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

On Wed, Feb 10, 2021 at 5:43 AM luojiaxing [off-list ref] wrote:

On 2021/2/9 17:42, Andy Shevchenko wrote:

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On Tue, Feb 9, 2021 at 11:24 AM luojiaxing [off-list ref] wrote:

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On 2021/2/8 21:28, Andy Shevchenko wrote:

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On Mon, Feb 8, 2021 at 11:11 AM luojiaxing [off-list ref] wrote:

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On 2021/2/8 16:56, Luo Jiaxing wrote:

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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*.

ooh, sorry, now I got your point.

As to me, I don't think another CPU can serve the IRQ when one CPU
runing hard IRQ handler,

Why is it so?
Each CPU can serve IRQs separately.

except it's a per CPU interrupts.

I didn't get how it is related.

The following is a simple call logic when IRQ come.

elx_irq -> handle_arch_irq -> __handle_domain_irq -> desc->handle_irq ->
handle_irq_event

What is `elx_irq()`? I haven't found any mention of this in the kernel
source tree.
But okay, it shouldn't prevent our discussion.

Assume that two CPUs receive the same IRQ and enter the preceding
process. Both of them will go to desc->handle_irq().

Ah, I'm talking about the same IRQ by number (like Linux IRQ number,
means from the same source), but with different sequence number (means
two consequent events).

In handle_irq(), raw_spin_lock(&desc->lock) always be called first.
Therefore, even if two CPUs are running handle_irq(),

only one can get the spin lock. Assume that CPU A obtains the spin lock.
Then CPU A will sets the status of irq_data to

IRQD_IRQ_INPROGRESS in handle_irq_event() and releases the spin lock.
Even though CPU B gets the spin lock later and

continue to run handle_irq(), but the check of irq_may_run(desc) causes
it to exit.


so, I think we don't own the situation that two CPU server the hard IRQ
handler at the same time.

Okay. Assuming your analysis is correct, have you considered the case
when all IRQ handlers are threaded? (There is a kernel command line
option to force this)

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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@hisilicon.com/ (local)

Here are some excerpts from the discussion:

I think the code disabling irq in hardIRQ is simply wrong.

Why?

I mention the following call before.

elx_irq -> handle_arch_irq -> __handle_domain_irq -> desc->handle_irq ->
handle_irq_event


__handle_domain_irq() will call irq_enter(), it ensures that the IRQ
processing of the current CPU can not be preempted.

So I think this is the reason why Song baohua said it's not need to
disable IRQ in hardIRQ handler.

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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

yes, you are right.

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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

I check [1], quite useful description about locking, thanks. But you can
see Table of locking Requirements

Between IRQ handler A and IRQ handle A, it's no need for a SLIS.

Right, but it's not the case in the patches you provided.

--
With Best Regards,
Andy Shevchenko