Re: [patch V6 07/37] x86/entry: Provide helpers for execute on irqstack

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> On Wed, May 20, 2020 at 5:35 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>
>> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>> > On Mon, May 18, 2020 at 4:53 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> >>
>> >> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>> >> > Actually, I revoke my ack.  Can you make one of two changes:
>> >> >
>> >> > Option A: Add an assertion to run_on_irqstack to verify that irq_count
>> >> > was -1 at the beginning?  I suppose this also means you could just
>> >> > explicitly write 0 instead of adding and subtracting.
>> >> >
>> >> > Option B: Make run_on_irqstack() just call the function on the current
>> >> > stack if we're already on the irq stack.
>> >> >
>> >> > Right now, it's too easy to mess up and not verify the right
>> >> > precondition before calling run_on_irqstack().
>> >> >
>> >> > If you choose A, perhaps add a helper to do the if(irq_needs_irqstack)
>> >> > dance so that users can just do:
>> >> >
>> >> > run_on_irqstack_if_needed(...);
>> >> >
>> >> > instead of checking everything themselves.
>> >>
>> >> I'll have a look tomorrow morning with brain awake.
>> >
>> > Also, reading more of the series, I suspect that asm_call_on_stack is
>> > logically in the wrong section or that the noinstr stuff is otherwise
>> > not quite right.  I think that objtool should not accept
>> > run_on_irqstack() from noinstr code.  See followups on patch 10.
>>
>> It's in entry.text which is non-instrumentable as well.
>
> Hmm.  I suppose we can chalk this up to the noinstr checking not being
> entirely perfect.

objtool considers both entry.text and noinstr.text. We just can't stick
everything into entry.text for these !%@#45@# reasons.