Re: [Linuxarm] Re: [PATCH for next v1 1/2] gpio: omap: Replace raw_spin_lock_irqsave with raw_spin_lock in omap_gpio_irq_handler()

[Grygorii Strashko <grygorii.strashko@ti.com>]

On 12/02/2021 13:29, Song Bao Hua (Barry Song) wrote:
> 
> 
>> -----Original Message-----
>> From: Andy Shevchenko [mailto:andy.shevchenko@gmail.com]
>> Sent: Friday, February 12, 2021 11:57 PM
>> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
>> Cc: Grygorii Strashko <grygorii.strashko@ti.com>; Arnd Bergmann
>> <arnd@kernel.org>; luojiaxing <luojiaxing@huawei.com>; Linus Walleij
>> <linus.walleij@linaro.org>; Santosh Shilimkar <ssantosh@kernel.org>; Kevin
>> Hilman <khilman@kernel.org>; open list:GPIO SUBSYSTEM
>> <linux-gpio@vger.kernel.org>; linux-kernel@vger.kernel.org;
>> linuxarm@openeuler.org
>> Subject: Re: [Linuxarm] Re: [PATCH for next v1 1/2] gpio: omap: Replace
>> raw_spin_lock_irqsave with raw_spin_lock in omap_gpio_irq_handler()
>>
>> On Fri, Feb 12, 2021 at 10:42:19AM +0000, Song Bao Hua (Barry Song) wrote:
>>>> From: Grygorii Strashko [mailto:grygorii.strashko@ti.com]
>>>> Sent: Friday, February 12, 2021 11:28 PM
>>>> On 12/02/2021 11:45, Arnd Bergmann wrote:
>>>>> On Fri, Feb 12, 2021 at 6:05 AM Song Bao Hua (Barry Song)
>>>>> <song.bao.hua@hisilicon.com> wrote:
>>
>>>>>>> Note. there is also generic_handle_irq() call inside.
>>>>>>
>>>>>> So generic_handle_irq() is not safe to run in thread thus requires
>>>>>> an interrupt-disabled environment to run? If so, I'd rather this
>>>>>> irqsave moved into generic_handle_irq() rather than asking everyone
>>>>>> calling it to do irqsave.
>>>>>
>>>>> In a preempt-rt kernel, interrupts are run in task context, so they clearly
>>>>> should not be called with interrupts disabled, that would defeat the
>>>>> purpose of making them preemptible.
>>>>>
>>>>> generic_handle_irq() does need to run with in_irq()==true though,
>>>>> but this should be set by the caller of the gpiochip's handler, and
>>>>> it is not set by raw_spin_lock_irqsave().
>>>>
>>>> It will produce warning from __handle_irq_event_percpu(), as this is IRQ
>>>> dispatcher
>>>> and generic_handle_irq() will call one of handle_level_irq or
>> handle_edge_irq.
>>>>
>>>> The history behind this is commit 450fa54cfd66 ("gpio: omap: convert to
>> use
>>>> generic irq handler").
>>>>
>>>> The resent related discussion:
>>>> https://lkml.org/lkml/2020/12/5/208
>>>
>>> Ok, second thought. irqsave before generic_handle_irq() won't defeat
>>> the purpose of preemption too much as the dispatched irq handlers by
>>> gpiochip will run in their own threads but not in the thread of
>>> gpiochip's handler.
>>>
>>> so looks like this patch can improve by:
>>> * move other raw_spin_lock_irqsave to raw_spin_lock;
>>> * keep the raw_spin_lock_irqsave before generic_handle_irq() to mute
>>> the warning in genirq.
>>
>> Isn't the idea of irqsave is to prevent dead lock from the process context when
>> we get interrupt on the *same* CPU?
> 
> Anyway, gpiochip is more tricky as it is also a irq dispatcher. Moving
> spin_lock_irq to spin_lock in the irq handler of non-irq dispatcher
> driver is almost always correct.
> 
> But for gpiochip, would the below be true though it is almost alway true
> for non-irq dispatcher?
> 
> 1. While gpiochip's handler runs in hardIRQ, interrupts are disabled, so no more
> interrupt on the same cpu -> No deadleak.
> 
> 2. While gpiochip's handler runs in threads
> * other non-threaded interrupts such as timer tick might come on same cpu,
> but they are an irrelevant driver and thus they are not going to get the
> lock gpiochip's handler has held. -> no deadlock.
> * other devices attached to this gpiochip might get interrupts, since
> gpiochip's handler is running in threads, raw_spin_lock can help avoid
> messing up the critical data by two threads -> still no deadlock.

The worst RT case I can imagine is when gpio API is still called from hard IRQ context by some
other device driver - some toggling for example.
Note. RT or "threadirqs" does not mean gpiochip become sleepable.

In this case:
  threaded handler
    raw_spin_lock
	IRQ from other device
           hard_irq handler
             gpiod_x()
		raw_spin_lock_irqsave() -- oops

But in general, what are the benefit of such changes at all, except better marking call context annotation,
so we are spending so much time on it?


-- 
Best regards,
grygorii