Ways to reset or reboot the system

  • Adb shell Reboot
  • Kernel Panic
  • Watchdog Timeout

Adb shell Reboot

From windows commands prompt run

 C:\>adb shell reboot

Analysis

  • “adb shell reboot” is a shell command
  • This invokes a system call from userspace, that will eventually call the kernel_restart in the kernel.
  • Kernel_restart is the function that handles restart in the kernel.
  • This in turn will call arch_reset() This will do the target specific functionality of rebooting the target.
  • arch_reset will save the reset_reason and then invokes the Watchdog Timer for resetting the target.

Kernel Panic

What is Kernal panic

A kernel panic is an action taken by an operating system upon detecting an internal fatal error from which it cannot safely recover.

What will it do?

The kernel routines that

  • handle panics are outputs an error message to the console
  • dump an image of kernel memory to disk for post-mortem debugging 
  • then either wait for the system to be manually rebooted, or initiate an automatic reboot.(kernel/panic.c)

How?

The panic timeout sets how long (in seconds) the target will wait before resetting automatically when a Linux kernel panic occurs. Currently on the Android mainline this is set to 5,

  • so if a panic occurs, the panic message and backtrace are sent to the dmesg log, the target waits 5 seconds, then flushes caches and resets.
  • If you set it to 0, the target will never reset as a result of a panic. The reset on panic functionality can be disabled by:
    • 1) echo 0 to /proc/sys/kernel/panic (on the target at runtime)
    • 2) adding panic=0 to the kernel command line
    • 3) From “make kernelconfig” ,general setup submenu, and setting default panic timeout to 0

Watchdog Timeout

Why

Watchdog Timeout happens if the processor is struck in some thread infinitely.

Apps watchdog timer will get fired if its not refreshed in 4 seconds. (watchdog driver handles all)

How

Hardware watchdog is driven by the apps processor.

The watchdog driver currently schedules itself to pet the watchdog every 3 seconds.

  • If the hardware watchdog goes without being pet for 4 seconds, an interrupt is asserted which will panic the Linux kernel and print a back trace (triggering the panic timeout and reset if it is configured,). This is known as the "dog bark."
  • If the watchdog goes for 5 seconds without being pet, the target will reset. This is the "dog bite."

Note that the dog bark will only be received if the target is not stuck with interrupts disabled and the hardware is still in a functional state.

If the dog bark cannot be processed, and the system remains wedged for whatever reason, the dog bite will occur an reset the target.