17 Linux kernel modes

Structure of kernel gives rise to two main modes for kernel code :

• process context : kernel code working for user programs by executing a system call.
• interrupt context : kernel code handling an interrupt. has access to user data only in process context. Any code running in process context may be pre-empted at any time by an interrupt. Interrupts have priority levels. Interrupts of lower priority are pre-empted by interrupts of higher priority.

Kernel modules

• can add code to running kernel.
• Useful for providing device drivers which are required only if hardware present.
• modprobe inserts module into running kernel.
• rmmod removes module from running kernel (if unused).
• lsmod lists currently running module.

Concurrency issues in the kernel

Correct handling concurrency in the kernel is important. Manipulation of data structures which are shared between :

• code running in process mode and code running in interrupt mode.
• code running in interrupt mode. Must happen only withing critical regions. In multi-processor system even manipulation of data structures shared between code running in process context must happen only within critical sections.

Achieving mutual exclusion

Two ways :

• Semaphores/Mutex : when entering critical section fails, current process is put to sleep until critical region is available
  $\Rightarrow$ only usable if all critical regions are in process context
  Functions : DEFINE_MUTEX(), mutex_lock(), mutex_unlock()
• Spinlock : processor tries repeatedly to enter critical section. Usable anywhere
  Disadvantage : Have busy waiting Functions : spin_lock_init(), spin_lock(), spin_unlock()

Programming data transfer between userspace and kernel

Linux maintains a directory called proc as interface between user space and kernel. Files in this directory do not exists on disk. Read-and write-operations on these files translated into kernel operations, together with data transfer between user space and kernel. Useful mechanism for information exchange between kernel and user space,