'''Why do a lot of #defines in the kernel use do { ... } while(0)?'''

There are a couple of reasons: 
 * ''(from Dave Miller)'' Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).
 * ''(from Dave Miller)'' It gives you a basic block in which to declare local variables.
 * ''(from Ben Collins)'' It allows you to use more complex macros in conditional code. Imagine a macro of several lines of code like:
 {{{
#define FOO(x) \
        printf("arg is %s\n", x); \
        do_something_useful(x);
}}}
 Now imagine using it like:
 {{{
if (blah == 2)
        FOO(blah);
}}}
 This interprets to:
 {{{
if (blah == 2)
        printf("arg is %s\n", blah);
        do_something_useful(blah);;
}}}
 As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block like `do { ... } while(0)`, you would get this:
 {{{
if (blah == 2)
        do {
                printf("arg is %s\n", blah);
                do_something_useful(blah);
        } while (0);
}}}
 Which is exactly what you want.
 * ''(from Per Persson)'' As both Miller and Collins point out, you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:
 {{{
#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
}}}
 However that wouldn't work in some cases. The following code is meant to be an `if`-statement with two branches:
 {{{
if (x > y)
        exch(x,y);          // Branch 1
else  
        do_something();     // Branch 2
}}}
 But it would be interpreted as an `if`-statement with only one branch:
 {{{
if (x > y) {                // Single-branch if-statement!!!
        int tmp;            // The one and only branch consists
        tmp = x;            // of the block.
        x = y;
        y = tmp;
}
;                           // empty statement
else                        // ERROR!!! "parse error before else"
        do_something();
}}}
 The problem is the semi-colon (`;`) coming directly after the block.
 The solution for this is to sandwich the block between `do` and `while (0)`. Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler.
 Our `if`-statement now becomes:
 {{{
if (x > y)
        do {
                int tmp;
                tmp = x;
                x = y;
                y = tmp;
        } while(0);
else
        do_something();
}}}