Date: Wed, 18 Jan 2012 10:09:23 -0500 From: John Baldwin <jhb@freebsd.org> To: davidxu@freebsd.org Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r230201 - head/lib/libc/gen Message-ID: <201201181009.23221.jhb@freebsd.org> In-Reply-To: <4F1629D5.4020605@gmail.com> References: <201201160615.q0G6FE9r019542@svn.freebsd.org> <201201170957.47718.jhb@freebsd.org> <4F1629D5.4020605@gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201201181009.23221.jhb>