Subject: Re: [PATCH] irq: move some interrupt arch_* functions into struct irq_chip.

From: Ian Campbell <[email-masked]>
Date: Tue, 16 Mar 2010 08:50:12 +0000

On Sat, 2010-03-13 at 00:29 +0000, Eric W. Biederman wrote: 
> [...]
> > after that xen could use
> > irq_to_desc_alloc_node_f(irq, node, xen_init_chip_data);
> >
> > as need...
> >
> > at last we don't need to call x86_init_chip_data everywhere.

This was one of the things I was considering. It seems like one of the
easiest solutions to make work correctly with the current locking in
e.g. irq_to_desc_alloc_node since the callback would always happen under
the lock taken in that function.

> So trying to evaluate races.  The worse case for this particular piece
> of code appears to be create_irq_nr.  As this is the only place where
> we are setting up irqs and possibly repurposing the structure.

Yes, create_irq_nr was one of the functions I was struggling to solve
cleanly. There is a similar construct in the Xen code as well.

Part of the problem I'm having is the combination of lookup and allocate
in irq_to_desc_alloc_node but also the kind of "implicit" repurposing is
tricky to deal with. (by implicit I just mean that I can't find where
the previous user explicitly says they are finished with it, if you see
what I mean)

What do you think of adding an explicit free operation for the irq_desc
structs? (does one already exist? I couldn't find it). This would go
along with some tracking of allocation state, trivial in the sparse case
where you can treat a NULL node in the radix tree as unallocated, I
guess a flag would suffice in the static array non-sparse case?

Going further could we split the alloc and lookup functions into
separate operations instead of combining them in irq_to_desc_alloc_node?
We already have irq_to_desc for the lookup portion so this would largely
involve changes to the semantics of irq_to_desc_alloc_node, perhaps
returning ERR_PTR(-EBUSY) if the node was already allocated.

Having a variant which found a free IRQ rather than operating on a
specific requested IRQ could also be useful for create_irq_nr as well as
find_unbound_irq on the Xen side. I'm not convinced irq_alloc_virt on
powerpc isn't implementing broadly the same concept as well, although it
seems to work very differently from the other two.

>   Today
> we figure out if an irq has been assigned by looking at irq_cfg->vector,
> and if it is non-zero the irq has been assigned.

Which is tricky to move into generic code hence my suggestions of
explicitly freeing the irq_desc and tracking the allocation status in
the generic code. 

> The logic in x86_init_chip_data is correct we only assign desc->chip_data
> if the generic layers are above it.  However we need a lock to ensure that
> two paths don't race in that comparison and that assignment.  There is
> no lock in x86_init_chip_data.  Which unfortunately means as it stands
> this patchset is buggy.

Yes, unfortunately I think you are right. The callback idea fixes this.
I'll respin with that.

Ian.