On Wed, May 04, 2016 at 10:25:03AM +0200, Juergen Gross wrote:
> On 03/05/16 17:06, Anthony PERARD wrote:
> > On Thu, Mar 10, 2016 at 04:19:30PM +0100, Juergen Gross wrote:
> >> +static void usbback_bh(void *opaque)
> >> +{
> >> + struct usbback_info *usbif;
> >> + struct usbif_urb_back_ring *urb_ring;
> >> + struct usbback_req *usbback_req;
> >> + RING_IDX rc, rp;
> >> + unsigned int more_to_do;
> >> +
> >> + usbif = opaque;
> >> + if (usbif->ring_error) {
> >> + return;
> >> + }
> >> +
> >> + urb_ring = &usbif->urb_ring;
> >> + rc = urb_ring->req_cons;
> >> + rp = urb_ring->sring->req_prod;
> >
> > Maybe use atomic_read() here to avoid req_prod been read more than once.
>
> Hmm. This isn't done in the other backends.
>
> TBH: what would happen if req_prod would be read multiple times? In the
> worst case we would see a new request from the guest which we would have
> missed without the atomic_read().
If the guest is misbehaving, it maybe could provoke QEMU to handle more
request. I'm not sure.
For this use of atomic_read, I'm mostly refering to XSA-155[1] and a
conversation[2].
[1] http://xenbits.xen.org/xsa/advisory-155.html
[2] <570cfa45.7070...@citrix.com>
http://lists.xen.org/archives/html/xen-devel/2016-04/msg01696.html
> >> + xen_rmb(); /* Ensure we see queued requests up to 'rp'. */
> >> +
--
Anthony PERARD
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