[PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list iteration

Niklas Schnelle
2022-10-18T14:52:02Z

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 <schnelle@linux.ibm.com>
---
arch/s390/include/asm/pci.h | 1 +
arch/s390/pci/pci.c | 2 +-
drivers/iommu/s390-iommu.c | 31 ++++++++++++++++---------------
3 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
index 07361e2fd8c5..e4c3e4e04d30 100644
--- a/arch/s390/include/asm/pci.h
+++ b/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;
diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
index a703dcd94a68..ef38b1514c77 100644
--- a/arch/s390/pci/pci.c
+++ b/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,
diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index a4c2e9bc6d83..4e90987be387 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/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;
@@ -61,7 +63,7 @@ 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;
}
@@ -70,7 +72,9 @@ 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));
+ rcu_read_unlock();
dma_cleanup_tables(s390_domain->dma_table);
kfree(s390_domain);
}
@@ -84,7 +88,7 @@ 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 +131,7 @@ 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;
@@ -203,17 +207,16 @@ 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;
int rc;

- 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) {
rc = zpci_refresh_trans((u64)zdev->fh << 32, zdev->start_dma,
zdev->end_dma - zdev->start_dma + 1);
if (rc)
break;
}
- spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+ rcu_read_unlock();
}

static void s390_iommu_iotlb_sync(struct iommu_domain *domain,
@@ -222,21 +225,20 @@ 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;
int rc;

/* If gather was never added to there is nothing to flush */
if (gather->start == ULONG_MAX)
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) {
rc = zpci_refresh_trans((u64)zdev->fh << 32, gather->start,
size);
if (rc)
break;
}
- spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+ rcu_read_unlock();
}

static void s390_iommu_iotlb_sync_map(struct iommu_domain *domain,
@@ -244,11 +246,10 @@ static void s390_iommu_iotlb_sync_map(struct iommu_domain *domain,
{
struct s390_domain *s390_domain = to_s390_domain(domain);
struct zpci_dev *zdev;
- unsigned long flags;
int rc;

- 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;
rc = zpci_refresh_trans((u64)zdev->fh << 32,
@@ -256,7 +257,7 @@ static void s390_iommu_iotlb_sync_map(struct iommu_domain *domain,
if (rc)
break;
}
- spin_unlock_irqrestore(&s390_domain->list_lock, flags);
+ rcu_read_unlock();
}

static int s390_iommu_update_trans(struct s390_domain *s390_domain,
--
2.34.1