Messages in this thread | | | Date | Sat, 11 Mar 2023 03:03:22 +0800 | Subject | Re: [PATCH 3/4] maple_tree: Fix a potential concurrency bug in RCU mode | From | Peng Zhang <> |
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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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