NICK CANNON & EMINEM GO AT IT OVER MARIAH CAREY!

Who is probably the only person in the world who can erase a smile off the always good-natured face of Nick Cannon? One guess. He's known as rap's Mr. No-Holds-Barred: Eminem.

Cannon has taken major offense to a leaked record from Em's Relapse, "Bagpipes From Baghdad," in which Em raps about taking Mariah Carey "back" from Nick Cannon. (Carey has denied Eminem's claims that they dated for six months.)

"Nick Cannon better back the f--- up," Em warns the actor/musician on the track. "I'm not playing, I want her back, you punk."

Later in the song, Em leaves a parting salvo: "Nick Cannon, you pr---, I wish you luck with that f---in' whore."

Cannon had a millions things to say in response on his blog on Friday.

"A mediocre (at best) Eminem record that sounds like it was written in 2001," Cannon blogged. "At first, I thought it was old material that had been dug up from when dude 'fantasized' about having a pretend fling with Mariah. ... But all of a sudden I hear my name in the verse! My first reaction was like, 'This is his new sh--? Wow, that's too bad.' "

Cannon said that Eminem's insults amounted to "racist bigotry." "She has had enough difficulty in her life dealing with racial and ethnicity issues," he wrote. "Believe it or not, Mariah is the same racial mixture as our beloved president, Barack Obama. (Black Father + White Mother= BLACK)

"Maybe I'm going too far, but I thought we got past the days where white men could spew vulgar obscenities at our beautiful queens and get away with it," Cannon continued. "What's next? Are we going to let this trash say something horrible about our lovely first lady, Mrs. Michelle Obama? Or would Marshall have talked sideways out of his neck like this about Oprah Winfrey? This act of racist bigotry cannot go unnoticed. Calling my wife a 'c---' and a 'whore' is way worse than anything Don Imus could have ever said. So trust, repercussions will be served. Anybody got Al Sharpton's number?! LOL Let's not forget about Eminem's amateur mixtape rants of calling African Americans 'ni----' and how he hates 'black bi---es.' "

The actor basically vowed revenge on Slim Shady, joking that he would begin rapping again and threatening comedic retribution.

"So, Miss Marshall, I'm going to make you wish you never spoke my name and regret the ungodly things you said about my wife," Cannon continued. "This is going to be fun! It's however you want it! Just remember, you did this to yourself! Your legacy has now been tainted from this day forth! You will now be known as the rapper who lost to corny-ass Nick Cannon!"