linux-kernel - Re: [PATCH] irqchip/sifive-plic: enable interrupt if needed before EOI

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Date: Fri, 26 Jan 2024 18:31:19 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Nam Cao <namcao@...utronix.de>, Thomas Gleixner <tglx@...utronix.de>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Samuel Holland <samuel@...lland.org>,
 Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org, Guo Ren <guoren@...nel.org>
Cc: stable@...r.kernel.org
Subject: Re: [PATCH] irqchip/sifive-plic: enable interrupt if needed before
 EOI

Hi,

On 2024-01-26 4:38 PM, Nam Cao wrote:
> RISC-V PLIC cannot EOI disabled interrupts, as explained in the
> description of Interrupt Completion in the PLIC spec:
> 
> "The PLIC signals it has completed executing an interrupt handler by
> writing the interrupt ID it received from the claim to the claim/complete
> register. The PLIC does not check whether the completion ID is the same
> as the last claim ID for that target. If the completion ID does not match
> an interrupt source that *is currently enabled* for the target, the
> completion is silently ignored."
> 
> Commit 69ea463021be ("irqchip/sifive-plic: Fixup EOI failed when masked")
> ensured that by enabling the interrupt if needed before EOI.
> 
> Commit a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask
> operations") removed the interrupt enabling code from the previous
> commit, because it assumes that interrupt should be enabled at the point
> of EOI. However, this is incorrect: there is a small window after a hart
> claiming an interrupt and before irq_desc->lock getting acquired,
> interrupt can be disabled during this window. Thus, EOI can be invoked
> while the interrupt is disabled, effectively nullify this EOI.
> 
> Make sure that interrupt is really enabled before EOI.

Could you please try the patch I previously sent for this issue[1]? It
should fix the bug without complicating the IRQ hot path.

Thanks,
Samuel

[1]: https://lore.kernel.org/lkml/20230717185841.1294425-1-samuel.holland@sifive.com/

> Fixes: a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask operations")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Nam Cao <namcao@...utronix.de>
> ---
>  drivers/irqchip/irq-sifive-plic.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
> index 5b7bc4fd9517..0857a516c35b 100644
> --- a/drivers/irqchip/irq-sifive-plic.c
> +++ b/drivers/irqchip/irq-sifive-plic.c
> @@ -148,7 +148,13 @@ static void plic_irq_eoi(struct irq_data *d)
>  {
>  	struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
>  
> -	writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +	if (irqd_irq_disabled(d)) {
> +		plic_toggle(handler, d->hwirq, 1);
> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +		plic_toggle(handler, d->hwirq, 0);
> +	} else {
> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +	}
>  }
>  
>  #ifdef CONFIG_SMP