[PATCH 3/4] maple_tree: Fix a potential concurrency bug in RCU mode

[Peng Zhang]

在 2023/3/11 02:27, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00 at 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 at bytedance.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett at 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
>>