Kernel Debugging Options

Multiple configure options that you can set during compile to aid in debugging and testing kernel code are available. These options are in the Kernel Hacking menu of the Kernel Configuration Editor. They all depend on CONFIG_DEBUG_KERNEL. When hacking on the kernel, consider enabling as many of these options as practical.

Some of the options are rather useful, enabling slab layer debugging, high-memory debugging, I/O mapping debugging, spin-lock debugging, and stack-overflow checking. One of the most useful settings, however, is sleep-inside-spinlock checking, which actually does much more.

Atomicity Debugging

Starting with 2.5, the kernel has an excellent infrastructure for detecting all sorts of atomicity violations. Recall from Chapter 8, "Kernel Synchronization Introduction," that atomic refers to something's capability to execute without division; the code completes without interruption or it does not complete at all. Code that holds a spin lock or has disabled kernel preemption is atomic. Code cannot sleep while atomicsleeping while holding a lock is a recipe for deadlock.

Thanks to kernel preemption, the kernel has a central atomicity counter. The kernel can be set such that if a task sleeps while atomic, or even does something that might sleep, the kernel will print a warning and provide a back trace. Potential bugs that are detectable include calling schedule() while holding a lock, issuing a blocking memory allocation while holding a lock, or sleeping while holding a reference to per-CPU data. This debugging infrastructure catches a lot of bugs and is highly recommended.

The following options make the best use of this feature:

CONFIG_PREEMPT=y
CONFIG_DEBUG_KERNEL=y
CONFIG_KALLSYMS=y
CONFIG_SPINLOCK_SLEEP=y