netdev - Re: [PATCH bpf-next 02/17] bpf: allow RCU-protected lookups to happen from bh context

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Date:   Thu, 10 Jun 2021 11:38:55 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH bpf-next 02/17] bpf: allow RCU-protected lookups to happen
 from bh context

On Wed, Jun 9, 2021 at 7:24 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> XDP programs are called from a NAPI poll context, which means the RCU
> reference liveness is ensured by local_bh_disable(). Add
> rcu_read_lock_bh_held() as a condition to the RCU checks for map lookups so
> lockdep understands that the dereferences are safe from inside *either* an
> rcu_read_lock() section *or* a local_bh_disable() section. This is done in
> preparation for removing the redundant rcu_read_lock()s from the drivers.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  kernel/bpf/hashtab.c  | 21 ++++++++++++++-------
>  kernel/bpf/helpers.c  |  6 +++---
>  kernel/bpf/lpm_trie.c |  6 ++++--
>  3 files changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 6f6681b07364..72c58cc516a3 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -596,7 +596,8 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
>         struct htab_elem *l;
>         u32 hash, key_size;
>
> -       WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
> +       WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
> +                    !rcu_read_lock_bh_held());

It's not clear to me whether rcu_read_lock_held() is still needed.
All comments sound like rcu_read_lock_bh_held() is a superset of rcu
that includes bh.
But reading rcu source code it looks like RCU_BH is its own rcu flavor...
which is confusing.