Re: [PATCH v5 4/7] genirq: Integrate GSIM into interrupt flow

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Overloading IRQD_IRQ_INPROGRESS for GSIM breaks __synchronize_hardirq() by introducing prolonged busy-waiting and potential deadlocks.
--

--- Patch [4]: [PATCH v5 4/7] genirq: Integrate GSIM into interrupt flow ---
commit 8749b57516c52ba231f3249d6f52173005693e96
Author: Luigi Rizzo <[email protected]>
Subject: genirq: Integrate GSIM into interrupt flow

This commit integrates Generic Software Interrupt Moderation (GSIM) into
the handle_edge_irq() and handle_fasteoi_irq() flows. It introduces
irq_moderation_allow() to validate IRQ eligibility, defaults moderation to
disabled during setup, and ensures the state is cleared during IRQ teardown.

> diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
> index de754db414d1d..04a30ba04e78b 100644
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
[ ... ]
> @@ -500,6 +501,10 @@ static bool irq_can_handle_pm(struct irq_desc *desc)
>  		return false;
>  	}
>  
> +	/* Moderated interrupts have IRQD_IRQ_INPROGRESS and need early return. */
> +	if (irqd_is_moderated(irqd))
> +		return false;
> +

[Severity: High]
The comment mentions that moderated interrupts have IRQD_IRQ_INPROGRESS
set. Could this cause a prolonged busy-wait or deadlock in
__synchronize_hardirq() when a driver calls disable_irq() or free_irq()
while the moderation timer is active?

__synchronize_hardirq() uses a tight loop to wait for this flag to clear:

kernel/irq/manage.c:__synchronize_hardirq() {
    ...
    while (irqd_irq_inprogress(&desc->irq_data))
        cpu_relax();
    ...
}

If the flag remains active for the entire duration of the moderation
hrtimer, any teardown or suspend/resume operation would busy-spin
for that entire duration.

Additionally, if synchronize_irq() is called with local interrupts disabled
on the CPU where the moderation timer is pinned, would this hard-deadlock
the system since the hrtimer can never fire to clear the flag?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=4