The current author of the mutex code has no right to relicense it. Period. It's a derived work, and I (as a copyright owner of the original code) have a MONOPOLY on what can be done with ALL derived works. And the only thing I have authorized before getting out of touch is to license them under the GPL.
IMHO, the thing stretches even more: as lots of things in the rest of the kernel are derived works from my work, those can't be relicensed too. More: imagine, graphically, if you "yank" the current mutex implementation from the kernel. You would have to write (clean-room) something that fits in the "void/hole" left behind. But to do this, you would have probably to use other code that is derived on my code... gotcha! Your code now is derived from mine and you have no other option to distribute it than doing it under the terms of the GPL.
Do you work for SCO ? This is exactly what they try claim, you know. The truth is that the only thing you can compare is your code before Tibet expedition and current code in kernel. And you can not appeal to "ideas": you need something textually similar (trivial modifications like variables renaming is Ok, complete rewrite is not). It's difference between copyright and patent... It's not so hard to create clean-room implementation if really needed: you ask some fellow to write API starting from point where mutex code is still in kernel and then you ask other person to reimplement API. What you'll get if API was without code snippets and without parts of you code big enough to be copyrightable is new code without any relation to original mutex implementation.
The real problem lies not in bare need to reimplement something but in sheer number of things included in kernel...
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