Although I've used Google Code search for finding out algos, implementations, generic API's for some typical scenarios and areas of interests in development, but it works much faster than tracing it locally for Open source code, even the kernel code and other system level implementation like in the above case.
What else, you get some really cool re-implementation of Linux code; some with improvements and customizations that can be useful.
The search results that we got also displayed implementations for some important related commands like ifconfig, iproute etc.
But then there's lot more to GoogleCode Search than searching just linux source code.
For similar code searches I tried koders
The one that gives a close fight to GoogleCodeSearch is Krugle.
With a search result providing different levels of implementations, modules to look for on this topic and one that also offers technical help, white papers and other related links on the searched context, this one sure is a better place to code-search. To promote it further, these guys offer an enterprise version that can sit on your desktop and provide code-searches both for internal and open source indexed code.
Other code search engine that can be referred:
Oreilly, Ucodit
I found another review for these search engines at: 5-great-code-search-engines
Code search can be fun too!