Re: read_barrier_depends() usage in vhost.c

[Jason Wang]

On 2019/12/18 äå5:19, Herbert Xu wrote:
> Will Deacon <will@xxxxxxxxxx> wrote:
> > > --->8
> > >
> > > // drivers/vhost/vhost.c
> > > static int get_indirect(struct vhost_virtqueue *vq,
> > >                        struct iovec iov[], unsigned int iov_size,
> > >                        unsigned int *out_num, unsigned int *in_num,
> > >                        struct vhost_log *log, unsigned int *log_num,
> > >                        struct vring_desc *indirect)
> > > {
> > >        [...]
> > >
> > >        /* We will use the result as an address to read from, so most
> > >         * architectures only need a compiler barrier here. */
> > >        read_barrier_depends();
> > >
> > > --->8
> > >
> > > Unfortunately, although the barrier is commented (hurrah!), it's not
> > > particularly enlightening about the accesses making up the dependency
> > > chain, and I don't understand the supposed need for a compiler barrier
> > > either (read_barrier_depends() doesn't generally provide this).
> > >
> > > Does anybody know which accesses are being ordered here? Usually you'd need
> > > a READ_ONCE()/rcu_dereference() beginning the chain, but I haven't managed
> > > to find one...
> I think what it's trying to separate is using indirect->addr as a
> base and then reading from that through copy_from_iter.
>
> Cheers,


The question is that there's a smp_rmb() before in vhost_get_vq_desc(), 
isn't it sufficient to do this?

Thanks