linux-kernel - Re: How to know if there is a pending interrupt when they are masked?

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Message-ID: <9ff685f9-d378-1942-3678-7aff8f5b3aa3@linaro.org>
Date:   Thu, 14 Feb 2019 17:16:07 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: How to know if there is a pending interrupt when they are masked?


Hi Thomas,

thanks for the answer,

On 14/02/2019 16:47, Marc Zyngier wrote:
> On 14/02/2019 15:35, Thomas Gleixner wrote:
>> On Thu, 14 Feb 2019, Daniel Lezcano wrote:
>>> today we have the possibility to mask the local interrupt with the
>>> command local_irq_disable / local_irq_enable.
>>>
>>> The power management path disables the local interrupt in order to
>>> initiate a power down sequence. If there is a pending interrupt this one
>>> will make the power down function to abort, thus exiting right after
>>> calling the shutdown function after costly operations. It could be
>>> interesting to check if there is a pending interrupt before initiating
>>> the power down sequence.
>>>
>>> Is there a way to know if there is a pending interrupt on the current
>>> CPU when the local interrupt are disabled? Something like,
>>> local_irq_pending() function ?
>>
>> We have nothing like that today, but it would be possible to implement this
>> at least on x86 by peeking the local APIC registers.
> 
> On arm64, reading ISR_EL1 would give some indication of that, but...

Does 'some indication' means as soon as we read the ISR_EL1 it may be
outdated right after ?

>> OTOH, the question is whether it's worth the trouble because the interrupt
>> could come in right after the query and the same issue which you want to
>> address persists. It only makes sense if it reduces the time window so
>> significantly that it actually matters.
> 
> as Thomas explains, the potential race is pretty tight.

Is a simple hack possible on ARM64 to check if it is worth or not?

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