If I'm not mistaking, you can put public domain or at least BSD/MIT code into the Linus kernel which is licensed under GPLv2. Other people are then free to use that specific code under that specific license instead of GPLv2.
Also, Linus Torvalds wrote long time ago (before v2.6.12) some simple assembly routines in linux/include/asm-i386/string.h [1] which are public domain:
NO Copyright (C) 1991, 1992 Linus Torvalds,
consider these trivial functions to be PD.