drm/amdgpu: Optimize mutex usage (v4)

In original function amdgpu_bo_list_get, the waiting
for result->lock can be quite long while mutex
bo_list_lock was holding. It can make other tasks
waiting for bo_list_lock for long period.

Secondly, this patch allows several tasks(readers of idr)
to proceed at the same time.

v2: use rcu and kref (Dave Airlie and Christian König)
v3: update v1 commit message (Michel Dänzer)
v4: rebase on upstream (Alex Deucher)
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -869,6 +869,8 @@ struct amdgpu_fpriv {
 
 struct amdgpu_bo_list {
 	struct mutex lock;
+	struct rcu_head rhead;
+	struct kref refcount;
 	struct amdgpu_bo *gds_obj;
 	struct amdgpu_bo *gws_obj;
 	struct amdgpu_bo *oa_obj;

drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c

@@ -41,6 +41,20 @@ static int amdgpu_bo_list_set(struct amdgpu_device *adev,
 				     struct drm_amdgpu_bo_list_entry *info,
 				     unsigned num_entries);
 
+static void amdgpu_bo_list_release_rcu(struct kref *ref)
+{
+	unsigned i;
+	struct amdgpu_bo_list *list = container_of(ref, struct amdgpu_bo_list,
+						   refcount);
+
+	for (i = 0; i < list->num_entries; ++i)
+		amdgpu_bo_unref(&list->array[i].robj);
+
+	mutex_destroy(&list->lock);
+	kvfree(list->array);
+	kfree_rcu(list, rhead);
+}
+
 static int amdgpu_bo_list_create(struct amdgpu_device *adev,
 				 struct drm_file *filp,
 				 struct drm_amdgpu_bo_list_entry *info,
@@ -57,7 +71,7 @@ static int amdgpu_bo_list_create(struct amdgpu_device *adev,
 
 	/* initialize bo list*/
 	mutex_init(&list->lock);
-
+	kref_init(&list->refcount);
 	r = amdgpu_bo_list_set(adev, filp, list, info, num_entries);
 	if (r) {
 		kfree(list);
@@ -83,14 +97,9 @@ static void amdgpu_bo_list_destroy(struct amdgpu_fpriv *fpriv, int id)
 
 	mutex_lock(&fpriv->bo_list_lock);
 	list = idr_remove(&fpriv->bo_list_handles, id);
-	if (list) {
-		/* Another user may have a reference to this list still */
-		mutex_lock(&list->lock);
-		mutex_unlock(&list->lock);
-		amdgpu_bo_list_free(list);
-	}
-
 	mutex_unlock(&fpriv->bo_list_lock);
+	if (list)
+		kref_put(&list->refcount, amdgpu_bo_list_release_rcu);
 }
 
 static int amdgpu_bo_list_set(struct amdgpu_device *adev,
@@ -185,11 +194,17 @@ amdgpu_bo_list_get(struct amdgpu_fpriv *fpriv, int id)
 {
 	struct amdgpu_bo_list *result;
 
-	mutex_lock(&fpriv->bo_list_lock);
+	rcu_read_lock();
 	result = idr_find(&fpriv->bo_list_handles, id);
-	if (result)
-		mutex_lock(&result->lock);
-	mutex_unlock(&fpriv->bo_list_lock);
+
+	if (result) {
+		if (kref_get_unless_zero(&result->refcount))
+			mutex_lock(&result->lock);
+		else
+			result = NULL;
+	}
+	rcu_read_unlock();
+
 	return result;
 }
 
@@ -227,6 +242,7 @@ void amdgpu_bo_list_get_list(struct amdgpu_bo_list *list,
 void amdgpu_bo_list_put(struct amdgpu_bo_list *list)
 {
 	mutex_unlock(&list->lock);
+	kref_put(&list->refcount, amdgpu_bo_list_release_rcu);
 }
 
 void amdgpu_bo_list_free(struct amdgpu_bo_list *list)