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

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Message-ID: <20190201173650.GC29111@jcartwri.amer.corp.natinst.com>
Date:   Fri, 1 Feb 2019 17:36:51 +0000
From:   Julia Cartwright <julia@...com>
To:     Julien Grall <julien.grall@....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:  Re: [PATCH v3 1/3] KVM: arm/arm64: vgic: Make vgic_irq->irq_lock a
 raw_spinlock

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;
	
		/* ...*/
	}

   Julia