handler_second(struct notifier_block *this, unsigned long action,
        void *data)
{
    printk(KERN_INFO "second reboot notifier called
");
    return 0;
}

static struct notifier_block reboot_notifier_second = {
    .notifier_call = reboot_handler_second,
    .next = NULL,
    .priority = INT_MAX,
};

static struct notifier_block reboot_notifier_first = {
    .notifier_call = reboot_handler_first,
    .next = &reboot_notifier_second,
    .priority = INT_MAX,
};

Here, the reboot_handler_first() gets called but I do not see any log from reboot_handler_second(). I was expecting that reboot_notifer_first() will trigger reboot_notifier_second() but that does not seem to be the case. Am I missing anything?

Below are the logs that I see with this kernel and busybox rootfs:

/ # reboot
/ # umount: devtmpfs busy - remounted read-only
swapoff: can't open '/etc/fstab': No such file or directory
The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL to all processes
Requesting system reboot
[   20.158023] first reboot notifier called
[   20.232718] reboot: Restarting system
[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd070]

EDIT

This is how my module_init(hello_init) function looks like. I have registered only the first reboot notifier reboot_notifier_first and then set its next to point to the reboot_notifier_second. Please correct me. My understanding is that I need to register the first notifier with register_reboot_notifier() and then I just need to set the .next of current notifier_block to add new nodes to the linked list. This way I will get a chain of notifier blocks.

static int __init hello_init(void)
{
    int rc;

    rc = register_reboot_notifier(&reboot_notifier_first);
    printk(KERN_INFO "This is from init
");
    return 0;
}

question from:https://stackoverflow.com/questions/65650339/how-does-linux-reboot-notifier-list-work

Answer 1

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