Re: [PATCH] serial: 8250: fix shared interrupts issues with SMP and RT kernels

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> > again, please let the -rt maintainers sort out which patches need to be 
> > propagated to upstream maintainers.
> 
> This appears to be not only RT issue though. In theory, this can be

Agreed - RT is showing up a real bug here.

> triggered on SMP also. Thanks to Daniel Walker for pointing this out.

It looks correct to me except that you cannot use spin_lock/disable_irq
in that way safely. You must always disable_irq before taking the lock,
or prove it is safe and use disable_irq_nosync

The reason:
		CPU#0 spin_lock_... [taken]
		CPU#1 IRQ
		CPU#1 spin_lock [waits]
		CPU#0 disable_irq (deadlock)

Note that is also not generally safe to do

		disable IRQ on device
		spin_lock
		disable_irq

because IRQ propogation occurs asynchronously to PCI bus traffic even on
PC class systems (especially Pentium-PII era boxes with SMP). You can
disable the device IRQ and still have an IRQ 'in flight' that arrives
afterwards.

So the fix needs some reworking in its ordering I think

Alan