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

[Alexei Starovoitov]

On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger
<borntraeger@xxxxxxxxxx> wrote:
>
> Anyone with a new propopal? ;-)                                        ^

one more proposal :)
#define __ACCESS_ONCE(x) ({ typeof(x) __var = 0; (volatile typeof(x) *)&(x); })
#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))

works as lvalue...
the basic idea is the same:
constant zero can be used to initialize any scalar
(including pointers), but unions and structs will fail to compile as:
"error: invalid initializer"

If I'm reading pr58145 gcc bug report correctly, it
miscompiles only structs, so we can let ACCESS_ONCE
to work on unions. Then the following will rejects structs only:
#define __ACCESS_ONCE(x) ({ (typeof(x))0; (volatile typeof(x) *)&(x); })
#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
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