On Sun, Mar 25, 2007 at 08:24:18PM -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?
No. If you look at commit 9623b3732d11b0a18d9af3419f680d27ea24b014, you can see that it defines mb/rmb/wmb as barrier() on both ARM UP and SMP systems. The UP part is obviously fine, the SMP part is what is under debate here. > 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. Given the semantics above, mb/rmb/wmb can obviously be just barrier()s on ARM UP systems.. I don't think anyone ever disagreed about that. > > 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. 'almost certainly'? That sounds like there is a possibility that it wouldn't have to? What does this depend on? At least David and Catalin seem to disagree with the statement that mb/rmb/wmb should order accesses from different CPUs. And memory-barriers.txt is pretty vague about this.. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
