iommu/s390: Use RCU to allow concurrent domain_list iteration

The s390_domain->devices list is only added to when new devices are
attached but is iterated through in read-only fashion for every mapping
operation as well as for I/O TLB flushes and thus in performance
critical code causing contention on the s390_domain->list_lock.
Fortunately such a read-mostly linked list is a standard use case for
RCU. This change closely follows the example fpr RCU protected list
given in Documentation/RCU/listRCU.rst.

Signed-off-by: Niklas Schnelle <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

Author: niklas88

arch/s390/include/asm/pci.h

@@ -119,6 +119,7 @@ struct zpci_dev {
 	struct list_head entry;		/* list of all zpci_devices, needed for hotplug, etc. */
 	struct list_head iommu_list;
 	struct kref kref;
+	struct rcu_head rcu;
 	struct hotplug_slot hotplug_slot;
 
 	enum zpci_state state;

arch/s390/pci/pci.c

@@ -996,7 +996,7 @@ void zpci_release_device(struct kref *kref)
 		break;
 	}
 	zpci_dbg(3, "rem fid:%x\n", zdev->fid);
-	kfree(zdev);
+	kfree_rcu(zdev, rcu);
 }
 
 int zpci_report_error(struct pci_dev *pdev,

drivers/iommu/s390-iommu.c

@@ -10,6 +10,8 @@
 #include <linux/iommu.h>
 #include <linux/iommu-helper.h>
 #include <linux/sizes.h>
+#include <linux/rculist.h>
+#include <linux/rcupdate.h>
 #include <asm/pci_dma.h>
 
 static const struct iommu_ops s390_iommu_ops;
@@ -20,6 +22,7 @@ struct s390_domain {
 	unsigned long		*dma_table;
 	spinlock_t		dma_table_lock;
 	spinlock_t		list_lock;
+	struct rcu_head		rcu;
 };
 
 static struct s390_domain *to_s390_domain(struct iommu_domain *dom)
@@ -61,18 +64,28 @@ static struct iommu_domain *s390_domain_alloc(unsigned domain_type)
 
 	spin_lock_init(&s390_domain->dma_table_lock);
 	spin_lock_init(&s390_domain->list_lock);
-	INIT_LIST_HEAD(&s390_domain->devices);
+	INIT_LIST_HEAD_RCU(&s390_domain->devices);
 
 	return &s390_domain->domain;
 }
 
+static void s390_iommu_rcu_free_domain(struct rcu_head *head)
+{
+	struct s390_domain *s390_domain = container_of(head, struct s390_domain, rcu);
+
+	dma_cleanup_tables(s390_domain->dma_table);
+	kfree(s390_domain);
+}
+
 static void s390_domain_free(struct iommu_domain *domain)
 {
 	struct s390_domain *s390_domain = to_s390_domain(domain);
 
+	rcu_read_lock();
 	WARN_ON(!list_empty(&s390_domain->devices));
-	dma_cleanup_tables(s390_domain->dma_table);
-	kfree(s390_domain);
+	rcu_read_unlock();
+
+	call_rcu(&s390_domain->rcu, s390_iommu_rcu_free_domain);
 }
 
 static void __s390_iommu_detach_device(struct zpci_dev *zdev)
@@ -84,7 +97,7 @@ static void __s390_iommu_detach_device(struct zpci_dev *zdev)
 		return;
 
 	spin_lock_irqsave(&s390_domain->list_lock, flags);
-	list_del_init(&zdev->iommu_list);
+	list_del_rcu(&zdev->iommu_list);
 	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
 
 	zpci_unregister_ioat(zdev, 0);
@@ -127,7 +140,7 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
 	zdev->s390_domain = s390_domain;
 
 	spin_lock_irqsave(&s390_domain->list_lock, flags);
-	list_add(&zdev->iommu_list, &s390_domain->devices);
+	list_add_rcu(&zdev->iommu_list, &s390_domain->devices);
 	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
 
 	return 0;
@@ -203,14 +216,13 @@ static void s390_iommu_flush_iotlb_all(struct iommu_domain *domain)
 {
 	struct s390_domain *s390_domain = to_s390_domain(domain);
 	struct zpci_dev *zdev;
-	unsigned long flags;
 
-	spin_lock_irqsave(&s390_domain->list_lock, flags);
-	list_for_each_entry(zdev, &s390_domain->devices, iommu_list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(zdev, &s390_domain->devices, iommu_list) {
 		zpci_refresh_trans((u64)zdev->fh << 32, zdev->start_dma,
 				   zdev->end_dma - zdev->start_dma + 1);
 	}
-	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+	rcu_read_unlock();
 }
 
 static void s390_iommu_iotlb_sync(struct iommu_domain *domain,
@@ -219,35 +231,33 @@ static void s390_iommu_iotlb_sync(struct iommu_domain *domain,
 	struct s390_domain *s390_domain = to_s390_domain(domain);
 	size_t size = gather->end - gather->start + 1;
 	struct zpci_dev *zdev;
-	unsigned long flags;
 
 	/* If gather was never added to there is nothing to flush */
 	if (!gather->end)
 		return;
 
-	spin_lock_irqsave(&s390_domain->list_lock, flags);
-	list_for_each_entry(zdev, &s390_domain->devices, iommu_list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(zdev, &s390_domain->devices, iommu_list) {
 		zpci_refresh_trans((u64)zdev->fh << 32, gather->start,
 				   size);
 	}
-	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+	rcu_read_unlock();
 }
 
 static void s390_iommu_iotlb_sync_map(struct iommu_domain *domain,
 				      unsigned long iova, size_t size)
 {
 	struct s390_domain *s390_domain = to_s390_domain(domain);
 	struct zpci_dev *zdev;
-	unsigned long flags;
 
-	spin_lock_irqsave(&s390_domain->list_lock, flags);
-	list_for_each_entry(zdev, &s390_domain->devices, iommu_list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(zdev, &s390_domain->devices, iommu_list) {
 		if (!zdev->tlb_refresh)
 			continue;
 		zpci_refresh_trans((u64)zdev->fh << 32,
 				   iova, size);
 	}
-	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+	rcu_read_unlock();
 }
 
 static int s390_iommu_update_trans(struct s390_domain *s390_domain,