Which stack is used by Interrupt Handler - Linux

Question

In a multitasking system when any hardware generates a interrupt to a particular CPU, where CPU can be performing either of below cases unless it is already serving a ISR:

1. User mode process is executing on CPU
2. Kernel mode process is executing on CPU

Would like to know which stack is used by interrupt handler in above two situations and why ?

Answer 1

All interrupts are handled by kernel. That is done by interrupt handler written for that particular interrupt. For Interrupt handler there is IRQ stack. The setup of an interrupt handler’s stacks is configuration option. The size of the kernel stack might not always be enough for the kernel work and the space required by IRQ processing routines. Hence 2 stack comes into picture.

1. Hardware IRQ Stack.
2. Software IRQ Stack.

In contrast to the regular kernel stack that is allocated per process, the two additional stacks are allocated per CPU. Whenever a hardware interrupt occurs (or a softIRQ is processed), the kernel needs to switch to the appropriate stack. Historically, interrupt handlers did not receive their own stacks. Instead, interrupt handlers would share the stack of the running process, they interrupted. The kernel stack is two pages in size; typically, that is 8KB on 32-bit architectures and 16KB on 64-bit architectures. Because in this setup interrupt handlers share the stack, they must be exceptionally frugal with what data they allocate there. Of course, the kernel stack is limited to begin with, so all kernel code should be cautious.