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From: Jeff Johnson <quic_jjohnson@quicinc.com>
To: Masahiro Yamada <masahiroy@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Cc: Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nicolas Schier <nicolas@fjasle.eu>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-modules <linux-modules@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	clang-built-linux <llvm@lists.linux.dev>
Subject: Re: [PATCH v4 03/14] modpost: split the section mismatch checks into section-check.c
Date: Wed, 11 May 2022 12:51:36 -0700	[thread overview]
Message-ID: <8babc23b-5e24-f662-6c4a-eb1c30e0e6da@quicinc.com> (raw)
In-Reply-To: <CAK7LNAR5preB59HJH1-F_ZfEmoF3N8b9z4eRhYzsEVwu_XUH4Q@mail.gmail.com>

On 5/11/2022 12:27 PM, Masahiro Yamada wrote:
> On Thu, May 12, 2022 at 3:48 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> On Mon, May 9, 2022 at 11:57 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>>>
>>>>> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
>>>>> index a78b75f0eeb0..e7e2c70a98f5 100644
>>>>> --- a/scripts/mod/modpost.c
>>>>> +++ b/scripts/mod/modpost.c
>>>>> @@ -31,7 +31,7 @@ static bool external_module;
>>>>>   /* Only warn about unresolved symbols */
>>>>>   static bool warn_unresolved;
>>>>>
>>>>> -static int sec_mismatch_count;
>>>>> +int sec_mismatch_count;
>>>>
>>>> ^ this should go in modpost.h if it is to be used in two translation
>>>> units, rather than forward declaring it in section-check.c.  You did
>>>> this for the functions.
>>>
>>>
>>> Sorry, I do not understand.
>>>
>>>
>>> In modpost.h, I put the declaration:
>>>
>>>    extern int sec_mismatch_count;
>>>
>>> If I moved it to the header without 'extern'
>>> I would get multiple definitions.
>>
>> Yeah, you need to _declare_ it w/ extern in the header, then _define_
>> it in one source file.
>>
>> That way, if the type ever changes, the sources will agree on type in
>> all source files. You will get a redefinition error if the definition
>> changes the type of the variable since the last declaration.
>>
>> What you're doing is forward declaring, which works, and is a common
>> pattern for (bloated) C++, but is less type safe than sharing a single
>> common declaration between multiple source files via a single common
>> shared header. (Sorry I didn't respond before you sent v5)
> 
> Sorry, I still do not understand your suggestion.
> 
> 
> Could you provide me with a code diff
> showing how to do this better?

I think you are doing exactly what he's asking for:
declare it with extern in the header (modpost.h change)
define it in one source file (modpost.c change)


WARNING: multiple messages have this Message-ID
From: Jeff Johnson <quic_jjohnson@quicinc.com>
To: Masahiro Yamada <masahiroy@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-s390 <linux-s390@vger.kernel.org>,
	Nicolas Schier <nicolas@fjasle.eu>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	clang-built-linux <llvm@lists.linux.dev>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-modules <linux-modules@vger.kernel.org>
Subject: Re: [PATCH v4 03/14] modpost: split the section mismatch checks into section-check.c
Date: Wed, 11 May 2022 12:51:36 -0700	[thread overview]
Message-ID: <8babc23b-5e24-f662-6c4a-eb1c30e0e6da@quicinc.com> (raw)
In-Reply-To: <CAK7LNAR5preB59HJH1-F_ZfEmoF3N8b9z4eRhYzsEVwu_XUH4Q@mail.gmail.com>

On 5/11/2022 12:27 PM, Masahiro Yamada wrote:
> On Thu, May 12, 2022 at 3:48 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> On Mon, May 9, 2022 at 11:57 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>>>
>>>>> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
>>>>> index a78b75f0eeb0..e7e2c70a98f5 100644
>>>>> --- a/scripts/mod/modpost.c
>>>>> +++ b/scripts/mod/modpost.c
>>>>> @@ -31,7 +31,7 @@ static bool external_module;
>>>>>   /* Only warn about unresolved symbols */
>>>>>   static bool warn_unresolved;
>>>>>
>>>>> -static int sec_mismatch_count;
>>>>> +int sec_mismatch_count;
>>>>
>>>> ^ this should go in modpost.h if it is to be used in two translation
>>>> units, rather than forward declaring it in section-check.c.  You did
>>>> this for the functions.
>>>
>>>
>>> Sorry, I do not understand.
>>>
>>>
>>> In modpost.h, I put the declaration:
>>>
>>>    extern int sec_mismatch_count;
>>>
>>> If I moved it to the header without 'extern'
>>> I would get multiple definitions.
>>
>> Yeah, you need to _declare_ it w/ extern in the header, then _define_
>> it in one source file.
>>
>> That way, if the type ever changes, the sources will agree on type in
>> all source files. You will get a redefinition error if the definition
>> changes the type of the variable since the last declaration.
>>
>> What you're doing is forward declaring, which works, and is a common
>> pattern for (bloated) C++, but is less type safe than sharing a single
>> common declaration between multiple source files via a single common
>> shared header. (Sorry I didn't respond before you sent v5)
> 
> Sorry, I still do not understand your suggestion.
> 
> 
> Could you provide me with a code diff
> showing how to do this better?

