PERFORCE change 126859 for review

[John Baldwin]

On Thursday 27 September 2007 05:57:57 am John Birrell wrote:
> http://perforce.freebsd.org/chv.cgi?CH=126859
> 
> Change 126859 by jb at jb_freebsd1 on 2007/09/27 09:57:33
> 
> 	To avoid infecting the GENERIC kernel with Sun's CDDL, the
> 	additional fields that would have just been added to
> 	'struct thread' have to be made opaque.
> 	
> 	The easiest way I can think of to do this is to allocate
> 	extra space when the thread structure is allocated. The
> 	value of KDTRACE_THREAD_SIZE is the extra space required.
> 	It is then left to the DTrace modules to do the pointer
> 	arithmetic to offset 'struct thread *td' to point to the
> 	DTrace-specific thread data.
> 	
> 	I get conflicting opinions about what is protected by
> 	copyright and what is not. None of these opinions come
> 	from lawyers. However, even if they were from lawyers
> 	they would still just be opinions which have no legal
> 	precedent.
> 	
> 	I am accused by one person from Sun of "effectively making
> 	up [my] own law, and then complaining that [I'm]
> 	forcing [myself] to follow it".
> 	
> 	I am also accused of making Sun a "straw man" and that I'm
> 	"more interested in having the problem than solving it".
> 	
> 	I have asked Sun for legal clarity. It's notable that,
> 	of all the emails I've received from Sun, exactly ZERO
> 	have come from anyone with any legal responsibility in
> 	Sun.
> 	
> 	So this is the way forward... obfuscate the bloody code
> 	so that it doesn't directly reference any CDDL code.
> 	
> 	And life goes on.

How about doing something like this:

	struct thread {
		...
		char td_dtrace_data[KDTRACE_THREAD_SIZE];
	};

Then you don't have to allocate it separately or maintain specific offsets
variables, etc. and using it can become easier.  In the dtrace code itself
you could have:

	struct thread_dtrace {
		/* per-thread dtrace fields */
		void	*foo;
		int	bar;
		struct blah baz;
	};

	CTASSERT(sizeof(struct thread_dtrace) <= KDTRACE_THREAD_SIZE);

	...

	int
	some_dtrace_func(struct thread *td, ...)
	{
		struct thread_dtrace *dt;

		dt = (struct thread_dtrace *)td->td_dtrace_data;
		...
	}

-- 
John Baldwin