On Tuesday, January 17, 2012 9:09:25 pm David Xu wrote: > On 2012/1/17 22:57, John Baldwin wrote: > > On Monday, January 16, 2012 1:15:14 am David Xu wrote: > >> Author: davidxu > >> Date: Mon Jan 16 06:15:14 2012 > >> New Revision: 230201 > >> URL: http://svn.freebsd.org/changeset/base/230201 > >> > >> Log: > >> Insert read memory barriers. > > I think using atomic_load_acq() on sem->nwaiters would be clearer as it > > would > > indicate which variable you need to ensure is read after other operations. > > In > > general I think raw rmb/wmb usage should be avoided when possible as it is > > does not describe the programmer's intent as well. > > > Yes, I had considered that I may use atomic_load_acq(), but at that time, > I thought it emits a bus locking, right ? so I just picked up rmb() which > only affects current cpu. maybe atomic_load_acq() does same thing with > rmb() ? > it is still unclear to me.
atomic_load_acq() is the same as rmb(). Right now it uses a locked instruction on amd64, but it could easily switch to lfence/sfence instead. I had patches to do that but I think bde@ had done some benchmarks that showed that change made no difference. -- John Baldwin _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
