linux-kernel - Re: [PATCH 1/3] genirq: Simplify cond_unmask_eoi

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Date:   Wed, 30 Nov 2022 15:48:18 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Samuel Holland <samuel@...lland.org>,
        Marc Zyngier <maz@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     linux-riscv@...ts.infradead.org,
        Samuel Holland <samuel@...lland.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] genirq: Simplify cond_unmask_eoi_irq()

Samuel!

On Sat, Nov 26 2022 at 17:41, Samuel Holland wrote:

> This function calls .irq_eoi in three places, making the logic hard to
> follow. Rearrange the function so that .irq_eoi is called only once.
>
> The only time .irq_eoi is not called is when all three if checks fail,
> so return early in that case. threads_oneshot can only be nonzero if
> IRQS_ONESHOT is set, so the IRQS_ONESHOT check can be omitted there.
>
> The IRQS_ONESHOT condition from the first if statement must then be
> copied to the unmask_irq() condition.
>
> Furthermore, if IRQS_ONESHOT is set, mask_irq() must have been called
> in the parent function, so the irqd_irq_masked() check is redundant.

Not really convinced that all this is functionaly equivalent.

>  static void cond_unmask_eoi_irq(struct irq_desc *desc, struct irq_chip *chip)
>  {
> -	if (!(desc->istate & IRQS_ONESHOT)) {
> -		chip->irq_eoi(&desc->irq_data);
> +	/*
> +	 * Do not send an EOI if the thread will do it later in
> +	 * unmask_threaded_irq().
> +	 */
> +	if ((chip->flags & IRQCHIP_EOI_THREADED) && desc->threads_oneshot)
>  		return;
> -	}
> +
> +	chip->irq_eoi(&desc->irq_data);

This now issues EOI when the interrupt is in disabled state, which was
not done before. That's probably a non-issue, but clearly a undocumented
change.

> +
>  	/*
>  	 * We need to unmask in the following cases:
>  	 * - Oneshot irq which did not wake the thread (caused by a
> @@ -669,12 +674,8 @@ static void cond_unmask_eoi_irq(struct irq_desc *desc, struct irq_chip *chip)
>  	 *   completely).
>  	 */
>  	if (!irqd_irq_disabled(&desc->irq_data) &&
> -	    irqd_irq_masked(&desc->irq_data) && !desc->threads_oneshot) {
> -		chip->irq_eoi(&desc->irq_data);
> +	    (desc->istate & IRQS_ONESHOT) && !desc->threads_oneshot)
>  		unmask_irq(desc);

This breaks the mask logic of handle_fasteoi_mask_irq() for an interrupt
which does not have IRQS_ONESHOT set.

So no, it's not the same and it even breaks stuff.

Thanks,

        tglx