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Date: Mon, 11 May 2015 16:07:00 -0700
Subject: Re: [PATCH 1/5] selftests: Add futex functional tests
From: Darren Hart <dvhart@linux.intel.com>
To: Shuah Khan <shuahkh@osg.samsung.com>, <linux-api@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>
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        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Message-ID: <D1767EA1.CC3DA%dvhart@linux.intel.com>
Thread-Topic: [PATCH 1/5] selftests: Add futex functional tests
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On 5/11/15, 3:21 PM, "Shuah Khan" <shuahkh@osg.samsung.com> wrote:

>>>>No need for a new pull request. Have you seen these errors before:
>>>>
>>>>make[2]: Entering directory
>>>>'/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex'
>>>>for DIR in functional; do make -C $DIR all ; done
>>>>make[3]: Entering directory
>>>>'/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functiona
>>>>l'
>>>>gcc  -g -O2 -Wall -D_GNU_SOURCE -I../include -I../../   -lpthread -lrt
>>>>futex_requeue_pi.c ../include/futextest.h   -o futex_requeue_pi
>>>>/tmp/cc2UgUVs.o: In function `create_rt_thread':
>>>>/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functional
>>>>/fu
>>>>tex_requeue_pi.c:102:
>>>>undefined reference to `pthread_create'
>>>>/tmp/cc2UgUVs.o: In function `unit_test':
>>>>/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functional
>>>>/fu
>>>>tex_requeue_pi.c:342:
>>>>undefined reference to `pthread_join'
>>>>/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functional
>>>>/fu
>>>>tex_requeue_pi.c:347:
>>>>undefined reference to `pthread_join'
>>>>/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functional
>>>>/fu
>>>>tex_requeue_pi.c:346:
>>>>undefined reference to `pthread_join'
>>>>collect2: error: ld returned 1 exit status
>>>><builtin>: recipe for target 'futex_requeue_pi' failed
>>>>make[3]: *** [futex_requeue_pi] Error 1
>>>>make[3]: Leaving directory
>>>>'/mnt/data/lkml/linux-kselftest/tools/testing/selftests/futex/functiona
>>>>l'
>>>>Makefile:7: recipe for target 'all' failed
>>>>
>>>>I am running make kselftest target when I saw the above build failures.
>>>>kselftest run doesn't fail which is good, however futex tests won't
>>>>run with this failure.
>>>
>>>I have not seen these errors whilst developing with futextest for the
>>>kernel. That looks like you may be missing the pthread development
>>>headers
>>>from your build machine.
>>>
>>>Those are provided by libc6-dev on my Debian systems.
>>>
>>I do have lib6-dev installed. timers uses pthread compiles fine.
>>So does mqueue. I am not sure what is going on with futex tests
>>though. I am guessing it has to do with library link order.
>>The following fixed it for me:
>>-LDFLAGS := $(LDFLAGS) -lpthread -lrt
>>+LDFLAGS := $(LDFLAGS) -pthread -lrt
>>Could you please make this change and resend the patch series.
>>I prefer patch series over pull request.
>>thanks,
>>-- Shuah
>
>Sorry full diff:
>
>
>diff --git a/tools/testing/selftests/futex/functional/Makefile
>b/tools/testing/selftests/futex/functional/Makefile
>index e64d43b..b8a2e9b 100644
>--- a/tools/testing/selftests/futex/functional/Makefile
>+++ b/tools/testing/selftests/futex/functional/Makefile
>@@ -1,6 +1,6 @@
>INCLUDES := -I../include -I../../
>CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE $(INCLUDES)
>-LDFLAGS := $(LDFLAGS) -lpthread -lrt
>+LDFLAGS := $(LDFLAGS) -pthread -lrt


I'm happy to do that, but I would like to make sure I'm doing the right
thing.

I'm building with:
$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2


What's there now works for me:
	LDFLAGS := $(LDFLAGS) -lpthread -lrt


What you propose also works for me:
	LDFLAGS := $(LDFLAGS) -pthread -lrt


I notice that most other test cases list -lrt first, including timers:
	LDFLAGS += -lrt -lpthread

Which works for me for timers - does that work for you? I assume so. It
also works for futexes for me. I've found references out there suggesting
-lrt should always come before -lpthread.

I suspect you are correct as -pthread provides flags for both the
preprocessor and the linker, however, I'm still concerned about the
ordering.

John - did you deliberately choose to use "-lrt -lpthread" ?

If ordering matters and -pthread is preferred over -lpthread, then perhaps
the "right way" is:

 LDFLAGS += -lrt -pthread


Since -pthread also impacts the pre-processor, it seems that if used, it
should be present in both LDFLAGS as well as CFLAGS. However, I've also
read that -pthread is a non-standard compiler option and is best avoided.

If so, and if the reversed order works for you, "-lrt -lpthread", perhaps
that would be the "right fix".

I'd like to answer this once and for all for all tests we add to selftest.

-- 
Darren Hart
Intel Open Source Technology Center



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