drm: Add a hash-tab rcu-safe API

While hashtab should now be RCU-safe, Add a drm_ht_xxx_api for consumers
to use to make it obvious what locking mechanism is used.

Document the way the rcu-safe interface should be used.

Don't use rcu-safe list traversal in modify operations where we should use
a spinlock / mutex anyway.

Signed-off-by: Thomas Hellstrom <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/drm_hashtab.c

···
 	hashed_key = hash_long(key, ht->order);
 	DRM_DEBUG("Key is 0x%08lx, Hashed key is 0x%08x\n", key, hashed_key);
 	h_list = &ht->table[hashed_key];
-	hlist_for_each_entry_rcu(entry, list, h_list, head)
+	hlist_for_each_entry(entry, list, h_list, head)
 		DRM_DEBUG("count %d, key: 0x%08lx\n", count++, entry->key);
 }
 
 static struct hlist_node *drm_ht_find_key(struct drm_open_hash *ht,
 					  unsigned long key)
+{
+	struct drm_hash_item *entry;
+	struct hlist_head *h_list;
+	struct hlist_node *list;
+	unsigned int hashed_key;
+
+	hashed_key = hash_long(key, ht->order);
+	h_list = &ht->table[hashed_key];
+	hlist_for_each_entry(entry, list, h_list, head) {
+		if (entry->key == key)
+			return list;
+		if (entry->key > key)
+			break;
+	}
+	return NULL;
+}
+
+static struct hlist_node *drm_ht_find_key_rcu(struct drm_open_hash *ht,
+					      unsigned long key)
 {
 	struct drm_hash_item *entry;
 	struct hlist_head *h_list;
···
 	return NULL;
 }
 
-
 int drm_ht_insert_item(struct drm_open_hash *ht, struct drm_hash_item *item)
 {
 	struct drm_hash_item *entry;
···
 	hashed_key = hash_long(key, ht->order);
 	h_list = &ht->table[hashed_key];
 	parent = NULL;
-	hlist_for_each_entry_rcu(entry, list, h_list, head) {
+	hlist_for_each_entry(entry, list, h_list, head) {
 		if (entry->key == key)
 			return -EINVAL;
 		if (entry->key > key)
···
 {
 	struct hlist_node *list;
 
-	list = drm_ht_find_key(ht, key);
+	list = drm_ht_find_key_rcu(ht, key);
 	if (!list)
 		return -EINVAL;
 

include/drm/drm_hashtab.h

···
 extern int drm_ht_remove_item(struct drm_open_hash *ht, struct drm_hash_item *item);
 extern void drm_ht_remove(struct drm_open_hash *ht);
 
+/*
+ * RCU-safe interface
+ *
+ * The user of this API needs to make sure that two or more instances of the
+ * hash table manipulation functions are never run simultaneously.
+ * The lookup function drm_ht_find_item_rcu may, however, run simultaneously
+ * with any of the manipulation functions as long as it's called from within
+ * an RCU read-locked section.
+ */
+#define drm_ht_insert_item_rcu drm_ht_insert_item
+#define drm_ht_just_insert_please_rcu drm_ht_just_insert_please
+#define drm_ht_remove_key_rcu drm_ht_remove_key
+#define drm_ht_remove_item_rcu drm_ht_remove_item
+#define drm_ht_find_item_rcu drm_ht_find_item
 
 #endif