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    Subject[PATCH] radix_tree_tag_get() is not as safe as the docs make out
    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

    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

    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 <>

    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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      Last update: 2010-04-06 21:35    [W:0.021 / U:39.004 seconds]
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