Am 24.11.2014 um 20:42 schrieb Paul E. McKenney:
> On Mon, Nov 24, 2014 at 11:14:42AM -0800, Linus Torvalds wrote:
>> On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger
>> <borntraeger@xxxxxxxxxx> wrote:
>>>
>>> Looks really nice, but does not work with ACCESS_ONCE is on the left-hand side:
>>
>> Oh, I forgot about that. And that was indeed why I had done that whole
>> helper macro originally, with ACCESS_ONCE() itself just being the
>> dereference of the pointer.
>
> OK, how about the following?
>
> It complains if the variable is too large, for example, long long on
> 32-bit systems or large structures. It is OK loading from and storing
> to small structures as well, which I am having a hard time thinking of
> as a disadvantage.
Well, the motivation for this series was that gcc 4.6 and 4.7 might ignore volatile for
such a case, see the original thread and this data structure
union ipte_control {
unsigned long val;
struct {
unsigned long k : 1;
unsigned long kh : 31;
unsigned long kg : 32;
};
};
> ------------------------------------------------------------------------
>
> #define get_scalar_volatile_pointer(x) ({ \
> volatile typeof(x) *__vp = &(x); \
> BUILD_BUG_ON(sizeof(*__vp) != sizeof(char) && \
> sizeof(*__vp) != sizeof(short) && \
> sizeof(*__vp) != sizeof(int) && \
> sizeof(*__vp) != sizeof(long)); \
> __vp; })
> #define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))
>
This gives also several compiler errors when accessing u64 on a 32bit system. This is expected, but more widespread than expected - ouch.
Christian
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