netdev - Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage

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Message-ID: <86ee274b-a23c-25aa-6ec3-e1448667b6b4@fb.com>
Date:   Mon, 14 Sep 2020 22:25:23 -0700
From:   Yonghong Song <yhs@...com>
To:     Song Liu <song@...nel.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage_map
 iterator



On 9/14/20 2:28 PM, Song Liu wrote:
> On Mon, Sep 14, 2020 at 11:47 AM Yonghong Song <yhs@...com> 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]).
> 
> The paragraph above describes why we can use bucket_lock, which is more
> or less irrelevant to this change. Also, I am not sure why we refer to [1] here.

What I try to clarify here is unlike [1], we do not have bugs
with the current code.
But I guess no bugs is implicit so I probably do not need to say
anything. Will skip the above paragraph in the next revision.

> 
>>
>> 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
> 
> It will be great to include some performance comparison.

Sure. Let me give some try to collect some statistics.

> 
>> 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@...com>
>> Signed-off-by: Yonghong Song <yhs@...com>
> 
> Other than these,
> 
> Acked-by: Song Liu <songliubraving@...com>
> 
> [...]
>