I think you are doing exactly what he's asking for:
declare it with extern in the header (modpost.h change)
define it in one source file (modpost.c change)


  reply	other threads:[~2022-05-11 19:53 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-08 19:06 [PATCH v4 00/14] kbuild: yet another series of cleanups (modpost, LTO, MODULE_REL_CRCS, export.h) Masahiro Yamada
2022-05-08 19:06 ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 01/14] modpost: remove left-over cross_compile declaration Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 17:08   ` Nick Desaulniers
2022-05-09 17:08     ` Nick Desaulniers
2022-05-12  4:35   ` Masahiro Yamada
2022-05-12  4:35     ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 02/14] modpost: change the license of EXPORT_SYMBOL to bool type Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-12  4:36   ` Masahiro Yamada
2022-05-12  4:36     ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 03/14] modpost: split the section mismatch checks into section-check.c Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 17:19   ` Nick Desaulniers
2022-05-09 17:19     ` Nick Desaulniers
2022-05-10  6:55     ` Masahiro Yamada
2022-05-10  6:55       ` Masahiro Yamada
2022-05-11 18:47       ` Nick Desaulniers
2022-05-11 18:47         ` Nick Desaulniers
2022-05-11 19:27         ` Masahiro Yamada
2022-05-11 19:27           ` Masahiro Yamada
2022-05-11 19:51           ` Jeff Johnson [this message]
2022-05-11 19:51             ` Jeff Johnson
2022-05-11 20:01             ` Nick Desaulniers
2022-05-11 20:01               ` Nick Desaulniers
2022-05-08 19:06 ` [PATCH v4 04/14] modpost: add sym_find_with_module() helper Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 05/14] modpost: extract symbol versions from *.cmd files Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 21:52   ` Sami Tolvanen
2022-05-09 21:52     ` Sami Tolvanen
2022-05-08 19:06 ` [PATCH v4 06/14] kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 17:50   ` Nick Desaulniers
2022-05-09 17:50     ` Nick Desaulniers
2022-05-10 13:03     ` Masahiro Yamada
2022-05-10 13:03       ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 07/14] kbuild: stop merging *.symversions Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 08/14] genksyms: adjust the output format to modpost Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 09/14] kbuild: do not create *.prelink.o for Clang LTO or IBT Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 23:13   ` Sami Tolvanen
2022-05-09 23:13     ` Sami Tolvanen
2022-05-08 19:06 ` [PATCH v4 10/14] kbuild: check static EXPORT_SYMBOL* by script instead of modpost Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09 18:05   ` Nick Desaulniers
2022-05-09 18:05     ` Nick Desaulniers
2022-05-10 13:53     ` Masahiro Yamada
2022-05-10 13:53       ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 11/14] kbuild: make built-in.a rule robust against too long argument error Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 12/14] kbuild: make *.mod " Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 13/14] kbuild: add cmd_and_savecmd macro Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 14/14] kbuild: rebuild multi-object modules when objtool is updated Masahiro Yamada
2022-05-08 19:06   ` Masahiro Yamada
2022-05-09  4:24 ` [PATCH v4 00/14] kbuild: yet another series of cleanups (modpost, LTO, MODULE_REL_CRCS, export.h) Masahiro Yamada
2022-05-09  4:24   ` Masahiro Yamada
2022-05-09 22:13   ` Nathan Chancellor
2022-05-09 22:13     ` Nathan Chancellor
2022-05-10  6:55     ` Masahiro Yamada
2022-05-10  6:55       ` Masahiro Yamada
2022-05-09 22:19   ` Nathan Chancellor
2022-05-09 22:19     ` Nathan Chancellor

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