That whole "Paul McCartney playing with Nirvana" thing was also to promote Dave Grohl's new movie

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Turns out that whole Paul McCartney/Nirvana performance at this week’s 12.12.12 concert had a another motive beyond disaster relief and “let’s just get together and jam, man!” The song the group performed, “Cut Me Some Slack,” just so happens to appear on the soundtrack for Dave Grohl’s new documentary, Sound City, about the legendary California recording studio of the same name. While the film won’t be released until Feb. 1, followed by the soundtrack on March 12, the album’s pre-sale began today, and the aforementioned single is—gasp!—conveniently available for purchase right this very second.