Re: [Linuxarm] [PATCH for next v1 0/2] gpio: few clean up patches to replace spin_lock_irqsave with spin_lock

[Andy Shevchenko]

On Tue, Feb 9, 2021 at 11:24 AM luojiaxing <luojiaxing@xxxxxxxxxx> wrote:
> On 2021/2/8 21:28, Andy Shevchenko wrote:
> > On Mon, Feb 8, 2021 at 11:11 AM luojiaxing <luojiaxing@xxxxxxxxxx> wrote:
> >> Sorry, my operation error causes a patch missing from this patch set. I
> >> re-send the patch set. Please check the new one.
> > What is the new one?! You have to give proper versioning and change
> > log for your series.
>
> sure, I will send a new one later, but let me answer your question first.
>
> >> On 2021/2/8 16:56, Luo Jiaxing wrote:
> >>> There is no need to use API with _irqsave in hard IRQ handler, So replace
> >>> those with spin_lock.
> > How do you know that another CPU in the system can't serve the

The keyword here is: *another*.

> > following interrupt from the hardware at the same time?
>
> Yes, I have some question before.
>
> There are some similar discussion here,  please take a look, Song baohua
> explained it more professionally.
>
> https://lore.kernel.org/lkml/e949a474a9284ac6951813bfc8b34945@xxxxxxxxxxxxx/
>
> Here are some excerpts from the discussion:
>
> I think the code disabling irq in hardIRQ is simply wrong.

Why?

> Since this commit
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e58aa3d2d0cc
> genirq: Run irq handlers with interrupts disabled
>
> interrupt handlers are definitely running in a irq-disabled context
> unless irq handlers enable them explicitly in the handler to permit
> other interrupts.

This doesn't explain any changes in the behaviour on SMP.
IRQ line can be disabled on a few stages:
 a) on the source (IP that generates an event)
 b) on IRQ router / controller
 c) on CPU side

The commit above is discussing (rightfully!) the problem when all
interrupts are being served by a *single* core. Nobody prevents them
from being served by *different* cores simultaneously. Also, see [1].

[1]: https://www.kernel.org/doc/htmldocs/kernel-locking/cheatsheet.html

-- 
With Best Regards,
Andy Shevchenko