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

[liu ping fan]

On Thu, Sep 13, 2012 at 4:19 PM, Paolo Bonzini <address@hidden> wrote:
> 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.
>
Yes, the linux just leave this barrier() as caller's choice at the
exact point, not right in atomic_set(). And embed barrier() in
atomic_set() can easy the caller's job.

Thanks and regards,
pingfan

> 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