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

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, September 26, 2016 9:03 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: andrew.cooper3@xxxxxxxxxx; dario.faggioli@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxxxxx; Tian, Kevin <kevin.tian@xxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxx
> Subject: Re: [PATCH v4 6/6] VMX: Fixup PI descritpor when cpu is offline
> 
> >>> 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.

Yes, I also noted this. I will add a comment here, which is really needed.
Thanks a lot!

Thanks,
Feg

> 
> Apart from that the patch looks fine to me now.
> 
> Jan


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