Paul E. McKenney [off-list ref] 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?
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt