netdev - Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read

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Message-ID: <CAPhsuW7tVhR1oFtsZ9t0bQdBQtgAKsRpGH9N5ir7JhMBTm81PQ@mail.gmail.com>
Date:   Wed, 8 Jan 2020 17:21:30 -0800
From:   Song Liu <song@...nel.org>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()

On Wed, Jan 8, 2020 at 1:36 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> for preempt_disabled region to complete the rcu read critical section
> in __dev_map_flush() is no longer relevant.
>
> These originally ensured the map reference was safe while a map was
> also being free'd. But flush by new rules can only be called from
> preempt-disabled NAPI context. The synchronize_rcu from the map free
> path and the rcu_call from the delete path will ensure the reference
> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> pair to avoid any confusion around how this is being protected.
>
> If the rcu_read_lock was required it would mean errors in the above
> logic and the original patch would also be wrong.
>
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

Acked-by: Song Liu <songliubraving@...com>