On Wed, Jan 27, 2016 at 12:52:07AM +0800, Boqun Feng wrote: > Hi Paul, > > On Mon, Jan 18, 2016 at 07:46:29AM -0800, Paul E. McKenney wrote: > > On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote: > > > Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > > > The reason for this is that smp_read_barrier_depends() must order the > > > > pointer load against any subsequent read or write through a dereference > > > > of that pointer. For example: > > > > > > > > p = READ_ONCE(gp); > > > > smp_rmb(); > > > > r1 = p->a; /* ordered by smp_rmb(). */ > > > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > > > > > > In contrast: > > > > > > > > p = READ_ONCE(gp); > > > > smp_read_barrier_depends(); > > > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > > > > > > Again, if your hardware maintains local ordering for address > > > > and data dependencies, you can have read_barrier_depends() and > > > > smp_read_barrier_depends() be no-ops like they are for most > > > > architectures. > > > > > > > > Does that help? > > > > > > This is crazy! smp_rmb started out being strictly stronger than > > > smp_read_barrier_depends, when did this stop being the case? > > > > Hello, Herbert! > > > > It is true that most Linux kernel code relies only on the read-read > > properties of dependencies, but the read-write properties are useful. > > Admittedly relatively rarely, but useful. > > > > The better comparison for smp_read_barrier_depends(), especially in > > its rcu_dereference*() form, is smp_load_acquire(). > > Confused.. > > 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. Those examples involved read-to-write with conditionals, as in: if (READ_ONCE(a)) WRITE_ONCE(b, 1); Without the "if", no ordering is guaranteed on weakly ordered CPUs. (The volatile accesses keep ordering within the compiler for once... > 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? The semantics of smp_read_barrier_depends() has been both read-to-write and read-to-read for some time now, this patch just catches the documentation up with reality. Thanx, Paul -- 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
