Re: [PATCH 03/14] ARM: v6k: remove CPU_32v6K dependencies in asm/spinlock.h

[Dave Martin <dave.martin@linaro.org>]

On Wed, Jan 26, 2011 at 3:52 PM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Wed, Jan 26, 2011 at 10:42:41AM -0500, Nicolas Pitre wrote:
>> On Wed, 26 Jan 2011, Dave Martin wrote:
>> > Alternatively, I wonder .alt.smp.init could make weak references
>> > instead of normal references: then if the referenced section is
>> > discarded, the reference will revert to 0 and the fixup code can
>> > ignore it (at the expense of wasting a bit of space in the fixup table
>> > ... but it's no worse than the current situation).  From
>> > experimentation, it seems this doesn't work if the referenced symbol
>> > is defined in the same file, since the linker resolves the reference
>> > before discarding sections, but maybe there's a way around it...
>>
>> That's strange.  How can the linker resolve the reference if the
>> referenced symbol is not in the same section as the reference source?
>> What happens when the final link completes?
>
> $ cat t.s
>        .section ".exit.text", "ax", %progbits
>        mov     r0, r0
> a:      mov     r0, r0
>
>        .pushsection ".alt.smp.fixup", "a"
>        .weak   a
>        .long   a
>        mov     r0, r1
>        .popsection
> $ arm-linux-as -o t.o t.s
> $ arm-linux-nm t.o
> 00000004 W a
> $ arm-linux-objdump -Dr t.o
>
> Disassembly of section .exit.text:
>
> 00000000 <a-0x4>:
>   0:   e1a00000        nop                     (mov r0,r0)
>
> 00000004 <a>:
>   4:   e1a00000        .word   0xe1a00000
>
> Disassembly of section .alt.smp.fixup:
>
> 00000000 <.alt.smp.fixup>:
>   0:   00000000        .word   0x00000000
>                        0: R_ARM_ABS32  a
>   4:   e1a00001        mov     r0, r1
>
> which when linked with:
>
> SECTIONS
> {
>        .alt.smp.init : { *(.alt.smp.init) }
>
>        /DISCARD/ : { *(.exit.text) }
> }
>
> $ arm-linux-ld -T t.lds -r -o t.oo t.o
> `a' referenced in section `.alt.smp.fixup' of t.o: defined in discarded section `.exit.text' of t.o
>
> So I don't think weak symbols work like we want them to.
>

That was the conclusion I came to also ... the linker seems to resolve
references in each object before discarding sections, so the weak
reference has already become concrete and section discard breaks it.

---Dave
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