Re: [Qemu-devel] [PATCH v2 02/12] util: add atomic64

[Murilo Opsfelder Araujo]

On Tue, Sep 11, 2018 at 04:43:04PM -0400, Emilio G. Cota wrote:
> On Tue, Sep 11, 2018 at 05:43:38 -0700, Richard Henderson wrote:
> > On 09/10/2018 04:27 PM, Emilio G. Cota wrote:
> > > +#define GEN_READ(name, type)                    \
> > > +    type name(const type *ptr)                  \
> > > +    {                                           \
> > > +        QemuSpin *lock = addr_to_lock(ptr);     \
> > > +        type ret;                               \
> > > +                                                \
> > > +        qemu_spin_lock(lock);                   \
> > > +        ret = *ptr;                             \
> > > +        qemu_spin_unlock(lock);                 \
> > > +        return ret;                             \
> > > +    }
> > > +
> > > +GEN_READ(atomic_read_i64, int64_t)
> > > +GEN_READ(atomic_read_u64, uint64_t)
> >
> > Is there really a good reason to have two external
> > functions instead of having one of them inline and
> > perform a cast?
>
> Not really. I can do a follow-up patch if you want me to.
>
> > Is this any better than using libatomic?
>
> I didn't think of using libatomic. I just checked the source
> code and it's quite similar:
> - It uses 64 locks instead of 16 ($page_size/$cache_line,
>   but these are hard-coded for POSIX as 4096/64, respectively)
> - We compute the cacheline size and corresponding padding
>   at run-time, which is a little better than the above.
> - The locks are implemented as pthread_mutex instead of
>   spinlocks. I think spinlocks make more sense here because
>   we do not expect huge contention (systems without
>   !CONFIG_ATOMIC64 have few cores); for libatomic it makes
>   sense to use mutexes because it might be used in many-core
>   machines.
>
> So yes, we could have used libatomic. If you feel strongly
> about it, I can give it a shot.

Would allowing user to choose between libatomic or Emilio's implementation
at configure time be a bad idea?

One could pass, for example, --with-libatomic to configure script to use
libatomic instead of using Emilio's implementation, which could be the
default implementation - or vice-versa.

Murilo