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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 24.11.2014 um 21:29 schrieb Alexei Starovoitov:
> On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger
> <borntraeger@de.ibm.com> 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))

This seems to work. I only had to add an __always_unused to __var.


> 
> 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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