Re: [Xen-devel] [PATCH v4 6/6] VMX: Fixup PI descritpor when cpu is offline

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.09.16 at 04:37, <feng.wu@xxxxxxxxx> wrote:
> +void vmx_pi_desc_fixup(int cpu)
> +{
> +    unsigned int new_cpu, dest;
> +    unsigned long flags;
> +    struct arch_vmx_struct *vmx, *tmp;
> +    spinlock_t *new_lock, *old_lock = &per_cpu(vmx_pi_blocking, cpu).lock;
> +    struct list_head *blocked_vcpus = &per_cpu(vmx_pi_blocking, cpu).list;
> +
> +    if ( !iommu_intpost )
> +        return;
> +
> +    spin_lock_irqsave(old_lock, flags);
> +
> +    list_for_each_entry_safe(vmx, tmp, blocked_vcpus, pi_blocking.list)
> +    {
> +        /*
> +         * We need to find an online cpu as the NDST of the PI descriptor, it
> +         * doesn't matter whether it is within the cpupool of the domain or
> +         * not. As long as it is online, the vCPU will be woken up once the
> +         * notification event arrives.
> +         */
> +        new_cpu = cpumask_any(&cpu_online_map);
> +        new_lock = &per_cpu(vmx_pi_blocking, new_cpu).lock;
> +
> +        spin_lock(new_lock);

Without extra consideration this is a classical ABBA deadlock
scenario. Please add a comment (perhaps at the first lock location
above) at least briefly explaining why there can't be any deadlock
here.

Apart from that the patch looks fine to me now.

Jan


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