On Mon, Feb 15, 2016 at 9:58 AM, Paul E. McKenney
[off-list ref] wrote:

Two threads:

        int a, b;

        void thread0(void)
        {
                WRITE_ONCE(a, 1);
                smp_wmb();
                WRITE_ONCE(b, 2);
        }

        void thread1(void)
        {
                WRITE_ONCE(b, 1);
                smp_wmb();
                WRITE_ONCE(a, 2);
        }

        /* After all threads have completed and the dust has settled... */

        BUG_ON(a == 1 && b == 1);

So the more I look at that kind of litmus test, the less I think that
we should care, because I can't come up with a scenario in where that
kind of test makes sense. without even a possibility of any causal
relationship between the two, I can't say why we'd ever care about the
ordering of the (independent) writes to the individual variables.

If somebody can make up a causal chain, things differ. But as long as
all the CPU's are just doing locally ordered writes, I don't think we
need to care about a global store ordering.

              Linus