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

[Liam R. Howlett]

* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [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.

> 
> Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> ---
>  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
>