Date:	Mon, 11 Jan 2016 17:50:21 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	<acme@...nel.org>, <jolsa@...nel.org>, <namhyung@...nel.or>,
	<lizefan@...wei.com>, <pi3orama@....com>,
	<linux-kernel@...r.kernel.org>, Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 1/6] perf tools: Add -lutil in python lib list for broken
 python-config



On 2016/1/11 17:44, Wangnan (F) wrote:
>
>
> On 2016/1/11 17:28, Jiri Olsa wrote:
>> On Fri, Jan 08, 2016 at 02:23:56PM +0000, Wang Nan wrote:
>>> On some system the perf-config is broken, causes link failure like 
>>> this:
>>>
>>>   /usr/lib64/python2.7/config/libpython2.7.a(posixmodule.o): In 
>>> function `posix_forkpty':
>>> /opt/wangnan/yocto-build/tmp-eglibc/work/x86_64-oe-linux/python/2.7.3-r0.3.1/Python-2.7.3/./Modules/posixmodule.c:3816: 
>>> undefined reference to `forkpty'
>>>   /usr/lib64/python2.7/config/libpython2.7.a(posixmodule.o): In 
>>> function `posix_openpty':
>>> /opt/wangnan/yocto-build/tmp-eglibc/work/x86_64-oe-linux/python/2.7.3-r0.3.1/Python-2.7.3/./Modules/posixmodule.c:3756: 
>>> undefined reference to `openpty'
>>>   collect2: error: ld returned 1 exit status
>>> make[1]: *** [/home/wangnan/kernel-hydrogen/tools/perf/out/perf] 
>>> Error 1
>>> make: *** [all] Error 2
>>>
>>>   $ python-config --libs
>>>   -lpthread -ldl -lpthread -lutil -lm -lpython2.7
>> so your 'python-config --libs' shows -lutil and you still get the 
>> build failure?
>>
>> after your fix I can see double -lutil in PYTHON_EMBED_LIBADD,
>> which is probably not a problem but could be ommited I think
>
> In my python-config:
>
>  # python-config
> -lpthread -ldl -lpthread -lutil -lm -lpython2.7
>
> There are multiple -lpthread already. In fact multiple '-l' options is 
> not
> a problem at all. Please see below.
>
>>> In this case a '-lutil' should be appended to -lpython2.7.
>>>
>>> (I know we have --start-group and --end-group. I can see them in
>>> command line of collect2 by strace. However it doesn't work. Seems
>>> I have a broken environment?)
>> hum, how is this related to start/end -group options?
>
> Please check man page of ld:
>
> <quote>
> --start-group archives --end-group
>     The archives should be a list of archive files. They may be either 
> explicit file names, or -l options.
>
>     The specified archives are searched repeatedly until no new 
> undefined references are created. Normally, an archive is searched 
> only once in the order that it is specified on the command line. If a 
> symbol in that archive is needed to resolve an undefined symbol 
> referred to by an object in an archive that appears later on the 
> command line, the linker would not be able to resolve that reference. 
> By grouping the archives, they all be searched repeatedly until all 
> possible references are resolved.
>
>     Using this option has a significant performance cost. It is best 
> to use it only when there are unavoidable circular references between 
> two or more archives.
> </quote>
>
> Without --start/end-group, if we provide -l in this order:
>
>  ... -la -lb -lc ...
>
> and libb.a requires a symbol in liba.a, link would fail. The simplest way
> to avoid this is to add another -la after -lb:
>
>  ... -la -lb -la -lc ...
>
> In my environment it works as if we don't provide -Wl,--start-group 
> and -Wl,--end-group.
> I don't know why.
>

I mean the python case before I add -lutil after python libs. 
-Wl,--start-group and -Wl,--end-group
seems not work for me. They are issued, I confirm this in strace output.

Thank you.

> Thank you.

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