This is basically a re-request to fix MDEV-14374 as it was described. On target architectures where nothing special is available (not IA-32, AMD64, POWER), the macro UT_RELAX_CPU() would perform a dummy compare-and-swap operation.
According to krunalbauskar, this extra operation is visible in some benchmarks.
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Marko,
On ARM as quoted on channel as per benchmarking switching to simple barrier (vs CAS) helps improve performance.
For other architecture, we should benchmark it and then decide.