They are using Lamport's algorithm to synchronize the clocks. However, Lamport's approach creates a _partial_ ordering, and to make that a _total_ ordering you need some mechanism to break "ties". For instance, the PID, or whatever.
The catch is that the relationship derived from this arbitrary tie-breaking mechanism has nothing to do with causality, and therefore the total order it imposes is only an artifact of the mechanism chosen.
Finding a tie-breaking mechanism that corresponds to the sending events is, in Lamport's own words, "not trivial".
The reason this works is that the only time we can have a tie is if there can be no causality between the events. I.e., the two sending events happen concurrently: the two ioctl calls overlap in time, so there would be no way for one to have caused the other.
What problem do you see with this?
PS. Neil Brown in the LWN article already conflates "global" and "total" order.
> a global order. In case two events happend concurrently, there can never be
> any inconsistency in which occurred before the other. By way of example,
> consider two peers sending one message each to two different peers, we are
> guaranteed that both the recipient peers receive the two messages in the same
> order, even though the order may be arbitrary.
How could that possibly work?