From: Martin KaFai Lau <martin.lau@linux.dev>
To: Kui-Feng Lee <sinquersw@gmail.com>, Kui-Feng Lee <thinker.li@gmail.com>
Cc: bpf@vger.kernel.org, ast@kernel.org, song@kernel.org,
kernel-team@meta.com, andrii@kernel.org, kuifeng@meta.com
Subject: Re: [PATCH bpf-next 6/6] selftests/bpf: test detaching struct_ops links.
Date: Fri, 3 May 2024 14:59:51 -0700 [thread overview]
Message-ID: <14a618ca-636f-420c-9356-034b1557e26d@linux.dev> (raw)
In-Reply-To: <33bded73-703d-443d-b428-48a03b3d395d@gmail.com>
On 5/3/24 2:34 PM, Kui-Feng Lee wrote:
>
>
> On 5/3/24 12:15, Martin KaFai Lau wrote:
>> On 5/3/24 11:34 AM, Kui-Feng Lee wrote:
>>>
>>>
>>> On 5/2/24 11:15, Martin KaFai Lau wrote:
>>>> On 4/29/24 2:36 PM, Kui-Feng Lee wrote:
>>>>> @@ -572,6 +576,12 @@ static int bpf_dummy_reg(void *kdata)
>>>>> if (ops->test_2)
>>>>> ops->test_2(4, ops->data);
>>>>> + if (ops->do_unreg) {
>>>>> + rcu_read_lock();
>>>>> + bpf_struct_ops_kvalue_unreg(kdata);
>>>>
>>>> Instead of unreg() immediately before the reg() has returned, the test
>>>> should reflect more on how the subsystem can use it in practice. The
>>>> subsystem does not do unreg() during reg().
>>>>
>>>> It also needs to test a case when the link is created and successfully
>>>> registered to the subsystem. The user space does BPF_LINK_DETACH first and
>>>> >> then the subsystem does link->ops->detach() by itself later.
>>
>>>
>>> agree
>>>
>>>>
>>>> It can create a kfunc in bpf_testmod.c to trigger the subsystem to do
>>>> link->ops->detach(). The kfunc can be called by a SEC("syscall") bpf prog
>>>> which is run by bpf_prog_test_run_opts(). The test_progs can then decide on
>>>> the timing when to do link->ops->detach() to test different cases.
>>>
>>> What is the purpose of this part?
>>> If it goes through link->ops->detach(), it should work just like to call
>>> bpf_link_detach() twice on the same link from the user space. Do you
>>> want to make sure detaching a link twice work?
>>
>> It is not quite what I meant and apparently link detach twice on the same
>> valid (i.e. refcnt non zero) link won't work.
>>
>> Anyhow, the idea is to show how the racing case may work in patch 3 (when
>> userspace tries to detach and the subsystem tries to detach/unreg itself
>> also). I was suggesting the kfunc idea such that the test_progs can have
>> better control on the timing on when to ask the subsystem to unreg/detach
>> itself instead of having to do the unreg() during the reg() as in patch 6
>> here. If kfunc does not make sense and there is a better way to do this, feel
>> free to ignore.
>>
>
> Ok! I think the case you are talking more like to happen when the link
> is destroyed, but bpf_struct_ops_map_link_dealloc() has not finished
> yet. Calling link->ops->detach() at this point may cause a racing since
> bpf_struct_ops_map_link_dealloc() doesn't acquire update_mutex.
Yes, adding link_dealloc() (i.e. close the link) in between will be a good test too.
With or without link_dealloc()/close(), the idea is to test this race (user
space detach and/or dealloc vs subsystem detach/unreg) or at least show how the
subsystem should do it. I was merely suggesting to use kfunc (may be there is a
better way and feel free to ignore). The details of the testing steps could be
adjusted based on how patch 3 will look like.
>
> Calling link->ops->detach() immediately after BPF_LINK_DETACH would not
> cause any racing since bpf_struct_ops_map_link_detach() always acquires
> update_mutex. They will be executed sequentially, and call
> st_map->ops->reg() sequentially as well.
I didn't meant the detach() itself is racy or not. That part is fine. It is more
about the link that the subsystem is holding. I feel how patch 3 will look like
may be something different from my current thinking. If this test does not make
sense based on how patch 3 will look like, feel free to ignore also.
>
> I will add a test case to call link->ops->detach() after close the fd of
> the link.
>
prev parent reply other threads:[~2024-05-03 21:59 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-29 21:36 [PATCH bpf-next 0/6] Notify user space when a struct_ops object is detached/unregisterd Kui-Feng Lee
2024-04-29 21:36 ` [PATCH bpf-next 1/6] bpf: add a pointer of the attached link to bpf_struct_ops_map Kui-Feng Lee
2024-05-01 17:01 ` Andrii Nakryiko
2024-05-01 22:15 ` Kui-Feng Lee
2024-04-29 21:36 ` [PATCH bpf-next 2/6] bpf: export bpf_link_inc_not_zero() Kui-Feng Lee
2024-04-29 21:36 ` [PATCH bpf-next 3/6] bpf: provide a function to unregister struct_ops objects from consumers Kui-Feng Lee
2024-05-01 18:48 ` Martin KaFai Lau
2024-05-01 22:15 ` Kui-Feng Lee
2024-05-01 23:06 ` Martin KaFai Lau
2024-05-02 17:56 ` Martin KaFai Lau
2024-05-02 18:29 ` Martin KaFai Lau
2024-05-03 0:41 ` Kui-Feng Lee
2024-05-03 16:19 ` Alexei Starovoitov
2024-05-03 18:09 ` Kui-Feng Lee
2024-05-03 17:17 ` Martin KaFai Lau
2024-04-29 21:36 ` [PATCH bpf-next 4/6] bpf: detach a bpf_struct_ops_map from a link Kui-Feng Lee
2024-04-29 21:36 ` [PATCH bpf-next 5/6] bpf: support epoll from bpf struct_ops links Kui-Feng Lee
2024-05-01 17:03 ` Andrii Nakryiko
2024-05-01 22:16 ` Kui-Feng Lee
2024-04-29 21:36 ` [PATCH bpf-next 6/6] selftests/bpf: test detaching " Kui-Feng Lee
2024-05-01 17:05 ` Andrii Nakryiko
2024-05-01 22:17 ` Kui-Feng Lee
2024-05-02 18:15 ` Martin KaFai Lau
2024-05-03 18:34 ` Kui-Feng Lee
2024-05-03 19:15 ` Martin KaFai Lau
2024-05-03 21:34 ` Kui-Feng Lee
2024-05-03 21:59 ` Martin KaFai Lau [this message]
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