Re: [Xen-devel] [PATCH 2/2] xen/arm: take the rank lock before accessing ipriority

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-06-24 at 19:11 +0100, Stefano Stabellini wrote:
> Currently we read ipriority from vgic_vcpu_inject_irq without taking the
> rank lock. Fix that by taking the rank lock and reading ipriority at the
> beginning of the function.

Since it is a byte read we'll always get either the before or after
value of any racing write, won't we?

The real danger would be the compiler deciding to read the value twice
for some reason, which it is entitled to do (e.g. under register
pressure).

The unlock has enough barriers in to prevent that I think (hope!). But I
think you could probably get away with an ACCESS_ONCE() type thing only.

> As vgic_vcpu_inject_irq is called from the irq.c upon receiving an
> interrupt, we need to change the implementation of vgic_lock/unlock_rank
> to spin_lock_irqsave to make it safe in irq context.
> 
> Also add a warning to point out that if the irq is already inflight with
> a different priority, we are not changing the irq priority for the
> second injection.

I think this matches the defined hardware behaviour, doesn't it? No need
for a warning in that case IMHO.

Ian.


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