The first and third are the stack pointers for hardware and software interrupts. It is obvious from the name of the variables, that these point to the top of stacks. The second - `irq_count` is used to check if a CPU is already on an interrupt stack or not. Initialization of the `hardirq_stack_ptr` is located in the `irq_init_percpu_irqstack` function in [arch/x86/kernel/irq\_64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/irq_64.c):
The first and third are the stack pointers for hardware and software interrupts. It is obvious from the name of the variables, that these point to the top of stacks. The second - `irq_count` is used to check if a CPU is already on an interrupt stack or not. Initialization of the `hardirq_stack_ptr` is located in the `irq_init_percpu_irqstack` function in [arch/x86/kernel/irq_64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/irq_64.c):
where `nmi` and `double_fault` are entry points created at [arch/x86/kernel/entry\_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S):
where `nmi` and `double_fault` are entry points created at [arch/x86/kernel/entry_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S):