netdev - Re: [PATCH bpf-next] bpf: move rcu lock management out of BPF_PROG

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Date:   Wed, 13 Apr 2022 12:23:16 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] bpf: move rcu lock management out of
 BPF_PROG_RUN routines

On Wed, Apr 13, 2022 at 11:33 AM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> Commit 7d08c2c91171 ("bpf: Refactor BPF_PROG_RUN_ARRAY family of macros
> into functions") switched a bunch of BPF_PROG_RUN macros to inline
> routines. This changed the semantic a bit. Due to arguments expansion
> of macros, it used to be:
>
>         rcu_read_lock();
>         array = rcu_dereference(cgrp->bpf.effective[atype]);
>         ...
>
> Now, with with inline routines, we have:
>         array_rcu = rcu_dereference(cgrp->bpf.effective[atype]);
>         /* array_rcu can be kfree'd here */
>         rcu_read_lock();
>         array = rcu_dereference(array_rcu);
>

So subtle difference, wow...

But this open-coding of rcu_read_lock() seems very unfortunate as
well. Would making BPF_PROG_RUN_ARRAY back to a macro which only does
rcu lock/unlock and grabs effective array and then calls static inline
function be a viable solution?

#define BPF_PROG_RUN_ARRAY_CG_FLAGS(array_rcu, ctx, run_prog, ret_flags) \
  ({
      int ret;

      rcu_read_lock();
      ret = __BPF_PROG_RUN_ARRAY_CG_FLAGS(rcu_dereference(array_rcu), ....);
      rcu_read_unlock();
      ret;
  })


where __BPF_PROG_RUN_ARRAY_CG_FLAGS is what
BPF_PROG_RUN_ARRAY_CG_FLAGS is today but with __rcu annotation dropped
(and no internal rcu stuff)?


> I'm assuming in practice rcu subsystem isn't fast enough to trigger
> this but let's use rcu API properly: ask callers of BPF_PROG_RUN
> to manage rcu themselves.
>
> Also, rename to lower caps to not confuse with macros. Additionally,
> drop and expand BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY.
>
> See [1] for more context.
>
>   [1] https://lore.kernel.org/bpf/CAKH8qBs60fOinFdxiiQikK_q0EcVxGvNTQoWvHLEUGbgcj1UYg@mail.gmail.com/T/#u
>
> Cc: Martin KaFai Lau <kafai@...com>
> Fixes: 7d08c2c91171 ("bpf: Refactor BPF_PROG_RUN_ARRAY family of macros into functions")
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  drivers/media/rc/bpf-lirc.c |  8 +++-
>  include/linux/bpf.h         | 70 ++++++-------------------------
>  kernel/bpf/cgroup.c         | 84 +++++++++++++++++++++++++++++--------
>  kernel/trace/bpf_trace.c    |  5 ++-
>  4 files changed, 90 insertions(+), 77 deletions(-)
>

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