netdev - Re: [PATCH bpf-next v2] bpf: move rcu lock management out of BPF_PROG

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Date:   Sat, 16 Apr 2022 01:28:05 +0000
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Network Development <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 v2] bpf: move rcu lock management out of
 BPF_PROG_RUN routines

On Thu, Apr 14, 2022 at 9:12 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> +static int
> +bpf_prog_run_array_cg_flags(const struct cgroup_bpf *cgrp,
> +                           enum cgroup_bpf_attach_type atype,
> +                           const void *ctx, bpf_prog_run_fn run_prog,
> +                           int retval, u32 *ret_flags)
> +{
> +       const struct bpf_prog_array_item *item;
> +       const struct bpf_prog *prog;
> +       const struct bpf_prog_array *array;
> +       struct bpf_run_ctx *old_run_ctx;
> +       struct bpf_cg_run_ctx run_ctx;
> +       u32 func_ret;
> +
> +       run_ctx.retval = retval;
> +       migrate_disable();
> +       rcu_read_lock();
> +       array = rcu_dereference(cgrp->effective[atype]);
> +       item = &array->items[0];
> +       old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
> +       while ((prog = READ_ONCE(item->prog))) {
> +               run_ctx.prog_item = item;
> +               func_ret = run_prog(prog, ctx);
...
> +       ret = bpf_prog_run_array_cg(&cgrp->bpf, CGROUP_GETSOCKOPT,
>                                     &ctx, bpf_prog_run, retval);

Did you check the asm that bpf_prog_run gets inlined
after being passed as a pointer to a function?
Crossing fingers... I suspect not every compiler can do that :(
De-virtualization optimization used to be tricky.