Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

[Linus Torvalds]

On Mon, Nov 24, 2014 at 10:02 AM, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> If the goal is to catch non-scalar users, the following is shorter:
> #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x))

Me likey. It probably works well in practice, although I think

 - the "(typeof(x))0)" seems unnecessary and wrong. Why not just "0"?
The typeof is not just longer, but it is incorrect for pointer types
(you can add 0 to a pointer, but you cannot add two pointers together)

 - it does mean that the resulting type ends up being upgraded to
"int", for the usual C type reasons.

Note that the "upgraded to 'int'" is true with or without the
"(typeof(x))0". If you add two 'char' values, the addition is still
done in 'int'.

Maybe you *meant* that typeof to fix the second problem, like so:

  (typeof(x)) (0 + *(volatile typeof(x) *)&(x))

Hmm? That casts the result of the addition, not the zero.

             Linus
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