Messages in this thread | | | Date | Sun, 25 Mar 2007 20:24:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: I/O memory barriers vs SMP memory barriers |
| |
On Sun, Mar 25, 2007 at 11:38:43PM +0200, Lennert Buytenhek wrote: > On Sun, Mar 25, 2007 at 02:15:42PM -0700, Paul E. McKenney wrote: > > > > > [ background: On ARM, SMP synchronisation does need barriers but device > > > > synchronisation does not. The question is that given this, whether > > > > mb() and friends can be NOPs on ARM or not (i.e. whether mb() is > > > > supposed to sync against other CPUs or not, or whether only smp_mb() > > > > can be used for this.) ] > > > > > > Hmmmm... > > > > > > [snip] > > > > 3. Orders memory accesses and device accesses, but not necessarily > > the union of the two -- mb(), rmb(), wmb(). > > If mb/rmb/wmb are required to order normal memory accesses, that means > that the change made in commit 9623b3732d11b0a18d9af3419f680d27ea24b014 > to always define mb/rmb/wmb as barrier() on ARM systems was wrong.
This was on UP ARM systems, right? Assuming that ARM CPUs respect the usual CPU-self-consistency semantics, and given the background that device accesses are ordered, then it might well be OK to have mb/rmb/wmb be barrier() on UP ARM systems.
Most likely not on SMP ARM systems, however.
> Does everybody agree on these semantics, though? At least David seems > to think that mb/rmb/wmb aren't required to order normal memory accesses > against each other..
Not on UP. On SMP, ordering is (almost certainly) required.
> > 4. Orders only device accesses, which is what seems to be looked > > for here. > > Yes. (As above, on ARM, SMP synchronisation does need barriers but > device synchronisation does not. If mb/rmb/wmb were only required to > synchronise device accesses, they could have been regular compiler > barriers on ARM, but if they are also required to synchronise normal > memory accesses against each other, they have to map to hardware > barriers.)
Again, for kernels built for UP, you might well be able to make the mb() primitives be barrier(). I don't see it for SMP, though.
Thanx, Paul - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |