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Date:   Mon, 24 Jul 2023 22:41:26 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
        "Arnd Bergmann" <arnd@kernel.org>
Cc:     "Yafang Shao" <laoar.shao@gmail.com>,
        "Alexei Starovoitov" <ast@kernel.org>,
        "Daniel Borkmann" <daniel@iogearbox.net>,
        "Andrii Nakryiko" <andrii@kernel.org>,
        "Hou Tao" <houtao1@huawei.com>,
        "Martin KaFai Lau" <martin.lau@linux.dev>,
        "Song Liu" <song@kernel.org>, "Yonghong Song" <yhs@fb.com>,
        "John Fastabend" <john.fastabend@gmail.com>,
        "KP Singh" <kpsingh@kernel.org>,
        "Stanislav Fomichev" <sdf@google.com>,
        "Hao Luo" <haoluo@google.com>, "Jiri Olsa" <jolsa@kernel.org>,
        "Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions
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On Mon, Jul 24, 2023, at 21:15, Alexei Starovoitov wrote:
> On Mon, Jul 24, 2023 at 11:30=E2=80=AFAM Arnd Bergmann <arnd@kernel.or=
g> 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) !=3D 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