linux-kernel - Re: [PATCH] bpf: force inc_active()/dec

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Message-Id: <3e202277-fe74-4105-93ec-b646efaaa956@app.fastmail.com>
Date:   Mon, 24 Jul 2023 22:41:26 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Alexei Starovoitov" <alexei.starovoitov@...il.com>,
        "Arnd Bergmann" <arnd@...nel.org>
Cc:     "Yafang Shao" <laoar.shao@...il.com>,
        "Alexei Starovoitov" <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "Andrii Nakryiko" <andrii@...nel.org>,
        "Hou Tao" <houtao1@...wei.com>,
        "Martin KaFai Lau" <martin.lau@...ux.dev>,
        "Song Liu" <song@...nel.org>, "Yonghong Song" <yhs@...com>,
        "John Fastabend" <john.fastabend@...il.com>,
        "KP Singh" <kpsingh@...nel.org>,
        "Stanislav Fomichev" <sdf@...gle.com>,
        "Hao Luo" <haoluo@...gle.com>, "Jiri Olsa" <jolsa@...nel.org>,
        "Kumar Kartikeya Dwivedi" <memxor@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions

On Mon, Jul 24, 2023, at 21:15, Alexei Starovoitov wrote:
> On Mon, Jul 24, 2023 at 11:30 AM Arnd Bergmann <arnd@...nel.org> wrote:
>> On Mon, Jul 24, 2023, at 20:13, Arnd Bergmann wrote:
>>
>> I have a minimized test case at https://godbolt.org/z/hK4ev17fv
>> that shows the problem happening with all versions of gcc
>> (4.1 through 14.0) if I force the dec_active() function to be
>> inline and force inc_active() to be non-inline.
>
> That's a bit of cheating, but I see your point now.
> How about we do:
> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> index 51d6389e5152..3fa0944cb975 100644
> --- a/kernel/bpf/memalloc.c
> +++ b/kernel/bpf/memalloc.c
> @@ -183,11 +183,11 @@ static void inc_active(struct bpf_mem_cache *c,
> unsigned long *flags)
>         WARN_ON_ONCE(local_inc_return(&c->active) != 1);
>  }
>
> -static void dec_active(struct bpf_mem_cache *c, unsigned long flags)
> +static void dec_active(struct bpf_mem_cache *c, unsigned long *flags)
>  {
>         local_dec(&c->active);
>         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -               local_irq_restore(flags);
> +               local_irq_restore(*flags);
>  }


Sure, that's fine. Between this and the two suggestions I had
(__always_inline or passing the flags from  inc_active as a
return code), I don't have a strong preference, so pick whichever
you like.

      Arnd