linux-kernel - Re: [PATCH v2 4/4] dynamic_debug: add jump label support

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Date:	Mon, 13 Jun 2016 16:32:15 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<akpm@...ux-foundation.org>, <joe@...ches.com>,
	<peterz@...radead.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 4/4] dynamic_debug: add jump label support



On 06/13/2016 04:23 PM, Arnd Bergmann wrote:
> On Monday, June 13, 2016 6:05:22 PM CEST Arnd Bergmann wrote:
>> On Friday, June 10, 2016 11:33:07 AM CEST Jason Baron wrote:
>>> On 06/10/2016 05:54 AM, Arnd Bergmann wrote:
>>>> On Friday, May 20, 2016 5:16:36 PM CEST Jason Baron wrote:
>>>>> Although dynamic debug is often only used for debug builds, sometimes its
>>>>> enabled for production builds as well. Minimize its impact by using jump
>>>>> labels. This reduces the text section by 7000+ bytes in the kernel image
>>>>> below. It does increase data, but this should only be referenced when
>>>>> changing the direction of the branches, and hence usually not in cache.
>>>>>
>>>>>    text         data     bss     dec     hex filename
>>>>> 8194852      4879776  925696 14000324         d5a0c4 vmlinux.pre
>>>>> 8187337      4960224  925696 14073257         d6bda9 vmlinux.post
>>>>>
>>>>> Signed-off-by: Jason Baron <jbaron@...mai.com>
>>>>> ---
>>>>
>>>> This causes problems for some of my randconfig builds, when a dynamic
>>>> debug call is used inside of an __exit function:
>>>>
>>>> `.exit.text' referenced in section `__jump_table' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o
>>>> `.exit.text' referenced in section `__jump_table' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o
>>>>
>>>
>>> I stuck pr_debug() in a few functions marked with __exit, but did not
>>> reproduce yet. Can you share your .config and gcc --version.
>>>
>>
>> I found these on ARM randconfig builds e.g. this one
>> http://pastebin.com/raw/KjWHxnwU
>>
>> I also have some other patches applied that could have interacted with your
>> change, so if you can't reproduce it easily, let me try it on a plain linux-next
>> kernel.
>>
>> The compiler I use is  arm-linux-gnueabi-gcc (GCC) 6.0.0 20160323 (experimental)
> 
> Update: on ARM, I have been able to reproduce this with gcc-4.6
> and gcc-4.8, so I'm pretty confident that this is independent of the
> toolchain. However, I have so far failed to reproduce this on x86.
> 
> Looking at the exit_ceph() function, I get these two assembly outputs,
> ARM fails with the link error above:
> 

ok, does this fix things up?

--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -44,7 +44,7 @@
 #endif

 #if (defined(CONFIG_SMP_ON_UP) && !defined(CONFIG_DEBUG_SPINLOCK)) || \
-       defined(CONFIG_GENERIC_BUG)
+       defined(CONFIG_GENERIC_BUG) || defined(CONFIG_JUMP_LABEL)
 #define ARM_EXIT_KEEP(x)       x
 #define ARM_EXIT_DISCARD(x)
 #else


Thanks,

-Jason