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 15:12, Song Bao Hua (Barry Song) wrote:
> 
> 
>> -----Original Message-----
>> From: Grygorii Strashko [mailto:grygorii.strashko@ti.com]
>> Sent: Saturday, February 13, 2021 12:53 AM
>> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; Andy Shevchenko
>> <andy.shevchenko@gmail.com>
>> Cc: 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 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
> 
> Actually no oops here. other drivers don't hold the same
> spinlock of this driver.

huh.
driver/module A requests gpio and uses it in its hard_irq handler by calling GPIO API
(Like gpiod_set_value()), those will go to this driver and end up in omap_gpio_set().

> 
>>
>> 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?
> 
> TBH, the benefit is really tiny except code cleanup. just curious how things could
> be different while it happens in an irq dispatcher's handler.


-- 
Best regards,
grygorii