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

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Date:   Sun, 23 Jul 2023 09:46:41 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hou Tao <houtao1@...wei.com>, Arnd Bergmann <arnd@...db.de>,
        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 Sun, Jul 23, 2023 at 7:25 AM Yafang Shao <laoar.shao@...il.com> wrote:
>
> On Sat, Jul 22, 2023 at 3:48 PM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > Splitting these out into separate helper functions means that we
> > actually pass an uninitialized variable into another function call
> > if dec_active() happens to not be inlined, and CONFIG_PREEMPT_RT
> > is disabled:
>
> Do you mean that the compiler can remove the flags automatically when
> dec_active() is inlined, but can't remove it automatically when
> dec_active() is not inlined ?
> If so, why can't we improve the compiler ?

Agree.
Sounds like a compiler bug.

> If we have to change the kernel, what about the change below?

To workaround the compiler bug we can simply init flag=0 to silence
the warn, but even that is silly. Passing flag=0 into irqrestore is buggy.