Re: [Qemu-devel] [PATCH V3 01/11] atomic: introduce atomic operations

[Paolo Bonzini]

Il 13/09/2012 10:14, Avi Kivity ha scritto:
>>>> >>> +static inline void atomic_set(Atomic *v, int i)
>>>> >>> +{
>>>> >>> +    v->counter = i;
>>>> >>> +}
>>>> >>> +
>>>> >>> +static inline int atomic_read(Atomic *v)
>>>> >>> +{
>>>> >>> +    return v->counter;
>>>> >>> +}
>>>> >>>
>>> >>
>>> >> So these two operations don't get mangled by the optimizer.
>>> >>
>> > Browsing linux code and reading lkml, find some similar material. But
>> > they have moved volatile from ->counter to function - atomic_read().
>> > As to atomic_read(), I think it need to prevent optimizer from
>> > refetching issue, but as to atomic_set(), do we need ?
> I think so, to prevent reordering.

Agreed.  Alternatively, stick a barrier() before and after the
assignment and read.

But I don't really see the point in wrapping atomically-accessed
variables in a struct.  Are we going to add a variant for long, a
variant for pointers, etc.?

I already proposed my version of this at
http://github.com/bonzini/qemu/commit/1b439343.

Paolo