On Sat, Nov 23, 2013 at 12:24:50PM +0100, Ingo Molnar wrote: > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > x86 does have that extra "Memory ordering obeys causality (memory > > > ordering respects transitive visibility)." rule, and the example > > > in the architecture manual (section 8.2.3.6 "Stores Are > > > Transitively Visible") seems to very much about this, but your > > > particular example is subtly different, so.. > > > > Indeed, my example needs CPU 1's -load- from y to be transitively > > visible, so I am nervous about this one as well. > > > > > I will have to ruminate on this. > > > > The rules on the left-hand column of page 5 of the below URL apply > > to this example more straightforwardly, but I don't know that Intel > > and AMD stand behind them: > > > > http://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf > > > > My guess is that x86 does guarantee this ordering, but at this point > > I would have to ask someone from Intel and AMD. > > An additional option might be to create a user-space testcase > engineered to hit all the exotic ordering situations, one that might > disprove any particular assumptions we have about the behavior of > hardware. (Back a decade ago when the x86 space first introduced quad > core CPUs with newfangled on-die cache coherency I managed to > demonstrate a causality violation by simulating kernel locks in > user-space, which turned out to be a hardware bug. Also, when > Hyperthreading/SMT was new it demonstrated many interesting bugs never > seen in practice before. So running stuff on real hardware is useful.) > > And a cache coherency (and/or locking) test suite would be very useful > anyway, for so many other purposes as well: such as a new platform/CPU > bootstrap, or to prove the correctness of some fancy new locking > scheme people want to add. Maybe as an extension to rcutorture, or a > generalization of it? I have the locking counterpart of rcutorture on my todo list. ;-) Of course, we cannot prove locks correct via testing, but a quick test can often find a bug faster and more reliably than manual inspection. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>