Date: Mon, 10 Nov 2008 16:47:11 -0500 From: John Baldwin <jhb@freebsd.org> To: "Attilio Rao" <attilio@freebsd.org> Cc: src-committers@freebsd.org, Kip Macy <kmacy@freebsd.org>, svn-src-user@freebsd.org Subject: Re: svn commit: r184759 - user/kmacy/HEAD_fast_multi_xmit/sys/net Message-ID: <200811101647.12258.jhb@freebsd.org> In-Reply-To: <3bbf2fe10811080853j3ebf5b2em29ebdd2a7187ff67@mail.gmail.com> References: <200811080202.mA822D0W098283@svn.freebsd.org> <20081108163006.GA5256@zim.MIT.EDU> <3bbf2fe10811080853j3ebf5b2em29ebdd2a7187ff67@mail.gmail.com>
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On Saturday 08 November 2008 11:53:53 am Attilio Rao wrote:
> 2008/11/8, David Schultz <das@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.).
--
John Baldwin
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