Re: [Xen-devel] [PATCH v3 6/6] VMX: Fixup PI descritpor when cpu is offline

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.08.16 at 05:56, <feng.wu@xxxxxxxxx> wrote:
> +void vmx_pi_desc_fixup(int cpu)

unsigned int

> +{
> +    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.
> +         */
> +restart:

I'd prefer if you did this without label and goto, but in any case
labels should be indented by at least one space. Yet ...

> +        new_cpu = cpumask_any(&cpu_online_map);
> +        new_lock = &per_cpu(vmx_pi_blocking, new_cpu).lock;
> +
> +        spin_lock(new_lock);
> +
> +        /*
> +         * If the new_cpu is not online, that means it became offline between
> +         * we got 'new_cpu' and acquiring its lock above, we need to find
> +         * another online cpu instead. Such as, this fucntion is being called
> +         * on 'new_cpu' at the same time. Can this happen??
> +         */
> +        if ( !cpu_online(new_cpu) )
> +        {
> +            spin_unlock(new_lock);
> +            goto restart;
> +        }

... I think this too has been discussed before: Is this case really
possible? You're in the context of a CPU_DEAD or CPU_UP_CANCELED
notification, which both get issued with cpu_add_remove_lock held.
How can a second CPU go down in parallel?

Jan


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