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

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Message-Id: <39444a4e-70da-4d17-a40a-b51e05236d23@app.fastmail.com>
Date:   Mon, 24 Jul 2023 20:29:59 +0200
From:   "Arnd Bergmann" <arnd@...nel.org>
To:     "Arnd Bergmann" <arnd@...db.de>,
        "Alexei Starovoitov" <alexei.starovoitov@...il.com>
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 20:13, Arnd Bergmann wrote:

>>> 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=0 to silence
>>> > the warn, but even that is silly. Passing flag=0 into irqrestore is buggy.
>>>
>>> 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?
>
> I think I only tried versions 8 through 13 for this one, but
> can check others as well.

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.

With clang, I only see the warning if I turn dec_active() into
a macro instead of an inline function. This is the expected
behavior in clang.

     Arnd