[DTrace-devel] [PATCH 3/3] Verify the activity state before calling each clause

[Kris Van Hees]

On Wed, Sep 30, 2020 at 09:47:54AM -0700, Eugene Loh wrote:
> Reviewed-by: Eugene Loh <eugene.loh at oracle.com>
> with the usual minor comment/question:
> 
> On 09/29/2020 04:14 PM, Kris Van Hees wrote:
> > When a clause contains an exit() action, processing of all other
> > clauses (even for the same probe firing) must not be executed
> > anymore.  The activity state check therefore needs to not just happen
> > in the trampoline, but also before every clause within the program.
> 
> Arguably, the activity check is no longer needed in the trampoline at 
> all, right?  (Leaving it in is a minor implementation choice.  It saves 
> the execution of a couple of instructions if probes fire at the end of a 
> run.)  Maybe change the last sentence to:
> 
> "The activity state check therefore needs to happen before every clause 
> within the program."

Actually, it isn't left in the trampoline jsut to save a few instructions
from being executed since that gain is minimal).  It is there because not
having it could result in rather unpredictable behaviour in which clauses
end up being executed for a probe if the state is set to ACTIVE *after* a
probe fires but before we've iterated over all its clauses.  If a given
probe has n clauses, you could end up with clauses [0 .. i] being skipped
while clauses [i .. n] get executed.  Given the way clauses are executed
in a strict first to last order and script writers depend on that), having
only some of the clauses be executed in the beginning can lead to strange
results.

We have a similar situation when probing ends, where only [0 .. i] might be
executed, and not [i .. n] but that is specifically implied by the semantics
of the exit() action.