Re: [Xen-devel] [PATCH 0/2] xen/arm: make accesses to desc->status flags atomic

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 13 Jun 2014, Jan Beulich wrote:
> >>> Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> 06/13/14 1:26 PM >>>
> >The IRQ line status flags are stored as bits in the status field of
> >struct irq_desc. Accesses to the status flags could be atomic but at the
> >moment they are not.
> >
> >Make them atomic on ARM. This allows us to avoid taking the desc
> >lock when clearing the IRQ_INPROGRESS flag for irqs routed to the guest.
> 
> So patch 2 doesn't drop any acquire/release pair of a lock - was it a bug then
> that the lock wasn't taken before?

Yes: after clearing an LR register (after the guest EOIs an irq), we
clear IRQ_INPROGRESS without taking the irq_desc lock.


In practice is not much of an issue because:

- at that point irqs are disabled so the same irq can only be received
  simultaneously if it is routed to another pcpu;

- when we receive an irq to be routed to a guest the only thing we do
  with IRQ_INPROGRESS is:
  
  desc->status |= IRQ_INPROGRESS;

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