iommu/ioasid: Add ioasid references

Let IOASID users take references to existing ioasids with ioasid_get().
ioasid_put() drops a reference and only frees the ioasid when its
reference number is zero. It returns true if the ioasid was freed.
For drivers that don't call ioasid_get(), ioasid_put() is the same as
ioasid_free().

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20201106155048.997886-2-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/intel/iommu.c

@@ -5198,7 +5198,7 @@ static void auxiliary_unlink_device(struct dmar_domain *domain,
 	domain->auxd_refcnt--;
 
 	if (!domain->auxd_refcnt && domain->default_pasid > 0)
-		ioasid_free(domain->default_pasid);
+		ioasid_put(domain->default_pasid);
 }
 
 static int aux_domain_add_dev(struct dmar_domain *domain,
@@ -5259,7 +5259,7 @@ attach_failed:
 	spin_unlock(&iommu->lock);
 	spin_unlock_irqrestore(&device_domain_lock, flags);
 	if (!domain->auxd_refcnt && domain->default_pasid > 0)
-		ioasid_free(domain->default_pasid);
+		ioasid_put(domain->default_pasid);
 
 	return ret;
 }

drivers/iommu/intel/svm.c

@@ -598,7 +598,7 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
 		if (mm) {
 			ret = mmu_notifier_register(&svm->notifier, mm);
 			if (ret) {
-				ioasid_free(svm->pasid);
+				ioasid_put(svm->pasid);
 				kfree(svm);
 				kfree(sdev);
 				goto out;
@@ -616,7 +616,7 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
 		if (ret) {
 			if (mm)
 				mmu_notifier_unregister(&svm->notifier, mm);
-			ioasid_free(svm->pasid);
+			ioasid_put(svm->pasid);
 			kfree(svm);
 			kfree(sdev);
 			goto out;
@@ -689,7 +689,7 @@ static int intel_svm_unbind_mm(struct device *dev, u32 pasid)
 			kfree_rcu(sdev, rcu);
 
 			if (list_empty(&svm->devs)) {
-				ioasid_free(svm->pasid);
+				ioasid_put(svm->pasid);
 				if (svm->mm) {
 					mmu_notifier_unregister(&svm->notifier, svm->mm);
 					/* Clear mm's pasid. */

drivers/iommu/ioasid.c

@@ -2,7 +2,7 @@
 /*
  * I/O Address Space ID allocator. There is one global IOASID space, split into
  * subsets. Users create a subset with DECLARE_IOASID_SET, then allocate and
- * free IOASIDs with ioasid_alloc and ioasid_free.
+ * free IOASIDs with ioasid_alloc and ioasid_put.
  */
 #include <linux/ioasid.h>
 #include <linux/module.h>
@@ -15,6 +15,7 @@ struct ioasid_data {
 	struct ioasid_set *set;
 	void *private;
 	struct rcu_head rcu;
+	refcount_t refs;
 };
 
 /*
@@ -314,6 +315,7 @@ ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
 
 	data->set = set;
 	data->private = private;
+	refcount_set(&data->refs, 1);
 
 	/*
 	 * Custom allocator needs allocator data to perform platform specific
@@ -346,13 +348,36 @@ exit_free:
 EXPORT_SYMBOL_GPL(ioasid_alloc);
 
 /**
- * ioasid_free - Free an IOASID
+ * ioasid_get - obtain a reference to the IOASID
- * @ioasid: the ID to remove
  */
-void ioasid_free(ioasid_t ioasid)
+void ioasid_get(ioasid_t ioasid)
 {
 	struct ioasid_data *ioasid_data;
 
+	spin_lock(&ioasid_allocator_lock);
+	ioasid_data = xa_load(&active_allocator->xa, ioasid);
+	if (ioasid_data)
+		refcount_inc(&ioasid_data->refs);
+	else
+		WARN_ON(1);
+	spin_unlock(&ioasid_allocator_lock);
+}
+EXPORT_SYMBOL_GPL(ioasid_get);
+
+/**
+ * ioasid_put - Release a reference to an ioasid
+ * @ioasid: the ID to remove
+ *
+ * Put a reference to the IOASID, free it when the number of references drops to
+ * zero.
+ *
+ * Return: %true if the IOASID was freed, %false otherwise.
+ */
+bool ioasid_put(ioasid_t ioasid)
+{
+	bool free = false;
+	struct ioasid_data *ioasid_data;
+
 	spin_lock(&ioasid_allocator_lock);
 	ioasid_data = xa_load(&active_allocator->xa, ioasid);
 	if (!ioasid_data) {
@@ -360,6 +385,10 @@ void ioasid_free(ioasid_t ioasid)
 		goto exit_unlock;
 	}
 
+	free = refcount_dec_and_test(&ioasid_data->refs);
+	if (!free)
+		goto exit_unlock;
+
 	active_allocator->ops->free(ioasid, active_allocator->ops->pdata);
 	/* Custom allocator needs additional steps to free the xa element */
 	if (active_allocator->flags & IOASID_ALLOCATOR_CUSTOM) {
@@ -369,8 +398,9 @@ void ioasid_free(ioasid_t ioasid)
 
 exit_unlock:
 	spin_unlock(&ioasid_allocator_lock);
+	return free;
 }
-EXPORT_SYMBOL_GPL(ioasid_free);
+EXPORT_SYMBOL_GPL(ioasid_put);
 
 /**
  * ioasid_find - Find IOASID data

include/linux/ioasid.h

@@ -34,7 +34,8 @@ struct ioasid_allocator_ops {
 #if IS_ENABLED(CONFIG_IOASID)
 ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
 		      void *private);
-void ioasid_free(ioasid_t ioasid);
+void ioasid_get(ioasid_t ioasid);
+bool ioasid_put(ioasid_t ioasid);
 void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
 		  bool (*getter)(void *));
 int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
@@ -48,10 +49,15 @@ static inline ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min,
 	return INVALID_IOASID;
 }
 
-static inline void ioasid_free(ioasid_t ioasid)
+static inline void ioasid_get(ioasid_t ioasid)
 {
 }
 
+static inline bool ioasid_put(ioasid_t ioasid)
+{
+	return false;
+}
+
 static inline void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
 				bool (*getter)(void *))
 {