RE: [tip:irq/core] genirq: Fix race on spurious interrupt detection

[David Laight]

From: Lukas Wunner
> Sent: 19 October 2018 16:34
> 
> genirq: Fix race on spurious interrupt detection
> 
> Commit 1e77d0a1ed74 ("genirq: Sanitize spurious interrupt detection of
> threaded irqs") made detection of spurious interrupts work for threaded
> handlers by:
> 
> a) incrementing a counter every time the thread returns IRQ_HANDLED, and
> b) checking whether that counter has increased every time the thread is
>    woken.

That seems horribly broken.
What is it trying to achieve?

There are (at least) two common cases where IRQ_HANDLED doesn't get returned.
(Unless the driver always returns it to avoid the message.)

1) The IOW that causes the hardware to drop a level sensitive IRQ is posted
   on the bus (etc) and happens late enough that the IRQ line is still
   asserted when the iret executes.
   If this happens all the time you need to flush the IOW, but if only
   occasionally it doesn't matter and you don't want a message.

2) Typically an ethernet driver ISR has to enable the interrupt and then
   check the ring for work before returning from the interrupt.
   If a packet arrives at this time it might be processed by the 'old'
   ISR invocation but still generate another interrupt.
   If no more packets arrive the second ISR invocation will find no work.
   Again this is normal behaviour.
   (Deferring everything with NAPI might make this not happen - but other
   interrupts end up working the same way.)

If you are really trying to detect 'stuck' interrupts then you probably
want to count un-handled ones and zero the count on handled ones.
I'm also pretty sure you don't need an atomic counter.

	David

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