If a group of CPUs all do concurrent stores to a single variable,
the series of values seen by all CPUs will be consistent with at
least one global ordering.
For example, in the series of accesses shown in
Figure ,
CPU 1 sees the sequence {1,2},
CPU 2 sees the sequence {2},
CPU 3 sees the sequence {3,2},
and
CPU 4 sees the sequence {4,2}.
This is consistent with the global sequence {3,1,4,2},
but also with all five of the other sequence of these four numbers that end
in ``2''.
Had the CPUs used atomic operations (such as the Linux kernel's atomic_inc_return() primitive) rather than simple stores of unique values, their observations would be guaranteed to determine a single globally consistent sequence of values.