Herbert Xu writes:

So the point here is that if you don't mind getting a stale
value from the CPU cache when doing an atomic_read, then
surely you won't mind getting a stale value from the compiler
"cache".

No, that particular argument is bogus, because there is a cache
coherency protocol operating to keep the CPU cache coherent with
stores from other CPUs, but there isn't any such protocol (nor should
there be) for a register used as a "cache".

(Linux requires SMP systems to keep any CPU caches coherent as far as
accesses by other CPUs are concerned.  It doesn't support any SMP
systems that are not cache-coherent as far as CPU accesses are
concerned.  It does support systems with non-cache-coherent DMA.)

Paul.