c - How to read, understand, analyze, and debug a Linux kernel panic?

Question

Consider the following Linux kernel dump stack trace, you can trigger a panic from the kernel source code by calling panic("debugging a Linux kernel panic");:

[<001360ac>] (unwind_backtrace+0x0/0xf8) from [<00147b7c>] (warn_slowpath_common+0x50/0x60)
[<00147b7c>] (warn_slowpath_common+0x50/0x60) from [<00147c40>] (warn_slowpath_null+0x1c/0x24)
[<00147c40>] (warn_slowpath_null+0x1c/0x24) from [<0014de44>] (local_bh_enable_ip+0xa0/0xac)
[<0014de44>] (local_bh_enable_ip+0xa0/0xac) from [<0019594c>] (bdi_register+0xec/0x150)

• In unwind_backtrace+0x0/0xf8 what does +0x0/0xf8 stand for?
• How can I see the C code of unwind_backtrace+0x0/0xf8?
• How to interpret the panic's content?

Answer 1

It's just an ordinary backtrace, those functions are called in reverse order (first one called was called by the previous one and so on):

unwind_backtrace+0x0/0xf8
warn_slowpath_common+0x50/0x60
warn_slowpath_null+0x1c/0x24
ocal_bh_enable_ip+0xa0/0xac
bdi_register+0xec/0x150

The bdi_register+0xec/0x150 is the symbol + the offset/length there's more information about that in Understanding a Kernel Oops and how you can debug a kernel oops. Also there's this excellent tutorial on Debugging the Kernel

Note: as suggested below by Eugene, you may want to try addr2line first, it still needs an image with debugging symbols though, for example

addr2line -e vmlinux_with_debug_info 0019594c(+offset)