Writes, smp_wmb(), and transitivity?

[Linus Torvalds]

On Mon, Feb 15, 2016 at 9:58 AM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> 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