Question

I have hit BUG_ON in kernel code. I want to know what went wrong at that time.
BUG_ON will check for the condition and if it is true, it will panic.

Will BUG_ON print the messages in /var/log/messages?
Will it print the value it is checking before panicking?

Answer 1

Yes. From include/asm-generic/bug.h:

#ifndef HAVE_ARCH_BUG
#define BUG() do { \
    printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
    panic("BUG!"); \
} while (0)
#endif

#ifndef HAVE_ARCH_BUG_ON
#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)
#endif

Provided your kernel is compiled appropriately, BUG_ON will print a message before panicking.