[v1] Some fixes and cleanup for maple tree.

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

Posted by Peng Zhang 3 years, 1 month ago

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@bytedance.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

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

Posted by Liam R. Howlett 3 years, 1 month ago

* 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.

> 
> 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
>

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

Posted by Peng Zhang 3 years, 1 month ago

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
>>

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

Posted by Liam R. Howlett 3 years, 1 month ago

* Peng Zhang <zhangpeng.00@bytedance.com> [230310 14:03]:
> 
> 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.

This should have a fixes tag as well.

Fixes: 54a611b60590 ("Maple Tree: add new data structure")

> > 
> > > 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
> > >