Re: [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@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.
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@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