[RFC] reworking FreeBSD's SDT implementation

[Andriy Gapon]

on 27/07/2013 05:26 Mark Johnston said the following:
> On Thu, Jul 25, 2013 at 06:16:32PM +0300, Andriy Gapon wrote:
>> on 22/07/2013 05:28 markj at freebsd.org said the following:
>>> http://people.freebsd.org/~markj/patches/sdt-module-info/20130721-sdt-module-info.diff
>>
>> Mark,
>>
>> this is a minor suggestion only partially related to your patch.
>> I think that it would be nice if module loading and unloading events were posted
>> via EVENTHANDLER(9) mechanism.  Then instead of introducing yet more DTrace
>> related hooks in the kernel code, DTrace modules could just subscribe to those
>> events.  Also, those events could be potentially useful to other consumers
>> beyond DTrace.
>> What do you think?
> 
> Hm, now that I look at this, I'm not sure if it can work. The unload
> hooks need to be able to veto a module unload in the case that one of
> its probes is enabled. This is done by checking whether lf->nenabled > 0,
> and it needs to be done with the dtrace lock held to prevent races.
> 
> I've done this by having the unload hooks return a non-zero value if
> there are probes enabled, but EVENTHANDLER(9) doesn't give me a way to
> look at a handler's return value. Do you see a way to get around this?

Hmm, I didn't think about this problem in advance...
Having looked around I think that it should be possible to handle this situation
in a way similar to watchdog_list.  watchdog(9) documents how that works.  Of
course, all handlers will have to be careful to not override error if it's
already set.


-- 
Andriy Gapon