s390 is the only architecture in the kernel which makes use of gcc's
atomic builtin functions. Even though I don't see any technical
problem with that right now, remove this code and open-code
compare-and-swap loops again, like every other architecture is doing
it also.
We can switch to a generic implementation when other architectures are
doing that also.
See also https://lwn.net/Articles/586838/ for forther details.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
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