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

[Julien Grall]

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