netdev - Re: [PATCH v2] Add BPF_SYNCHRONIZE

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Date:   Sun, 29 Jul 2018 18:51:18 +0300
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Chenbo Feng <fengc@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v2] Add BPF_SYNCHRONIZE_MAPS bpf(2) command

On Thu, Jul 26, 2018 at 7:51 PM, Daniel Colascione <dancol@...gle.com> wrote:
> BPF_SYNCHRONIZE_MAPS waits for the release of any references to a BPF
> map made by a BPF program that is running at the time the
> BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command is
> to provide a means for userspace to replace a BPF map with another,
> newer version, then ensure that no component is still using the "old"
> map before manipulating the "old" map in some way.
>
> Signed-off-by: Daniel Colascione <dancol@...gle.com>
> ---
>  include/uapi/linux/bpf.h |  9 +++++++++
>  kernel/bpf/syscall.c     | 13 +++++++++++++
>  2 files changed, 22 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index b7db3261c62d..5b27e9117d3e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -75,6 +75,14 @@ struct bpf_lpm_trie_key {
>         __u8    data[0];        /* Arbitrary size */
>  };
>
> +/* BPF_SYNCHRONIZE_MAPS waits for the release of any references to a
> + * BPF map made by a BPF program that is running at the time the
> + * BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command

that doesn't sound right to me.
such command won't wait for the release of the references.
in case of map-in-map the program does not hold
the references to inner map (only to outer map).

> + * is to provide a means for userspace to replace a BPF map with
> + * another, newer version, then ensure that no component is still
> + * using the "old" map before manipulating the "old" map in some way.
> + */

that's correct, but the whole paragraph still reads too
'generic' which will lead to confusion,
whereas the use case is map-in-map only.
I think bpf_sync_inner_map or
bpf_sync_map_in_map would be better
choices for the name.

btw i don't have reliable internet access at the moment,
so pls excuse the delays.