Can an interrupt handler be preempted by the same interrupt handler?

Question

Does the CPU disable all interrupts on local CPU before calling the interrupt handler? Or does it only disable that particular interrupt line, which is being served?

Answer 1

We want ISR to be atomic and no one should be able to preempt the ISR.

Therefore, An ISR disables the local interrupts ( i.e. the interrupt on the current processor) and once the ISR calls ret_from_intr() function ( i.e. we have finished the ISR) , interrupts are again enabled on the current processor.

If an interrupt occurs, it will now be served by the other processor ( in SMP system) and ISR related to that interrupt will start running.

In SMP system , We also need to include the proper synchronization mechanism ( spin lock) in an ISR.