There is a concurrency bug that may cause the wrong value to be loaded
when a CPU is modifying the maple tree.
CPU1:
mtree_insert_range()
mas_insert()
mas_store_root()
...
mas_root_expand()
...
rcu_assign_pointer(mas->tree->ma_root, mte_mk_root(mas->node));
ma_set_meta(node, maple_leaf_64, 0, slot); <---IP
CPU2:
mtree_load()
mtree_lookup_walk()
ma_data_end();
When CPU1 is about to execute the instruction pointed to by IP,
the ma_data_end() executed by CPU2 may return the wrong end position,
which will cause the value loaded by mtree_load() to be wrong.
An example of triggering the bug:
Add mdelay(100) between rcu_assign_pointer() and ma_set_meta() in
mas_root_expand().
static DEFINE_MTREE(tree);
int work(void *p) {
unsigned long val;
for (int i = 0 ; i< 30; ++i) {
val = (unsigned long)mtree_load(&tree, 8);
mdelay(5);
pr_info("%lu",val);
}
return 0;
}
mt_init_flags(&tree, MT_FLAGS_USE_RCU);
mtree_insert(&tree, 0, (void*)12345, GFP_KERNEL);
run_thread(work)
mtree_insert(&tree, 1, (void*)56789, GFP_KERNEL);
In RCU mode, mtree_load() should always return the value before or after
the data structure is modified, and in this example mtree_load(&tree, 8)
may return 56789 which is not expected, it should always return NULL.
Fix it by put ma_set_meta() before rcu_assign_pointer().
Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
---
lib/maple_tree.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 4d15202a0692..de43ff19da72 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3635,10 +3635,9 @@ static inline int mas_root_expand(struct ma_state *mas, void *entry)
slot++;
mas->depth = 1;
mas_set_height(mas);
-
+ ma_set_meta(node, maple_leaf_64, 0, slot);
/* swap the new root into the tree */
rcu_assign_pointer(mas->tree->ma_root, mte_mk_root(mas->node));
- ma_set_meta(node, maple_leaf_64, 0, slot);
return slot;
}
--
2.20.1
