Re: [PATCH 1/2] kbuild: provide include/asm/asm-prototypes.h for ARM

[Russell King - ARM Linux]

On Sun, Nov 20, 2016 at 10:32:50AM -0800, Linus Torvalds wrote:
> On Sun, Nov 20, 2016 at 5:21 AM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxx> wrote:
> > On Tue, Oct 25, 2016 at 07:32:00PM +1100, Nicholas Piggin wrote:
> >>
> >> Michal, what's your thoughts? If you merge my patch 2/2 and skip 1/2, it
> >> should not give any new build warnings or errors, so then arch patches can
> >> go via arch trees. 1/2 could go in after everyone is up to date.
> >
> > So what's the conclusion on this?  I've just had a failure due to
> > CONFIG_TRIM_UNUSED_KSYMS reported on ARM, and it looks like (at
> > least some of) patch 1 could resolve it.
> 
> Hmm. I've got
> 
>   cc6acc11cad1 kbuild: be more careful about matching preprocessed asm
> ___EXPORT_SYMBOL
>   4efca4ed05cb kbuild: modversions for EXPORT_SYMBOL() for asm
> 
> in my tree. Is that sufficient, or do we still have issues?

Hmm, those seem to have gone in during the last week, so I haven't
tested it yet (build running, but it'll take a while).  However, I
don't think they'll solve _this_ problem.

Some of the issue here is that we use a mixture of assembly macros
and preprocessor for the ARM bitops - the ARM bitops are created
with an assembly macro which contains some pre-processor expanded
macros (eg, EXPORT_SYMBOL()).

This means that the actual symbol being exported is not known to
the preprocessor, so doing the "__is_defined(__KSYM_##sym)" inside
"EXPORT_SYMBOL(\name)" becomes "__is_defined(__KSYM_\name)" to the
preprocessor.  As "__KSYM_\name" is never defined, it always comes
out as zero, hence we always use __cond_export_sym_0, which omits
the symbol export from the assembly macro definition:

 .macro bitop, name, instr
.globl \name ; .align 0 ; \name:

...

.type \name, %function; .size \name, .-\name

 .endm

In other words, using preprocessor macros inside an assembly macro
may not work as expected, and now leads to config-specific failures.

-- 
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