Re: [PATCH v2 10/11] iommu/vt-d: Use xarray for global device_domain_info

[Lu Baolu]

Hi Jason,

On 2/14/22 10:00 PM, Jason Gunthorpe wrote:
> > +
> > +/* Convert device source ID into the index of device_domain_array. */
> > +static inline unsigned long devi_idx(unsigned long seg, u8 bus, u8 devfn)
> > +{
> > +	return (seg << 16) | PCI_DEVID(bus, devfn);
> > +}
> >   
> >   /*
> > - * Iterate over elements in device_domain_list and call the specified
> > + * Iterate over elements in device_domain_array and call the specified
> >    * callback @fn against each element.
> >    */
> >   int for_each_device_domain(int (*fn)(struct device_domain_info *info,
> >   				     void *data), void *data)
> >   {
> > -	int ret = 0;
> > -	unsigned long flags;
> >   	struct device_domain_info *info;
> > +	unsigned long index;
> > +	int ret = 0;
> >   
> > -	spin_lock_irqsave(&device_domain_lock, flags);
> > -	list_for_each_entry(info, &device_domain_list, global) {
> > +	rcu_read_lock();
> > +	xa_for_each(&device_domain_array, index, info) {
> >   		ret = fn(info, data);
> > -		if (ret) {
> > -			spin_unlock_irqrestore(&device_domain_lock, flags);
> > -			return ret;
> > -		}
> > +		if (ret)
> > +			break;
> And you probably shouldn't try to use RCU. It is really unclear how
> this function can be useful while racing against
> intel_iommu_release_device(), eg today the only user of this function
> does:
>
> static int search_pasid_table(struct device_domain_info *info, void *opaque)
> {
> 	struct pasid_table_opaque *data = opaque;
>
> 	if (info->iommu->segment == data->segment &&
> 	    info->bus == data->bus &&
> 	    info->devfn == data->devfn &&
>
> And even if you kfree_rcu(info) then 'info->iommu->' is still racy
> unlocked.
>
> RCU is complicated to use, it is not just a drop in replacement for a
> spinlock.

Thanks for your comments. I am going to stop this patch (and the next
11/11) and spend more time figuring them out.

Best regards,
baolu