More generally, Ack is great for doing "recursive grep's" on source code. By default it does a pretty good job of only listing matches from filetypes you're likely to care about.

I think most gitsters (this is the kernel, afterall) would say:

    $ git grep foo -- '*.c'

cscope (kscope if you want GUI) will be easier if you have a copy of kernel. lxr is useful when you do not have a local copy of kernel.