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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Mon, 24 Jul 2023 11:00:11 -0700
Message-ID: <CAADnVQ+p4wpd=tKJAiwB34O1y5vv4mibtkt9D-F7sG=rQapcew@mail.gmail.com>
Subject: Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Yafang Shao <laoar.shao@gmail.com>,
        Arnd Bergmann <arnd@kernel.org>,
        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>
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On Sun, Jul 23, 2023 at 11:32=E2=80=AFAM Arnd Bergmann <arnd@arndb.de> wrot=
e:
>
> >> If so, why can't we improve the compiler ?
> >
> > Agree.
> > Sounds like a compiler bug.
>
> I don't know what you might want to change in the compiler
> to avoid this. Compilers are free to decide which functions to
> inline in the absence of noinline or always_inline flags.

Clearly a compiler bug.
Compilers should not produce false positive warnings regardless
how inlining went and optimizations performed.


> One difference between gcc and clang is that gcc tries to
> be smart about warnings by using information from inlining
> to produce better warnings, while clang never uses information
> across function boundaries for generated warnings, so it won't
> find this one, but also would ignore an unconditional use
> of the uninitialized variable.
>
> >> If we have to change the kernel, what about the change below?
> >
> > To workaround the compiler bug we can simply init flag=3D0 to silence
> > the warn, but even that is silly. Passing flag=3D0 into irqrestore is b=
uggy.
>
> Maybe inc_active() could return the flags instead of modifying
> the stack variable? that would also result in slightly better
> code when it's not inlined.

Which gcc are we talking about here that is so buggy?