[RFC PATCH 08/12] iommu/vt-d: Use RCU for dev_pasids list updates in set/remove_dev_pasid()

[Tina Zhang <tina.zhang@intel.com>]

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. 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 eeb658d3bc6e..fe063e1250fa 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4694,7 +4694,7 @@ 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;
 		}
@@ -4703,7 +4703,7 @@ static void intel_iommu_remove_dev_pasid(struct device *dev, ioasid_t pasid)
 	spin_unlock_irqrestore(&dmar_domain->lock, flags);
 
 	domain_detach_iommu(dmar_domain, iommu);
-	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);
@@ -4751,8 +4751,14 @@ 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);
 
 	return 0;
-- 
2.39.3