Notice that this means they are resolved in opposite order.
The last IO set/clear to be executed wins (it's action has visible effect).
For tags, the first action executed wins (is not conflicting).

Previously, the valid counter would increase on the last cycle of a strobe.
Thus, if it were stalled forever, the counter would not increase.  That
posed a problem for software driven queues which need the counter to
increase in order to receive an MSI to pop the queue via lowering stall.