Having thought about this some more, I've realised that this also doesn't work
if when one thread tries to alter another thread's creds that other thread's
cred struct is currently overridden.
So, I think this has to be the best way to do things. It's the cleanest,
certainly and probably has the lowest overhead overall.
This also means that I don't need that horrible __current_cred() function that
Linus so objects to (I do too).
David
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