Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

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


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