Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage_map iterator

[Yonghong Song <yhs@fb.com>]

On 9/15/20 10:40 AM, Jakub Kicinski wrote:
> On Tue, 15 Sep 2020 10:35:50 -0700 Yonghong Song wrote:
>> On 9/15/20 8:33 AM, Jakub Kicinski wrote:
>>> On Mon, 14 Sep 2020 11:46:30 -0700 Yonghong Song wrote:
>>>> Currently, we use bucket_lock when traversing bpf_sk_storage_map
>>>> elements. Since bpf_iter programs cannot use bpf_sk_storage_get()
>>>> and bpf_sk_storage_delete() helpers which may also grab bucket lock,
>>>> we do not have a deadlock issue which exists for hashmap when
>>>> using bucket_lock ([1]).
>>>>
>>>> If a bucket contains a lot of sockets, during bpf_iter traversing
>>>> a bucket, concurrent bpf_sk_storage_{get,delete}() may experience
>>>> some undesirable delays. Using rcu_read_lock() is a reasonable
>>>> compromise here. Although it may lose some precision, e.g.,
>>>> access stale sockets, but it will not hurt performance of other
>>>> bpf programs.
>>>>
>>>> [1] https://lore.kernel.org/bpf/20200902235341.2001534-1-yhs@fb.com
>>>>
>>>> Cc: Martin KaFai Lau <kafai@fb.com>
>>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>>
>>> Sparse is not happy about it. Could you add some annotations, perhaps?
>>>
>>> include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_find_next' - unexpected unlock
>>> include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_stop' - unexpected unlock
>>
>> Okay, I will try.
>>
>> On my system, sparse is unhappy and core dumped....
>>
>> /data/users/yhs/work/net-next/include/linux/string.h:12:38: error: too
>> many errors
>> /bin/sh: line 1: 2710132 Segmentation fault      (core dumped) sparse
>> -D__linux__ -Dlinux -D__STDC__ -Dunix
>> -D__unix__ -Wbitwise -Wno-return-void -Wno-unknown-attribute
>> -D__x86_64__ --arch=x86 -mlittle-endian -m64 -W
>> p,-MMD,net/core/.bpf_sk_storage.o.d -nostdinc -isystem
>> ...
>> /data/users/yhs/work/net-next/net/core/bpf_sk_storage.c
>> make[3]: *** [net/core/bpf_sk_storage.o] Error 139
>> make[3]: *** Deleting file `net/core/bpf_sk_storage.o'
>>
>> -bash-4.4$ rpm -qf /bin/sparse
>> sparse-0.5.2-1.el7.x86_64
>> -bash-4.4$
> 
> I think you need to build from source, sadly :(
> 
> https://git.kernel.org/pub/scm//devel/sparse/sparse.git

Indeed, building sparse from source works. After adding some 
__releases(RCU) and __acquires(RCU), I now have:
   context imbalance in 'bpf_sk_storage_map_seq_find_next' - different 
lock contexts for basic block
I may need to restructure code to please sparse...

>