Comment 4 for bug 503185

Hi,

I noticed today that BLCR is failing to build on armel on Ubuntu. This
problem is reported in [1]. It would seem that ARMv7 no longer
includes swp/swpne instructions. It's relatively trivial to
conditionally replace this with appropriate ldrex/strex instructions
instead though. Before I produce a patch though I have a few
questions:

1) CR_KCODE___kuser_cmpxchg clearly isn't defined at build time here.
The test doesn't get run because we're only building userspace parts
at this point. Is that correct? Are we always expecting to be in the
#else oflibcr/arch/arm/cr_atomic.h when building userspace bits? I
can't help but feel I've missed something in my reading of this
somehow though.
2) Is a 2nd alternative for
cri_atomic_inc/cri_atomic_dec_and_test/cri_cmp_swap the correct way to
fix this? The comments in the source seem to suggest that using
ldrex/strex would be correct and better where they exist. They also
imply that we shouldn be using __kernel_cmpxchg on newer kernels
anyway though.

Thanks,
Alan

[1] https://bugs.launchpad.net/ubuntu/+source/blcr/+bug/503185