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

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