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()
From: Grygorii Strashko
Date: Fri Feb 12 2021 - 06:55:16 EST
On 12/02/2021 13:29, Song Bao Hua (Barry Song) wrote:
-----Original Message-----
From: Andy Shevchenko [mailto:andy.shevchenko@xxxxxxxxx]
Sent: Friday, February 12, 2021 11:57 PM
To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
Cc: Grygorii Strashko <grygorii.strashko@xxxxxx>; Arnd Bergmann
<arnd@xxxxxxxxxx>; luojiaxing <luojiaxing@xxxxxxxxxx>; Linus Walleij
<linus.walleij@xxxxxxxxxx>; Santosh Shilimkar <ssantosh@xxxxxxxxxx>; Kevin
Hilman <khilman@xxxxxxxxxx>; open list:GPIO SUBSYSTEM
<linux-gpio@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
linuxarm@xxxxxxxxxxxxx
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@xxxxxx]
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@xxxxxxxxxxxxx> 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