I'm assuming you're asking when BSD grep has to be released under the GPL? As the BSD license is slightly more restrictive than the GPL, I imagine that the GPL would have to be adopted if there was significant code usage, which there doesn't appear to be in this case.
The case where code is used would make it clear cut that you need to GPL it. You can relicense BSD (without advertisement clause) to GPL but not the other way around.
My doubt is at what point would you need to GPL it even though you haven't actually reused any code, just described the GPL code to someone who then reimplemented it into the BSD grep. This case still seems pretty benign as only general algorithms and approaches are shared but it made me consider where the line might be.
I'd be surprised if the GNU people claimed use of algorithms was sufficient to warrant the forced adoption of GPL, as that goes against everything they stand for. Rather, I suspect a direct transfer of code would be the minimum necessary condition for the FSF to raise a stink.
This case is clearly not over the line but it seems to me that the minimum can't be a direct use of code. If someone did enough documentation of the architecture and algorithms used in the Linux kernel and handed that off to someone else who reimplemented I assume it would be hard to argue it wasn't a derived work.
When LWN does a description of how the VM or scheduler works they are effectively describing the code. But maybe that's the same as a research paper on algorithms where reimplementation is clearly possible. Maybe for it to be a derived work you'd have to describe the code so exactly you'd be effectively translating it in whole to another form.
I anticipated that response, but was on my way out the door and had no time to clarify.
From the point of view of the FSF, the BSD license is more restrictive, as it grants fewer freedoms to users. From the BSD point of view, GPL grants fewer freedoms to developers, and is thus more restrictive.
In the context of the question asked, I was approaching it from the POV of the GPL's authors, given that the question was whether the BSD code would have to adopt the GPL license. I apologize for my inarticulateness.
