Re: [Xen-devel] [PATCH v5 1/3] arm: remove irq from inflight, then change physical affinity

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 01/03/2017 22:15, Stefano Stabellini wrote:
> This patch fixes a potential race that could happen when
> gic_update_one_lr and vgic_vcpu_inject_irq run simultaneously.
>
> When GIC_IRQ_GUEST_MIGRATING is set, we must make sure that the irq has
> been removed from inflight before changing physical affinity, to avoid
> concurrent accesses to p->inflight, as vgic_vcpu_inject_irq will take a
> different vcpu lock.
>
> Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> ---
>  xen/arch/arm/gic.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
> index 9522c6c..16bb150 100644
> --- a/xen/arch/arm/gic.c
> +++ b/xen/arch/arm/gic.c
> @@ -503,6 +503,11 @@ static void gic_update_one_lr(struct vcpu *v, int i)
>              gic_raise_guest_irq(v, irq, p->priority);
>          else {
>              list_del_init(&p->inflight);
> +            /* Remove from inflight, then change physical affinity. It
> +             * makes sure that when a new interrupt is received on the
> +             * next pcpu, inflight is already cleared. No concurrent
> +             * accesses to inflight. */
Coding style:

/*
 * ...
 */

> +            smp_mb();
Barriers are working in pair. So where is the associated barrier?

Also, I am still unsure why you use a dmb(ish) (implementation of 
smp_mb) and not dmb(sy).
Cheers,

--
Julien Grall

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