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Date:   Wed, 20 Dec 2017 09:07:07 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Yang Shi <yang.s@...baba-inc.com>, Arnd Bergmann <arnd@...db.de>,
        Ingo Molnar <mingo@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Build error for 4.15-rc3 kernel caused by patch "kbuild:
 Add a cache for generated variables"

Hi,

On Tue, Dec 19, 2017 at 6:29 PM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> 2017-12-19 2:17 GMT+09:00 Doug Anderson <dianders@...omium.org>:
>> Hi,
>>
>> On Mon, Dec 18, 2017 at 7:50 AM, Masahiro Yamada
>> <yamada.masahiro@...ionext.com> wrote:
>>> 2017-12-18 23:56 GMT+09:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
>>>> 2017-12-17 7:35 GMT+09:00 Yang Shi <yang.s@...baba-inc.com>:
>>>>> Hi folks,
>>>>>
>>>>> I just upgraded gcc to 6.4 on my centos 7 machine by Arnd's suggestion. But,
>>>>> I ran into the below compile error with 4.15-rc3 kernel:
>>>>>
>>>>> In file included from ./include/uapi/linux/uuid.h:21:0,
>>>>>                  from ./include/linux/uuid.h:19,
>>>>>                  from ./include/linux/mod_devicetable.h:12,
>>>>>                  from scripts/mod/devicetable-offsets.c:2:
>>>>> ./include/linux/string.h:8:20: fatal error: stdarg.h: No such file or
>>>>> directory
>>>>>  #include <stdarg.h>
>>>>>
>>>>> I bisected to commit 3298b690b21cdbe6b2ae8076d9147027f396f2b1 ("kbuild: Add
>>>>> a cache for generated variables"). Once I revert this commit, kernel build
>>>>> is fine.
>>>>>
>>>>> gcc 4.8.5 is fine to build kernel with this commit.
>>>>>
>>>>> I'm not quite sure if this is a bug or my gcc install is skewed although it
>>>>> can build kernel without that commit since that commit might exacerbate the
>>>>> case.
>>>>>
>>>>> Any hint is appreciated
>>>>
>>>>
>>>> Today, I was also hit with the same error
>>>> when I was compiling linux-next.
>>>> I am not so sure why this error happens, but
>>>> "make clean" will probably fix the problem.
>>>>
>>>> You need to do "make clean" to blow .cache.mk
>>>> when you upgrade your compiler.
>>>> This is nasty, though...
>>>>
>>>
>>>
>>> I got it.
>>>
>>> The following line in the top-level Makefile.
>>>
>>> NOSTDINC_FLAGS += -nostdinc -isystem $(call shell-cached,$(CC)
>>> -print-file-name=include)
>>>
>>>
>>> If the stale result of -print-file-name is stored in the cache file,
>>> the compiler fails to find <stdarg.h>
>>
>> Nice catch!  Do you have any idea how we can fix it?  I suppose we
>> could add a single (non-cached) call to CC somewhere in there to get
>> CC's version and clobber the cache if the version changes.  Is that
>> the best approach here?
>>
>> In general I remember thinking about the gcc upgrade problem when I
>> was first experimenting with the cache.  At the time my assumption was
>> that if someone updated their gcc then they really ought to be doing a
>> clean anyway (I wasn't sure if the build system somehow enforced this,
>> but I didn't think so).  Doing an incremental build after a compiler
>> upgrade just seems (to me) to be asking for asking for trouble, or in
>> the very least seems like it's not what the user wanted (if you update
>> your compiler you almost certainly want it to be used to build all of
>> your code, don't you?)
>
> I agree.
> When you upgrade your compiler,
> you need to remove not only cache files, but also all object files.
> So, "make clean" is the most reasonable way.
>
>
>> Even if it's wise to do a clean after a compiler upgrade, it still
>> seems pretty non-ideal that a user has to decipher an arcane error
>> like this, so it seems like we should see what we can do to detect
>> this case for the user and help them out.  Perhaps rather than
>> clobbering the cache we should actually suggest that the user run a
>> "make clean"?
>>
>
> Right.  I think it's a good thing to do.

Are you planning on doing this, or is this something you'd like me to
attempt?  I'm a bit busy in the last two days before I go on Christmas
break, but I can try to squeeze something like this in since the root
of the issue is a patch that I authored.  Let me know.

If this is something you'd like me to do, let me know if you think the
right solution is to detect the problem and warn the user or if the
right solution is to just blow away the cache.  It would be up to you,
but I'd tend to go the route of warning the user because:

* The user should almost certainly do a "make clean" to really ensure
no mismatch between object files.

* I could imagine that trying to invoke "make clean" automatically
might be complicated.


> BTW, "sudo make install" or "sudo make modules_install" could
> add some cache entries by super user privilege?
>
> (For example, run build targets with CROSS_COMPILE,
> but run install targets without CROSS_COMPILE,
> install targets will produce different cache entries.)
>
>
> If so, "make clean" in normal user privilege
> can not remove cache files...

Hrm.  That doesn't sound nice.  I guess this could be solved by
something like your "no-compiler-targets" patch, but IIUC that didn't
include "install" or "module_install".  I guess the other option would
be to somehow detect "UID=0" specifically and not generate the cache?

-Doug

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