On Wed, Jan 27, 2016 at 12:52:07AM +0800, Boqun Feng wrote: > I recall that last time you and Linus came into a conclusion that even > on Alpha, a barrier for read->write with data dependency is unnecessary: > > http://article.gmane.org/gmane.linux.kernel/2077661 > > And in an earlier mail of that thread, Linus made his point that > smp_read_barrier_depends() should only be used to order read->read. > > So right now, are we going to extend the semantics of > smp_read_barrier_depends()? Can we just make smp_read_barrier_depends() > still only work for read->read, and assume all the architectures won't > reorder read->write with data dependency, so that the code above having > a smp_rmb() also works? That discussions was about control dependencies. So writes that _depend_ on a prior read having an explicit value. So something like: struct foo *x = READ_ONCE(*ptr); smp_read_barrier_depends() if (x->val == 5) x->bar = 5; In that case, the load of x->val must be complete and its value determined _before_ the store to x->bar can happen. This is distinct from: struct foo *x = READ_ONCE(*ptr); smp_read_barrier_depends(); x->bar = 5; And its the second case where smp_read_barrier_depends() read->write order matters. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
