[RFC] reworking FreeBSD's SDT implementation

[George Neville-Neil]

On Jul 4, 2013, at 23:25 , Mark Johnston <markj at freebsd.org> wrote:

> On Wed, Jul 03, 2013 at 12:10:23AM -0400, Mark Johnston wrote:
>> Hello,
>> 
>> There are a few problems with the way SDT is currently implemented in
>> FreeBSD. First, the DTrace framework isn't notified when modules are
>> unloaded, so any probes created by these modules are never destroyed
>> (this problem isn't specific to SDT though, FBT probes have the same
>> problem). Second, there is currently nothing preventing one from
>> unloading a module while some of its SDT probes are enabled; doing this
>> will generally cause a panic. Finally, providers are "tied" to modules
>> in the sense that dtrace_unregister() is called on each provider
>> declared in a module when that module is unloaded. This is inflexible -
>> probes already have a "module" field to indicate which module they're
>> defined in, and it would restrict the implementation of, say, a
>> hypothetical GEOM or netgraph provider, which would probably contain
>> some common probes for each GEOM or netgraph module. Plus a panic will
>> occur if a probe from one module is enabled and a second module
>> declaring the provider of the probe is unloaded.
>> 
>> I have a patch at [1] which tries to solve all of these problems. It
>> more or less completely reworks FreeBSD's SDT implementation (currently
>> contained in kern/kern_sdt.c and cddl/dev/sdt/sdt.c) and changes a
>> number of things:
>> 
>> [...]
>> 
>> I'm also writing a man page for the SDT macros so that there's some
>> guidance on how to create SDT probes and providers.
> 
> I've copied a draft SDT man page below. Any
> suggestions/corrections/comments/criticisms are most welcome.
> 
> I would also like to have man pages for individual providers, but this
> is a start. :)

Hello Mark,

Sorry to take to long to reply.  I think your approach to SDTs is sound and we should
move forward with that, in particular as you say it brings us closer to the implementation
ideas in Solaris.  I will try to test your patch on HEAD today and let you know if I have
any problems with it.  In the meantime feel free to test and commit it on your own,
don't wait for me.  The man page also looks good.

Best,
George