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radix-tree: fix race in gang lookup

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If the indirect_ptr bit is set on a slot, that indicates we need to redo
the lookup.  Introduce a new function radix_tree_iter_retry() which
forces the loop to retry the lookup by setting 'slot' to NULL and
turning the iterator back to point at the problematic entry.

This is a pretty rare problem to hit at the moment; the lookup has to
race with a grow of the radix tree from a height of 0.  The consequences
of hitting this race are that gang lookup could return a pointer to a
radix_tree_node instead of a pointer to whatever the user had inserted
in the tree.

Fixes: cebbd29e1c ("radix-tree: rewrite gang lookup using iterator")
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matthew Wilcox 2016-02-02 16:57:52 -08:00 committed by Linus Torvalds
parent 3c1da7beee
commit 46437f9a55
2 changed files with 26 additions and 2 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

16
include/linux/radix-tree.h
View file

@ -378,6 +378,22 @@ radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
void **radix_tree_next_chunk(struct radix_tree_root *root,
struct radix_tree_iter *iter, unsigned flags);
/**
* radix_tree_iter_retry - retry this chunk of the iteration
* @iter: iterator state
*
* If we iterate over a tree protected only by the RCU lock, a race
* against deletion or creation may result in seeing a slot for which
* radix_tree_deref_retry() returns true. If so, call this function
* and continue the iteration.
*/
static inline __must_check
void **radix_tree_iter_retry(struct radix_tree_iter *iter)
{
iter->next_index = iter->index;
return NULL;
}
/**
* radix_tree_chunk_size - get current chunk size
*

12
lib/radix-tree.c
View file

@ -1019,9 +1019,13 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
return 0;
radix_tree_for_each_slot(slot, root, &iter, first_index) {
results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
results[ret] = rcu_dereference_raw(*slot);
if (!results[ret])
continue;
if (radix_tree_is_indirect_ptr(results[ret])) {
slot = radix_tree_iter_retry(&iter);
continue;
}
if (++ret == max_items)
break;
}
@ -1098,9 +1102,13 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
return 0;
radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
results[ret] = rcu_dereference_raw(*slot);
if (!results[ret])
continue;
if (radix_tree_is_indirect_ptr(results[ret])) {
slot = radix_tree_iter_retry(&iter);
continue;
}
if (++ret == max_items)
break;
}