Re: [RFC v1 06/14] bus1: util - queue utility library

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 26, 2016 at 09:18:02PM +0200, David Herrmann wrote:

> A bus1 message queue is a FIFO, i.e., messages are linearly ordered by
> the time they were sent. Moreover, atomic delivery of messages to
> multiple queues are supported, without any global synchronization, i.e.,
> the order of message delivery is consistent across queues.
> 
> Messages can be destined for multiple queues, hence, we need to be
> careful that all queues get a consistent order of incoming messages.

So I read that to mean that if A and B both send a multi-cast message to
C and D, the messages will appear in the same order for both C and D.

Why is this important? It seem that this multi-cast ordering generates
much of the complexity of this patch while this Changelog fails to
explain why this is a desired property.


> We
> define the concept of `global order' to provide a basic set of
> guarantees. This global order is a partial order on the set of all
> messages. The order is defined as:
> 
> 1) If a message B was queued *after* a message A, then: A < B
> 
> 2) If a message B was queued *after* a message A was dequeued,
>    then: A < B
> 
> 3) If a message B was dequeued *after* a message A on the same queue,
>    then: A < B
> 
>     (Note: Causality is honored. `after' and `before' do not refer to
>      the same task, nor the same queue, but rather any kind of
>      synchronization between the two operations.)
> 
> The queue object implements this global order in a lockless fashion. It
> solely relies on a distributed clock on each queue. Each message to be
> sent causes a clock tick on the local clock and on all destination
> clocks. Furthermore, all clocks are synchronized, meaning they're
> fast-forwarded in case they're behind the highest of all participating
> peers. No global state tracking is involved.

Yet the code does compares on more than just timestamps. Why are these
secondary (and even tertiary) ordering required?