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SubjectRe: [PATCH 3/4] maple_tree: Fix a potential concurrency bug in RCU mode
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在 2023/3/11 02:27, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@bytedance.com> [230310 09:09]:
>> 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().
> Are you able to write a test case for this issue? I understand it's a
> race so it may be difficult to catch.
Yes it's hard to catch. I'll try to think of a test case next week.
It is difficult because of the need to expand the competitive area.
>
>> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
>
>> ---
>> 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
>>

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