sdt deadlock

[Mark Johnston]

On Thu, Aug 29, 2013 at 02:01:06PM +0300, Andriy Gapon wrote:
> 
> We've just experienced a deadlock between dtrace_lock and sdt_sx.
> 
> Mark, could you please take a look at the following debugging data if you have
> some time?  Does it look like something that could be fixed by your recent big
> change to the related code?

Hi Andriy,

It will definitely be fixed by my recent changes - the sdt rewrite
removed sdt_sx. :)

Now the sdt code relies only on the kld lock and the DTrace locks, and I
was careful to avoid LORs. In particular, the kld lock is always taken
before any DTrace locks.

I also just did a little test where I was loading and unloading a file
containing SDT probe definitions in a loop, and running DTrace scripts
in another terminal. I didn't have any problems, so I'm fairly confident
that this particular interaction works properly now.