linux-kernel - Re: [PATCH v3 1/3] KVM: arm/arm64: vgic: Make vgic_irq->irq_lock a raw

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Message-ID: <6eeb2d94-cdda-006b-f32f-7857b61b3eaa@arm.com>
Date:   Fri, 1 Feb 2019 17:55:43 +0000
From:   Julien Grall <julien.grall@....com>
To:     Julia Cartwright <julia@...com>
Cc:     Julien Thierry <julien.thierry@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/3] KVM: arm/arm64: vgic: Make vgic_irq->irq_lock a
 raw_spinlock

Hi Julia,

On 01/02/2019 17:36, Julia Cartwright wrote:
> On Fri, Feb 01, 2019 at 03:30:58PM +0000, Julien Grall wrote:
>> Hi Julien,
>>
>> On 07/01/2019 15:06, Julien Thierry wrote:
>>> vgic_irq->irq_lock must always be taken with interrupts disabled as
>>> it is used in interrupt context.
>>
>> I am a bit confused with the reason here. The code mention that ap_list_lock
>> could be taken from the timer interrupt handler interrupt. I assume it
>> speaks about the handler kvm_arch_timer_handler. Looking at the
>> configuration of the interrupt, the flag IRQF_NO_THREAD is not set, so the
>> interrupt should be threaded when CONFIG_PREEMPT_FULL is set. If my
>> understanding is correct, this means the interrupt thread would sleep if it
>> takes the spinlock.
>>
>> Did I miss anything? Do you have an exact path where the vGIC is actually
>> called from an interrupt context?
> 
> The part you're missing is that percpu interrupts are not force
> threaded:
> 
> 	static int irq_setup_forced_threading(struct irqaction *new)
> 	{
> 		if (!force_irqthreads)
> 			return 0;
> 		if (new->flags & (IRQF_NO_THREAD | IRQF_PERCPU | IRQF_ONESHOT))
> 			return 0;
> 	
> 		/* ...*/
> 	}

Thank you for the pointer! I think it would be worth mentioning in the commit 
message that per-cpu interrupts are not threaded.

Best regards,

-- 
Julien Grall