Why Interrupts can't sleep ? And Nested Interrupt

So Lets start with a simple and very important question, why Interrupt can't sleep.

First go back to our understanding to User Context, there we can sleep why? because when you sleep in it, you can call schedule() which can ask other processes to run.

But when you are in Interrupt context, the only thing residing on that Processor is the Interrupt handler(of the respective interrupt). So if you sleep now, there is nothing else to schedule. So its like processor itself is in sleep which is not good at all.

Nested Interrupt

Now there are few things associated with Interrupt, like nested interrupts. So Nested interrupt is when you are running and interrupt handler and more interrupts come on the same line. So in interrupt handler you are getting another interrupt.

When this situation arises ?
Suppose for some reason you need to call enable_irq() in interrupt handler, so it means you are telling your hardware that you are ready to receive the new interrupt.

How Kernel Handles it ?
For this situation, Kernel have third mode called System Mode( apart from user and kernel mode). This is also a privileged mode. So if you want to enable nested interrupt you should change mode to system, and then you should enable interrupt. Before enabling interrupt, you keep LR and SPSR of IRQ mode in Stack of System mode.
This way we save LR and SPSR from being corrupted. And once the new interrupt handled it can return to the previous interrupt and finally it can go back to original location.
you can refer this link for more clarity.