An interrupt is an event that alters the sequence of instructions executed by a processor and requires immediate attention. When the processor receives an interrupt signal, it may temporarily switch control to an inter- rupt service routine (ISR) and the suspended process (i.e., the previously running program) will be resumed as soon as the interrupt is being served. The generic term interrupt is oftentimes used synonymously for two terms, interrupts and exceptions. An exception is a synchronous event that occurs when the processor detects an error condition while executing an instruction. Such an error condition may be a devision by zero, a page fault, a protection violation, etc. An interrupt, on the other hand, is an asynchronous event that occurs at random times during execution of a pro- gram in response to a signal from hardware. A proper and timely handling of interrupts is critical to the performance, but also to the security of a computer system.

In general, interrupts can be emitted by hardware as well as by software. Software interrupts (e.g., via the INT n instruction of the x86 instruction set architecture (ISA)) are means to change the execution context of a program to a more privileged interrupt context in order to enter the kernel and, in contrast to hardware interrupts, occur synchronously to the currently running program. Consequently, such instructions are gates to the privileged operating-system kernel (e.g., ring 0 on x86) and thus are used to request services from the kernel (i.e., system calls). A hardware-triggered interrupt indicates that some device requires attention of the processor, and hence implements a means of communication between devices and the operating system. Interrupts from hardware can greatly improve a system’s performance when devices send interrupts (e.g., a keystroke on a keyboard) instead of expensive polling of devices (e.g., periodically polling a keyboard for stroked keys). Furthermore, hardware-emitted interrupts by timers are used for timing and time measurement in general, but also for time sharing as ticks can be used to schedule another task.

After arrival of an interrupt, the processor executes the interrupt ser- vice routine that is associated with this interrupt. The ISR is also referred to as the interrupt request (IRQ) handler. But as interrupts are disabled during execution of the IRQ handler, not all kinds of code can or should be executed in this IRQ context. For instance, if such routine goes to sleep with interrupts disabled, the system is likely to freeze. On the other hand, active waiting and blocking should be avoided since other interrupts with potentially urgent needs remain disabled and, hence, cannot be served. Furthermore, acquiring locks is likely to cause deadlocks. As a consequence, the Linux kernel offers various mechanisms and application programming interfaces (APIs) to implement interrupt handling in order to meet certain functional and non-functional requirements.

This report focuses on how the Linux operating-system kernel handles interrupts on the software side and aims to give brief background infor- mation as well as implementation details. Detailed information about exceptions and exception handling in the x86 architecture can be found in the CPU manual ”Intel 64 and IA-32 Architectures Software Developer’s Manual”. Notice that the report does not aim for completeness, neither does it target to introduce the general concept of interrupts. It rather tries to provide information for developers, researchers and students, familiar with operating systems and operating-system concepts, how Linux handles interrupts on the software side.