question about do_IRQ + 4k stacks

[Terence Ripperda <tripperda@nvidia.com>]

I'm investigating some 4k stack issues with our driver, and I noticed
this ordering in do_IRQ:

asmlinkage unsigned int do_IRQ(struct pt_regs regs)
{
	...

#ifdef CONFIG_DEBUG_STACKOVERFLOW
        /* Debugging check for stack overflow: is there less than 1KB free? */
        {
	...
        }
#endif

	...

#ifdef CONFIG_4KSTACKS

        for (;;) {
	... switch to interrupt stack
        }
#endif


Is the intention of this stack overflow check to catch a currently
running kernel thread that's getting low on stack space, or is the
intent to make sure there's enough stack space to handle the incoming
interrupt? if the later, wouldn't you want to potentially switch to
your interrupt stack to be more accurate? (I recognize that often you
will have switched to an empty stack, unless you have nested
interrupts)

Thanks,
Terence