svn commit: r230201 - head/lib/libc/gen

[John Baldwin]

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