linux-kernel - Re: [PATCH v3] genirq: avoid long loops in handle_edge

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Message-ID: <87r0m3q32t.ffs@tglx>
Date:   Mon, 09 Oct 2023 16:32:10 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Wei Gong <gongwei833x@...il.com>
Cc:     linux-kernel@...r.kernel.org, Wei Gong <gongwei833x@...il.com>
Subject: Re: [PATCH v3] genirq: avoid long loops in handle_edge_irq

On Thu, Sep 28 2023 at 18:06, Wei Gong wrote:
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
> @@ -831,7 +831,9 @@ void handle_edge_irq(struct irq_desc *desc)
>  		handle_irq_event(desc);
>  
>  	} while ((desc->istate & IRQS_PENDING) &&
> -		 !irqd_irq_disabled(&desc->irq_data));
> +		 !irqd_irq_disabled(&desc->irq_data) &&
> +		 cpumask_test_cpu(smp_processor_id(),
> +				   irq_data_get_effective_affinity_mask(&desc->irq_data)));

Ok. So now that mask part is correct, but what guarantees that this does
not lose interrupts?

Assume the following scenario:

   CPU 0                                CPU 1

   interrupt
        set IN_PROGRESS
        do {
                                        change_affinity_to(CPU1);
           handle_irq_event()
                ack_in_device()
                                        interrupt
                                           set PENDING
        } while (COND)

Now $COND is not true due to the affinity change and the edge handler
returns. As a consequence nothing acks the device and no further
interrupts are sent by the device.

That might not be true for your case, but that's a generic function and the
zoo of hardware which uses that is massive.

So no, we are not taking a risk here.

Thanks,

        tglx