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>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200811101647.12258.jhb>