On Thu, Dec 27, 2012 at 4:46 AM, Gleb Smirnoff <gleb...@freebsd.org> wrote: > Attilio, > > On Thu, Dec 27, 2012 at 12:36:58PM +0000, Attilio Rao wrote: > A> Author: attilio > A> Date: Thu Dec 27 12:36:58 2012 > A> New Revision: 244732 > A> URL: http://svnweb.freebsd.org/changeset/base/244732 > A> > A> Log: > A> br_prod_tail and br_cons_tail members are used as barrier to > A> signal bug_ring ownership. However, instructions can be reordered > A> around members write leading to stale values for ie. br_prod_bufs. > A> > A> Use correct memory barriers to ensure proper ordering of the > A> ownership tokens updates. > A> > A> Sponsored by: EMC / Isilon storage division > A> MFC after: 2 weeks > > Have you profiled this? > > After this change the buf_ring actually gains its own hand-rolled > mutex: > > while (atomic_load_acq_32(&br->br_prod_tail) != prod_head) > cpu_spinwait(); > > The only difference with mutex(9) is that this one isn't monitored > by WITNESS.
I think you are not correct. It doesn't disable interrupts (as spinlock do) and it doesn't sleep. So your analogy is completely off. Also, on x86 atomic_store_rel_*() is a simple write. The only thing that really changes is the atomic_load_acq_*() that introduces a locked instruction. > The idea behind buf_ring was lockless storing and lockless fetching > from a ring and now this vanished. > > What does your change actually fixes except precise accounting of > br_prod_bufs that are actually unused and should be better garbage > collected rather than fixed? The write of br_prod_tail must happens as very last thing, also after the whole buf setup. The only way you can enforce this is with a memory barrier. I can double-check if we can garbage collect br_prod_bufs but this should not be enough yet. Attilio -- Peace can only be achieved by understanding - A. Einstein _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
