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SubjectRe: [PATCH v3 08/16] irqchip/gic: Configure SGIs as standard interrupts
Hi James,

On Fri, 18 Sep 2020 10:58:45 +0100,
James Morse <james.morse@arm.com> wrote:
>
> Hi Marc,
>
> (CC: +Jon)
>
> On 01/09/2020 15:43, Marc Zyngier wrote:
> > Change the way we deal with GIC SGIs by turning them into proper
> > IRQs, and calling into the arch code to register the interrupt range
> > instead of a callback.
>
> Your comment "This only works because we don't nest SGIs..." on this
> thread tripped some bad memories from adding the irq-stack. Softirq
> causes us to nest irqs, but only once.
>
>
> (I've messed with the below diff to remove the added stuff:)
>
> > diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> > index 4ffd62af888f..4be2b62f816f 100644
> > --- a/drivers/irqchip/irq-gic.c
> > +++ b/drivers/irqchip/irq-gic.c
> > @@ -335,31 +335,22 @@ static void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
> > irqstat = readl_relaxed(cpu_base + GIC_CPU_INTACK);
> > irqnr = irqstat & GICC_IAR_INT_ID_MASK;
> >
> > - if (likely(irqnr > 15 && irqnr < 1020)) {
> > - if (static_branch_likely(&supports_deactivate_key))
> > - writel_relaxed(irqstat, cpu_base + GIC_CPU_EOI);
> > - isb();
> > - handle_domain_irq(gic->domain, irqnr, regs);
> > - continue;
> > - }
> > - if (irqnr < 16) {
> > writel_relaxed(irqstat, cpu_base + GIC_CPU_EOI);
> > - if (static_branch_likely(&supports_deactivate_key))
> > - writel_relaxed(irqstat, cpu_base + GIC_CPU_DEACTIVATE);
> > -#ifdef CONFIG_SMP
> > - /*
> > - * Ensure any shared data written by the CPU sending
> > - * the IPI is read after we've read the ACK register
> > - * on the GIC.
> > - *
> > - * Pairs with the write barrier in gic_raise_softirq
> > - */
> > smp_rmb();
> > - handle_IPI(irqnr, regs);
>
> If I read this right, previously we would EOI the interrupt before
> calling handle_IPI(). Where as now with the version of this series
> in your tree, we stuff the to-be-EOId value in a percpu variable,
> which is only safe if these don't nest.
>
> Hidden in irq_exit(), kernel/softirq.c::__irq_exit_rcu() has this:
> | preempt_count_sub(HARDIRQ_OFFSET);
> | if (!in_interrupt() && local_softirq_pending())
> | invoke_softirq();
>
> The arch code doesn't raise the preempt counter by HARDIRQ, so once
> __irq_exit_rcu() has dropped it, in_interrupt() returns false, and
> we invoke_softirq().
>
> invoke_softirq() -> __do_softirq() -> local_irq_enable()!
>
> Fortunately, __do_softirq() raises the softirq count first using
> __local_bh_disable_ip(), which in-interrupt() checks too, so this
> can only happen once per IRQ.
>
> Now the irq_exit() has moved from handle_IPI(), which ran after EOI,
> into handle_domain_irq(), which runs before. I think its possible
> SGIs nest, and the new percpu variable becomes corrupted.

I can't see how. The interrupt is active until we EOI/deactivate it,
and thus cannot be observed again by the CPU interface until this
happens.

Furthermore, irq_exit() in __handle_domain_irq() is *after* the EOI
anyway (generic_handle_irq_() directly calls the flow, which
immediately EOIs the interrupt). The only material change is that
irq_enter() happens before EOI. Is that what you are referring to?

Thanks,

M.

--
Without deviation from the norm, progress is not possible.

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