Instead of relying on magic memory barriers, document the pattern that
is being used. It is the one based on Dekker's algorithm, and in this
case it is embodied as follows:
enqueue request; sleeping = true;
smp_mb(); smp_mb();
if (sleeping) kick(); if (!have a request) yield();
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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