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

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Message-ID: <b8420944-03ec-e426-e1fa-789b17604686@linux.intel.com>
Date:   Tue, 15 Feb 2022 13:46:18 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Liu Yi L <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 10/11] iommu/vt-d: Use xarray for global
 device_domain_info

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