Re: [PATCH] irq: fasteoi handler re-runs on concurrent invoke

["Gowans, James" <jgowans@amazon.com>]

On Mon, 2023-05-29 at 10:47 +0800, Liao, Chang wrote:
> 
> > 
> > Wording may need a slight tweak, especially pointing out why PENDING is
> > set.
> 
> Sure, see the comment below:
> 
> "Ack another interrupt from the same source can occurs on new CPU even before
> the first one is handled on original CPU, IRQS_PENDING bit can be reused to
> indicate this situation, which will defer the execution of the interrupt handler
> function associated with the irq_desc until the first interrupt handler returns."
> 
> In summary, the IRQ_PENDINGS ensures that only one interrupt handler is ever
> running for a particular source at a time, and the major usages of IRQS_PENDING
> in kernel as follows:
> 
> 1. Used in irq flow handler to indicate that an acknowledged interrupt cannot be
> handled immediately due to three different reasons:
> - Case1: the interrupt handler function has been unregistered via free_irq().
> - Case2: the interrupt has been disabled via irq_disable().
> - Case3: the interrupt is an edge-triggered interrupt and its handler is already
>          running on the CPU.
> 
> In any of these cases, the kernel will defer the execution of the interrupt handler
> until the interrupt is enabled and new handler is established again via check_irq_resend(),
> or via the inside loop in handle_edge_irq() upon the previous handler returns.
> 
> 2. Used in the spurious interrupt detector, a few systems with misdescribed IRQ
> routing can cause an interrupt to be handled on the wrong CPU. In this situation,
> the spurious interrupt detector searches for a recovery handler for the interrupt.
> If the found handler is running on another CPU, the spurious interrupt detector
> also defers the execution of the recovery handler, similar to case 3 in #1.
> 
> I hope this is helpful.

Stunning! I tried to pillage some of this for the commit message, but I
think it was too much detail for this commit message - it probably belongs
in a doc or code comment somewhere though. Useful!

> > If you want I'm happy to make these tweaks and post as V2?
> > There are also some other comments I'm keen to add to the flag enums to
> > make it a bit clearer what some of the flags mean.
> 
> Don't see why not :)

Done, V2: https://lore.kernel.org/lkml/20230530213848.3273006-2-jgowans@amazon.com/

JG