netdev - Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability

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Message-ID: <aa6081aa-9741-be05-8051-e01909662ff1@mojatatu.com>
Date:   Wed, 3 Nov 2021 06:34:08 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Joe Burton <jevburton.kernel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     Petar Penkov <ppenkov@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Joe Burton <jevburton@...gle.com>
Subject: Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability

On 2021-11-01 22:14, Joe Burton wrote:
> From: Joe Burton<jevburton@...gle.com>
> 
> This is the third version of a patch series implementing map tracing.
> 
> Map tracing enables executing BPF programs upon BPF map updates. This
> might be useful to perform upgrades of stateful programs; e.g., tracing
> programs can propagate changes to maps that occur during an upgrade
> operation.
> 
> This version uses trampoline hooks to provide the capability.
> fentry/fexit/fmod_ret programs can attach to two new functions:
>          int bpf_map_trace_update_elem(struct bpf_map* map, void* key,
>                  void* val, u32 flags);
>          int bpf_map_trace_delete_elem(struct bpf_map* map, void* key);
> 
> These hooks work as intended for the following map types:
>          BPF_MAP_TYPE_ARRAY
>          BPF_MAP_TYPE_PERCPU_ARRAY
>          BPF_MAP_TYPE_HASH
>          BPF_MAP_TYPE_PERCPU_HASH
>          BPF_MAP_TYPE_LRU_HASH
>          BPF_MAP_TYPE_LRU_PERCPU_HASH
> 
> The only guarantee about the semantics of these hooks is that they execute
> before the operation takes place. We cannot call them with locks held
> because the hooked program might try to acquire the same locks. Thus they
> may be invoked in situations where the traced map is not ultimately
> updated.

Sorry, I may have missed something obvious while staring at the patches,
but:
Dont you want the notification if the command actually was successful
on the map? If the command failed for whatever reason theres nothing
to synchronize? Unless you use that as an indicator to re-read the map?

cheers,
jamal