Re: 3.1-rc1 link failure

[Stephen Boyd]

( Including full text for other lists)

On 08/08/11 12:58, Russell King - ARM Linux wrote:
> On Mon, Aug 08, 2011 at 01:49:31PM -0500, Rob Herring wrote:
>> Russell,
>>
>> This commit is causing some link failures:
>>
>>     ARM: vmlinux.lds: move discarded sections to beginning
>>
>>     Rather than scattering the discarded sections throughout the linker
>>     file, move them to the start.
>>
>>     Acked-by: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
>>     Tested-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>>     Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
>>
>> The error is this:
>>
>> `.exit.text' referenced in section `.alt.smp.init' of
>> drivers/built-in.o: defined in discarded section `.exit.text' of
>> drivers/built
>> -in.o
>> `.exit.text' referenced in section `.alt.smp.init' of net/built-in.o:
>> defined in discarded section `.exit.text' of net/built-in.o
>>
>> I traced the one in net/ to CONFIG_SMP_ON_UP=y and CONFIG_DCB=y.
>> dcbnl_exit calls dcb_flushapp which contains a spinlock. dcb_flushapp is
>> getting inlined into dcbnl_exit.
> Argh.  This is going to be an absolute _pig_ to fix.
>
> This is the relevent part of the linker script (reformatted to make it
> clearer):
>
> | SECTIONS
> | {
> |  /*
> |   * unwind exit sections must be discarded before the rest of the
> |   * unwind sections get included.
> |   */
> |  /DISCARD/ : {
> |   *(.ARM.exidx.exit.text)
> |   *(.ARM.extab.exit.text)
> |  }
> | ...
> |  .exit.text : {
> |   *(.exit.text)
> |   *(.memexit.text)
> |  }
> | ...
> |  /DISCARD/ : {
> |   *(.exit.text)
> |   *(.memexit.text)
> |   *(.exit.data)
> |   *(.memexit.data)
> |   *(.memexit.rodata)
> |   *(.exitcall.exit)
> |   *(.discard)
> |   *(.discard.*)
> |   }
> | }
>
> Now, this is what the linker manual says about discarded output sections:
>
> |    The special output section name `/DISCARD/' may be used to discard
> | input sections.  Any input sections which are assigned to an output
> | section named `/DISCARD/' are not included in the output file.
>
> No questions, no exceptions.  It doesn't say "unless they are listed
> before the /DISCARD/ section."  Now, this is what asn-generic/vmlinux.lds.S
> says:
>
> | /*
> |  * Default discarded sections.
> |  *
> |  * Some archs want to discard exit text/data at runtime rather than
> |  * link time due to cross-section references such as alt instructions,
> |  * bug table, eh_frame, etc.  DISCARDS must be the last of output
> |  * section definitions so that such archs put those in earlier section
> |  * definitions.
> |  */
>
> And guess what - the list _always_ includes .exit.text etc.
>
> Now, what's actually happening is that the linker is reading the script,
> and it finds the first /DISCARD/ output section at the beginning of the
> script.  It continues reading the script, and finds the 'DISCARD' macro
> at the end, which having been postprocessed results in another
> /DISCARD/ output section.  As the linker already contains the earlier
> /DISCARD/ output section, it adds it to that existing section, so it
> effectively is placed at the start.  This can be seen by using the -M
> option to ld:
>
> | Linker script and memory map
> | 
> |                 0xc037c080                jiffies = jiffies_64
> | 
> | /DISCARD/
> |  *(.ARM.exidx.exit.text)
> |  *(.ARM.extab.exit.text)
> |  *(.exit.text)
> |  *(.memexit.text)
> |  *(.exit.data)
> |  *(.memexit.data)
> |  *(.memexit.rodata)
> |  *(.exitcall.exit)
> |  *(.discard)
> |  *(.discard.*)
> | 
> |                 0xc0008000                . = 0xc0008000
> | 
> | .head.text      0xc0008000      0x1d0
> |                 0xc0008000                _text = .
> |  *(.head.text)
> |  .head.text     0xc0008000      0x1d0 arch/arm/kernel/head.o
> |                 0xc0008000                stext
> | 
> | .text           0xc0008200   0x2d78d0
> |                 0xc0008200                _stext = .
> |                 0xc0008200                __exception_text_start = .
> |  *(.exception.text)
> |  .exception.text
> | ...
>
> As you can see, all the discarded sections are grouped together - and
> as a result of it being the first output section, they all appear before
> any other section.
>
> The result is that not only is the unwind information discarded (as
> intended), but also the .exit.text, despite us wanting to have the
> .exit.text preserved.
>
> We can't move the unwind information elsewhere, because it'll then be
> included even when we do actually discard the .exit.text (and similar)
> sections.
>
> The only solution that I can think of is to stop using this
> asm-generic/vmlinux.lds.S and write our own fully conditionalized
> linker script, ensuring that no input section is mentioned more than
> once in the output sections.
>
> Or someone sorts out the asm-generic/vmlinux.lds.S stuff to actually
> conform to the linker manual, and stop relying on implementation defined
> behaviour of the linker - again by having it fully conditionalized.
>

Now that the generic bug patch has been merged to linux-next a lot of
ARM builds are failing like so:

`.exit.text' referenced in section `__bug_table' of net/built-in.o: defined in discarded section `.exit.text' of net/built-in.o
`.exit.text' referenced in section `__bug_table' of net/built-in.o: defined in discarded section `.exit.text' of net/built-in.o
`.exit.text' referenced in section `__bug_table' of net/built-in.o: defined in discarded section `.exit.text' of net/built-in.o


I'm not sure what the fix is here. Hopefully we can figure out how to
keep using the asm-generic stuff.

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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