svn commit: r184759 - user/kmacy/HEAD_fast_multi_xmit/sys/net

[Attilio Rao]

2008/11/10, John Baldwin <jhb at freebsd.org>:
> On Saturday 08 November 2008 11:53:53 am Attilio Rao wrote:
>  > 2008/11/8, David Schultz <das at freebsd.org>:
>  > > On Sat, Nov 08, 2008, Attilio Rao wrote:
>  > >  > Definitively, I'm not sure we need this.
>  > >  > We alredy have memory barriers you could exploit which just require a
>  > >  > "dummy" object.
>  > >  >
>  > >  > For example you could do:
>  > >  > flowtable_pcpu_unlock(struct flowtable *table, uint32_t hash)
>  > >  >  {
>  > >  >
>  > >  >         (void)atomic_load_acq_ptr(&dummy);
>  > >  >         ...
>  > >
>  > >
>  > > Memory barriers are cheaper than atomic ops.
>  >
>  > But this is an atomic op too.
>  >
>  > >  Furthermore, there's different types of memory barriers
>  > >  (store/store, load/store, etc.), not just a generic mb().  Some
>  > >  architectures like sparc64 define all four, but only actually
>  > >  implement the varieties that are useful in improving performance.
>  > >  Take a look at what Solaris has here.
>  > >
>  > >  I'm skeptical of trying to play clever tricks with these things
>  > >  outside of the code that implements synchronization
>  > >  primitives. Memory ordering is very hard to reason about, and we
>  > >  already have a lot of code, e.g., in libthr, that isn't correct
>  > >  under weak memory ordering. Moreover, the compiler can reorder
>  > >  loads and stores, and that just adds a whole new level of pain.
>  >
>  > _acq prefix is intended to not let reordering happening really.
>  > man 9 atomic can explain how the acq and rel memory barriers work.
>
>
> _acq is not a full barrier, it's more of an 'lfence'.  The mb() here is doing
>  more of a _rel barrier ('sfence', etc.).

Sure but the comment is still valid.
I don't see the point of such things when you can implement barriers
trough our atomic_* stuff.

Thanks,
Attilio


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