1 files changed, 101 insertions, 0 deletions

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index cbfa11537421..0deb842541ac 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -1,6 +1,7 @@
 /*
  * Copyright (C) 2001 Momchil Velikov
  * Portions Copyright (C) 2001 Christoph Hellwig
+ * Copyright (C) 2006 Nick Piggin
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -21,6 +22,35 @@
 
 #include <linux/preempt.h>
 #include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/rcupdate.h>
+
+/*
+ * A direct pointer (root->rnode pointing directly to a data item,
+ * rather than another radix_tree_node) is signalled by the low bit
+ * set in the root->rnode pointer.
+ *
+ * In this case root->height is also NULL, but the direct pointer tests are
+ * needed for RCU lookups when root->height is unreliable.
+ */
+#define RADIX_TREE_DIRECT_PTR   1
+
+static inline void *radix_tree_ptr_to_direct(void *ptr)
+{
+        return (void *)((unsigned long)ptr | RADIX_TREE_DIRECT_PTR);
+}
+
+static inline void *radix_tree_direct_to_ptr(void *ptr)
+{
+        return (void *)((unsigned long)ptr & ~RADIX_TREE_DIRECT_PTR);
+}
+
+static inline int radix_tree_is_direct_ptr(void *ptr)
+{
+        return (int)((unsigned long)ptr & RADIX_TREE_DIRECT_PTR);
+}
+
+/*** radix-tree API starts here ***/
 
 #define RADIX_TREE_MAX_TAGS 2
 
@@ -47,6 +77,77 @@ do {									\
         (root)->rnode = NULL;                                           \
 } while (0)
 
+/**
+ * Radix-tree synchronization
+ *
+ * The radix-tree API requires that users provide all synchronisation (with
+ * specific exceptions, noted below).
+ *
+ * Synchronization of access to the data items being stored in the tree, and
+ * management of their lifetimes must be completely managed by API users.
+ *
+ * For API usage, in general,
+ * - any function _modifying_ the the tree or tags (inserting or deleting
+ *   items, setting or clearing tags must exclude other modifications, and
+ *   exclude any functions reading the tree.
+ * - any function _reading_ the the tree or tags (looking up items or tags,
+ *   gang lookups) must exclude modifications to the tree, but may occur
+ *   concurrently with other readers.
+ *
+ * The notable exceptions to this rule are the following functions:
+ * radix_tree_lookup
+ * radix_tree_tag_get
+ * radix_tree_gang_lookup
+ * radix_tree_gang_lookup_tag
+ * radix_tree_tagged
+ *
+ * The first 4 functions are able to be called locklessly, using RCU. The
+ * caller must ensure calls to these functions are made within rcu_read_lock()
+ * regions. Other readers (lock-free or otherwise) and modifications may be
+ * running concurrently.
+ *
+ * It is still required that the caller manage the synchronization and lifetimes
+ * of the items. So if RCU lock-free lookups are used, typically this would mean
+ * that the items have their own locks, or are amenable to lock-free access; and
+ * that the items are freed by RCU (or only freed after having been deleted from
+ * the radix tree *and* a synchronize_rcu() grace period).
+ *
+ * (Note, rcu_assign_pointer and rcu_dereference are not needed to control
+ * access to data items when inserting into or looking up from the radix tree)
+ *
+ * radix_tree_tagged is able to be called without locking or RCU.
+ */
+
+/**
+ * radix_tree_deref_slot        - dereference a slot
+ * @pslot:      pointer to slot, returned by radix_tree_lookup_slot
+ * Returns:     item that was stored in that slot with any direct pointer flag
+ *              removed.
+ *
+ * For use with radix_tree_lookup_slot().  Caller must hold tree at least read
+ * locked across slot lookup and dereference.  More likely, will be used with
+ * radix_tree_replace_slot(), as well, so caller will hold tree write locked.
+ */
+static inline void *radix_tree_deref_slot(void **pslot)
+{
+        return radix_tree_direct_to_ptr(*pslot);
+}
+/**
+ * radix_tree_replace_slot      - replace item in a slot
+ * @pslot:      pointer to slot, returned by radix_tree_lookup_slot
+ * @item:       new item to store in the slot.
+ *
+ * For use with radix_tree_lookup_slot().  Caller must hold tree write locked
+ * across slot lookup and replacement.
+ */
+static inline void radix_tree_replace_slot(void **pslot, void *item)
+{
+        BUG_ON(radix_tree_is_direct_ptr(item));
+        rcu_assign_pointer(*pslot,
+                (void *)((unsigned long)item |
+                        ((unsigned long)*pslot & RADIX_TREE_DIRECT_PTR)));
+}
+
 int radix_tree_insert(struct radix_tree_root *, unsigned long, void *);
 void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
 void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);