Re: [PATCH 07/11] iommu/vt-d: Use RCU for dev_pasids list updates in set/remove_dev_pasid()

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2024/1/16 9:11, Tina Zhang wrote:
> Extend intel_iommu_remove_dev_pasid() and intel_iommu_set_dev_pasid() to
> support updating dev_pasids list concurrently with readers.
> 
> For default domain operations, the dev_pasids list accesses are protected
> by domain->lock and therefore all read/write accesses of default domain
> operations to dev_pasids list are performed sequentially. However, for sva
> domain, the dev_pasids list accesses could be performed concurrently.
> For example, the callbacks invoked by memory management notifier may run
> in a process which runs concurrently with another process wherein the
> intel_iommu_set/remove_dev_pasid operations are performed.
> 
> To extend intel_iommu_set/remove_dev_pasid() to have the ability to update
> the dev_pasids list concurrently with multiple readers (which is required
> by sva domain), RCU mechanism is being used here.
> 
> Signed-off-by: Tina Zhang <tina.zhang@intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 847e21117e7a..0eec5d971315 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -4553,7 +4553,7 @@ static void intel_iommu_remove_dev_pasid(struct device *dev, ioasid_t pasid)
>   	spin_lock_irqsave(&dmar_domain->lock, flags);
>   	list_for_each_entry(curr, &dmar_domain->dev_pasids, link_domain) {
>   		if (curr->dev == dev && curr->pasid == pasid) {
> -			list_del(&curr->link_domain);
> +			list_del_rcu(&curr->link_domain);
>   			dev_pasid = curr;
>   			break;
>   		}
> @@ -4563,7 +4563,7 @@ static void intel_iommu_remove_dev_pasid(struct device *dev, ioasid_t pasid)
>   
>   	domain_detach_iommu(dmar_domain, iommu);
>   	intel_iommu_debugfs_remove_dev_pasid(dev_pasid);
> -	kfree(dev_pasid);
> +	kfree_rcu(dev_pasid, rcu);
>   out_tear_down:
>   	intel_pasid_tear_down_entry(iommu, dev, pasid, false);
>   	intel_drain_pasid_prq(dev, pasid);
> @@ -4613,8 +4613,14 @@ static int intel_iommu_set_dev_pasid(struct iommu_domain *domain,
>   
>   	dev_pasid->dev = dev;
>   	dev_pasid->pasid = pasid;
> +
> +	/*
> +	 * Spin lock protects dev_pasids list from being updated concurrently with
> +	 * multiple updaters, while rcu ensures concurrency between one updater
> +	 * and multiple readers
> +	 */
>   	spin_lock_irqsave(&dmar_domain->lock, flags);
> -	list_add(&dev_pasid->link_domain, &dmar_domain->dev_pasids);
> +	list_add_rcu(&dev_pasid->link_domain, &dmar_domain->dev_pasids);
>   	spin_unlock_irqrestore(&dmar_domain->lock, flags);
>   
>   	if (domain->type & __IOMMU_DOMAIN_PAGING)

It appears you're using an RCU lock within another spinlock critical
region. Wouldn't that be redundant?

Best regards,
baolu