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:

quoted

Paul E. McKenney [off-list ref] wrote:

quoted

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