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Subject[PATCH] radix_tree_tag_get() is not as safe as the docs make out
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radix_tree_tag_get() is not safe to use concurrently with radix_tree_tag_set()
or radix_tree_tag_clear(). The problem is that the double tag_get() in
radix_tree_tag_get():

if (!tag_get(node, tag, offset))
saw_unset_tag = 1;
if (height == 1) {
int ret = tag_get(node, tag, offset);
may see the value change due to the action of set/clear. RCU is no protection
against this as no pointers are being changed, no nodes are being replaced
according to a COW protocol - set/clear alter the node directly.

The documentation in linux/radix-tree.h, however, proclaims that
radix_tree_tag_get() is an exception to the rule that "any function modifying
the tree or tags (...) must exclude other modifications, and exclude any
functions reading the tree".

To this end, remove radix_tree_tag_get() from that list, and comment on its
definition that the caller is responsible for preventing concurrent access with
set/clear.

Furthermore, radix_tree_tag_get() is not safe with respect to
radix_tree_delete() either as that also modifies the tags directly.

An alternative would be to drop the BUG_ON() from radix_tree_tag_get() and note
that it may produce an untrustworthy answer if not so protected.

Signed-off-by: David Howells <dhowells@redhat.com>
---
include/linux/radix-tree.h | 3 +--
lib/radix-tree.c | 4 ++++
2 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index c5da749..33daa70 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -100,14 +100,13 @@ do { \
* The notable exceptions to this rule are the following functions:
* radix_tree_lookup
* radix_tree_lookup_slot
- * radix_tree_tag_get
* radix_tree_gang_lookup
* radix_tree_gang_lookup_slot
* radix_tree_gang_lookup_tag
* radix_tree_gang_lookup_tag_slot
* radix_tree_tagged
*
- * The first 7 functions are able to be called locklessly, using RCU. The
+ * The first 6 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.
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 6b9670d..795a3bb 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -556,6 +556,10 @@ EXPORT_SYMBOL(radix_tree_tag_clear);
*
* 0: tag not present or not set
* 1: tag set
+ *
+ * The caller must make sure this function does not run concurrently with
+ * radix_tree_tag_set/clear() or radix_tree_delete() as these modify the nodes
+ * directly to alter the tags.
*/
int radix_tree_tag_get(struct radix_tree_root *root,
unsigned long index, unsigned int tag)


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