Il 18/06/2013 16:38, Richard Henderson ha scritto:
>>>> +#ifndef atomic_read
>>>> +#define atomic_read(ptr) (*(__typeof__(*ptr) *volatile) (ptr))
>>>> #endif
>>>>
>>>> +#ifndef atomic_set
>>>> +#define atomic_set(ptr, i) ((*(__typeof__(*ptr) *volatile) (ptr)) =
>>>> (i))
>>>> +#endif
>>>
>>> Use
>>>
>>> __atomic_load(..., __ATOMIC_RELAXED)
>>> __atomic_store(..., __ATOMIC_RELAXED)
>>>
>>> ?
>>
>> Same here, I didn't want proliferation of #ifdefs beyond what is actually
>> required.
>
> Not knowing exactly where these might be used within the code base, I'd be
> worried about someone applying them to a uint64_t, somewhere a 32-bit host
> might see it. At which point the above is going to be silently wrong, loaded
> with two 32-bit pieces.
>
> Given that we're not requiring gcc 4.8, and cannot guarantee use of
> __atomic_load, perhaps we ought to do something like
>
> #define atomic_read(ptr) \
> ({ if (sizeof(*(ptr)) > sizeof(ptr)) invalid_atomic_read(); \
> *(__typeof__(*ptr) *volatile) (ptr)); })
>
> which should generate a link error when reading a size we can't guarantee will
> Just Work.
Good idea.
>> The FAQ also has an "important note", however:
>>
>> Important Note: Note that it is important for both threads to access
>> the same volatile variable in order to properly set up the happens-before
>> relationship. It is not the case that everything visible to thread A
>> when it writes volatile field f becomes visible to thread B after it
>> reads volatile field g. The release and acquire have to "match" (i.e.,
>> be performed on the same volatile field) to have the right semantics.
>>
>> Is this final "important note" the difference between ACQ_REL and SEQ_CST?
>
> Yes, exactly.
I still have some confusion, so I sent another follow-up with the exact
differences in the generated code. But in any case, I will augment the
documentation with the text from the FAQ, seems safe.
Paolo
