Re: [PATCH v3 11/13] xen/pvcalls: implement poll command

[Stefano Stabellini]

On Tue, 12 Sep 2017, Boris Ostrovsky wrote:
> >>> +
> >>> +unsigned int pvcalls_front_poll(struct file *file, struct socket *sock,
> >>> +			       poll_table *wait)
> >>> +{
> >>> +	struct pvcalls_bedata *bedata;
> >>> +	struct sock_mapping *map;
> >>> +
> >>> +	if (!pvcalls_front_dev)
> >>> +		return POLLNVAL;
> >>> +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> >>> +
> >>> +	map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
> >> I just noticed this --- why is it READ_ONCE? Are you concerned that
> >> sk_send_head may change?
> > No, but I wanted to avoid partial reads. A caller could call
> > pvcalls_front_accept and pvcalls_front_poll on newsock almost at the
> > same time (it is probably not the correct way to use the API), I wanted
> > to make sure that "map" is either read correctly, or not read at all.
> 
> How can you have a partial read on a pointer?

I don't think that the compiler makes any promises on translating a
pointer read into a single read instruction. Of couse, I expect gcc to
actually do it without any need for READ/WRITE_ONCE.