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(). 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
