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. 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? Regards, Boqun
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