netdev - Re: [PATCH bpf] bpf: fix rcu lockdep warning for lpm_trie map

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Date:   Thu, 22 Feb 2018 09:49:39 -0800
From:   Yonghong Song <yhs@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>, <ast@...com>,
        <daniel@...earbox.net>, <edumazet@...gle.com>,
        <netdev@...r.kernel.org>
CC:     <kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: fix rcu lockdep warning for lpm_trie map_free
 callback



On 2/22/18 5:37 AM, Eric Dumazet wrote:
> On Wed, 2018-02-21 at 22:38 -0800, Yonghong Song wrote:
>> Commit 9a3efb6b661f ("bpf: fix memory leak in lpm_trie map_free callback function")
>> fixed a memory leak and removed unnecessary locks in map_free callback function.
>> Unfortrunately, it introduced a lockdep warning. When lockdep checking is turned on,
>> running tools/testing/selftests/bpf/test_lpm_map will have:
>>
>>    [   98.294321] =============================
>>    [   98.294807] WARNING: suspicious RCU usage
>>    [   98.295359] 4.16.0-rc2+ #193 Not tainted
>>    [   98.295907] -----------------------------
>>    [   98.296486] /home/yhs/work/bpf/kernel/bpf/lpm_trie.c:572 suspicious rcu_dereference_check() usage!
>>    [   98.297657]
>>    [   98.297657] other info that might help us debug this:
>>    [   98.297657]
>>    [   98.298663]
>>    [   98.298663] rcu_scheduler_active = 2, debug_locks = 1
>>    [   98.299536] 2 locks held by kworker/2:1/54:
>>    [   98.300152]  #0:  ((wq_completion)"events"){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0
>>    [   98.301381]  #1:  ((work_completion)(&map->work)){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0
>>
>> Since actual trie tree removal happens only after no other
>> accesses to the tree are possible, this patch simply converted all
>> rcu protected pointer access to normal access, which removed the
>> above warning.
>>
>> Fixes: 9a3efb6b661f ("bpf: fix memory leak in lpm_trie map_free callback function")
>> Reported-by: Eric Dumazet <edumazet@...gle.com>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>>   kernel/bpf/lpm_trie.c | 11 +++++------
>>   1 file changed, 5 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
>> index a75e02c..0c15813 100644
>> --- a/kernel/bpf/lpm_trie.c
>> +++ b/kernel/bpf/lpm_trie.c
>> @@ -552,7 +552,7 @@ static struct bpf_map *trie_alloc(union bpf_attr *attr)
>>   static void trie_free(struct bpf_map *map)
>>   {
>>   	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
>> -	struct lpm_trie_node __rcu **slot;
>> +	struct lpm_trie_node **slot;
>>   	struct lpm_trie_node *node;
>>   
>>   	/* Wait for outstanding programs to complete
>> @@ -569,23 +569,22 @@ static void trie_free(struct bpf_map *map)
>>   		slot = &trie->root;
>>   
>>   		for (;;) {
>> -			node = rcu_dereference_protected(*slot,
>> -					lockdep_is_held(&trie->lock));
>> +			node = *slot;
> 
> Hi Yonghong
> 
> It is not sparse compliant.
> 
> kernel/bpf/lpm_trie.c:573:30: warning: incorrect type in assignment (different address spaces)
> kernel/bpf/lpm_trie.c:573:30:    expected struct lpm_trie_node *node
> kernel/bpf/lpm_trie.c:573:30:    got struct lpm_trie_node [noderef] <asn:4>*<noident>
> 
> 
> In my local tree, I simply did
> 
> 	node = rcu_dereference_protected(*slot, 1);
> 
> Since we are the last user of the whole tree after the prior synchronize_rcu();

Emic,

Thanks for the fix suggestion. It does make sense.

I indeed ran sparse before my patch send-email. Unfortunately, my dev 
machine (sparse 0.5.0 + gcc 4.8.5) didn't issue warning like the above.

With the same kernel config and kernel tree, I just tried on another 
machine (a FC27 VM, sparse 0.5.1 + gcc 7.3.1), I did see the warning and
the above suggested fix makes warning went away.
Need to figure out why sparse is not happy with my dev machine.

Will send a follow patch soon.

Thanks!

Yonghong

> 
>