read_barrier_depends() usage in vhost.c

[Will Deacon <will@kernel.org>]

Hi all,

In an attempt to remove the remaining traces of [smp_]read_barrier_depends()
following my previous patches to strengthen READ_ONCE() for Alpha [1], I
ended up trying to decipher the read_barrier_depends() usage in the vhost
driver:

--->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...

Thanks,

Will

[1] c2bc66082e10 ("locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()")