Re: QUESTION: Network hangs with BP6 and 2.4.x kernels, hardware

[Linus Torvalds]

On Fri, 12 Jan 2001, Alan Cox wrote:

> > interrupt controllers (io-apic definitely included).  Drivers would
> > generally be better off if they disabled their own chip from sending
> > interrupts, rather than disabling the interrupt line the chip is on. 
> 
> That doesn't work very well because the device irq can arrive a measurable
> number of clocks after you disable it on the source and there is no way to
> say 'and be sure the stupid thing has propogated the apic bus'

I agree. Asynchronous buses are nasty that way. However, if your "locking"
is based on device state, you can still do it: you just have to have an
internal device protocol. The simplest example of this is:

	interrupt_handler()
	{
		status = readl(dev->status);
		if (status & MY_IRQ_DISABLE)
			return;

	}

	{
		status = readl(dev->status);
		writel(dev->status, status | MY_IRQ_DISABLE);
		spin_lock();
		.. critical region ..
		spin_unlock();
		writel(dev->status, status);
	}

Notice how the above does NOT guarantee that the physical interrupt might
not be "in flight". It does, however, synchronize other ways, simply by
virtue of using the _synchronous_ bus (PCI, in this case) to figure out
when the asynchronous bus (interrupts) has a bogus event.

		Linus