[PATCH] powerpc: Don't select HAVE_IRQ_EXIT_ON_IRQ_STACK

[Christophe Leroy]

Le 25/05/2022 à 05:26, Michael Ellerman a écrit :
> The HAVE_IRQ_EXIT_ON_IRQ_STACK option tells generic code that irq_exit()
> is called while still running on the hard irq stack (hardirq_ctx[] in
> the powerpc code).
> 
> Selecting the option means the generic code will *not* switch to the
> softirq stack before running softirqs, because the code is already
> running on the (mostly empty) hard irq stack.
> 
> But since commit 1b1b6a6f4cc0 ("powerpc: handle irq_enter/irq_exit in
> interrupt handler wrappers"), irq_exit() is now called on the regular task
> stack, not the hard irq stack.
> 
> That's because previously irq_exit() was called in __do_irq() which is
> run on the hard irq stack, but now it is called in
> interrupt_async_exit_prepare() which is called from do_irq() constructed
> by the wrapper macro, which is after the switch back to the task stack.
> 
> So drop HAVE_IRQ_EXIT_ON_IRQ_STACK from the Kconfig. This will mean an
> extra stack switch when processing some interrupts, but should
> significantly reduce the likelihood of stack overflow.
> 
> It also means the softirq stack will be used for running softirqs from
> other interrupts that don't use the hard irq stack, eg. timer interrupts.

One thing I have always wondered: what of the point of having both an 
hardirq stack and a softirq stack ?

__do_IRQ() doesn't switch of hardirq stack if we are on softirq stack.

do_softirq() voids if already in interrupts.
invoke_softirq() calls do_softirq_own_stack() on task_stack.

So we have neither situation where we switch from hardirq stack to 
softirq stack nor from softirq stack to hardirq stack.

So wouldn't it be enough to have only one common irq stack ?

christophe