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I have just watched "Top Five" played by Chris Rock. In this movie, there is a scene where Chris Rock saw a white actor and here is the conversation:

The actor (played by white male actor): Hey, Alfred Alen

Alfred Alen (played by Chris Rock): Hey, white bread, happy birthday!

That is in the movie, but is calling a white person "white bread" considered racist in everyday conversation?

"whitebread" is a wry or gently mocking expression originating in the African American communities of the United States which is most often used to describe a "white" person the user is on friendly terms with and the expression is not intended to offend or insult. "wry" adjective: 1. using or expressing dry, especially mocking, humor. synonyms: ironic, sardonic, satirical, mocking. see, google.com "wry" Link

A slightly more common but equivalent expression would be to refer to a "white" person as "Opie", as in Opie Taylor from Andy Griffith fame.

Belonging to the class of bland, clean-cut, middle-of-the-road suburbanite breeders. The Cleavers from the old TV show "Leave It To Beaver" are a familiar archetype of whitebread culture. see, The Urban Dictionary “white bread” Link

Most of the online dictionaries I've consulted don't seem to get the quality of "whitebread" I've included and refer to only the bland, insipid qualities which really connote to "lack of soul". But, in my personal experience, this is one of those, admittedly rare, occasions when the dictionary definitions are missing the more subtle nuances of the term - reporting as outsiders looking in - that said, "location, location, location!"

My understanding is that it is of American origin, and has to do with the mundane and uninteresting lifestyle which may be perceived of the white middle-class.

white bread

2. bland; conventional.
Random House Unabridged Dictionary

white bread
North American
informal
Blandly conventional
in a way that is regarded as characteristic of the white middle
classes.
Oxford Living Dictionaries

Check out the example sentences to get a feel of how it can be used.

Here is an explanation of the meaning at The Phrase Finder.

Pertaining to the US white middle classes.
The Phrase Finder

Billy Joel's famous 1983 song Uptown Girl features the phrase, where he talks about an uptown girl living in her white-bread world, here are the lyrics:
Link to lyrics in video

I'm gonna try for an uptown girl
She's been living in her white bread
world
As long as anyone with hot blood can
And now she's looking for a
downtown man
That's what I am
metrolyrics

It's interesting if you watch the video, a group of grease monkeys singing about a girl in her white-bread world who are working in what definitely looks like a rough part of the neighbourhood. You can obviously get the meaning intended by white bread here, though let's not forget this was 1983.

Here are two excerpts from an article from Huffington Post titled "How White Bread Became White Trash"

In truth, though, the pejorative “white bread” had earlier
antecedents. In the diverse ranks of 1960s counterculture activism,
the phrase had already come to signify everything bland, homogenous,
suburban, chemical, and corporate—everything that the counterculture
hoped to upend.

But, by the early 1980s, another usage had emerged. In this case,
“white bread” signified almost the opposite: not dull affluent
suburbia, but white trash. “White bread,” like broken-down trailers,
came to denote poverty of a white and rural kind—the world described
by residents of TV’s South Park as “a quiet, little, white-bread,
Podunk, white trash, redneck corner of the U.S.A.”
Huffington Post article

Also, you can look through this short discussion among users at The Phrase Finder giving their understanding of the meaning of white bread.

Thus "white bread" describes the bourgeoisie not only because its
members are racially white but also because they subsist on highly
processed commercial foods.
Phrase Finder discussion
NB: This is just the opinion of some internet user.

It's hard to tell whether today its primary meaning is related to white middle-class lifestyle, dull lifestyle, or as the Huffington Post article put it, to denote "white trash". There's no doubt that it can be used in a primarily racial way, as seen in your original quote. This can be said in light-hearted playfulness or as a hateful slur, depending on context.

I'm white (at least, that's the check box that applies to me on applications, etc.). If I had a good friend who teasingly called me "white bread" I guess that might be okay--although it would not feel friendly. But, aside from that situation, if someone calls me "white bread" my response would be "what do you know?" As the term is often used, it is offensive. It may not be at the level of the "n word," but it stereotypes, is dismissive of the person, and is generally meant in at least a mildly derogatory way. I have never heard it used as a compliment. Also, ironically, many people who use "white bread" as a demeaning label actually enjoy eating white bread sandwiches, etc.!

In British English 'white bread' is not commonly used in everyday conversation outside of its context of actual white bread. So, in terms of directly answering your question is saying "white bread" considered as racist in everyday conversation? I would say probably not as the term is seldom used to describe a white person. (I would imagine that most people would believe one would literally be talking about white bread.)

However, where the context is sufficient for it to be clear that white bread is being used to describe a person's skin colour then this is racist.

The terms' origins are from the UK but not originated in the UK; it is a life style that is middle of the road, boring and or uppercrust. Italians say manga cake, as reference to that culture.