Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling

[Oleksandr Andrushchenko]

On 04/24/2018 05:20 PM, Takashi Iwai wrote:
> On Mon, 16 Apr 2018 08:24:51 +0200,
> Oleksandr Andrushchenko wrote:
> > +static irqreturn_t evtchnl_interrupt_req(int irq, void *dev_id)
> > +{
> > +	struct xen_snd_front_evtchnl *channel = dev_id;
> > +	struct xen_snd_front_info *front_info = channel->front_info;
> > +	struct xensnd_resp *resp;
> > +	RING_IDX i, rp;
> > +	unsigned long flags;
> > +
> > +	if (unlikely(channel->state != EVTCHNL_STATE_CONNECTED))
> > +		return IRQ_HANDLED;
> > +
> > +	spin_lock_irqsave(&front_info->io_lock, flags);
> > +
> > +again:
> > +	rp = channel->u.req.ring.sring->rsp_prod;
> > +	/* ensure we see queued responses up to rp */
> > +	rmb();
> > +
> > +	for (i = channel->u.req.ring.rsp_cons; i != rp; i++) {
> I'm not familiar with Xen stuff in general, but through a quick
> glance, this kind of code worries me a bit.
>
> If channel->u.req.ring.rsp_cons has a bogus number, this may lead to a
> very long loop, no?  Better to have a sanity check of the ring buffer
> size.
In this loop I have:
resp = RING_GET_RESPONSE(&channel->u.req.ring, i);
and the RING_GET_RESPONSE macro is designed in the way that
it wraps around when *i* in the question gets bigger than
the ring size:

#define RING_GET_REQUEST(_r, _idx)ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ \
ÂÂÂ (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))

So, even if the counter has a bogus number it will not last long

> > +static irqreturn_t evtchnl_interrupt_evt(int irq, void *dev_id)
> > +{
> > +	struct xen_snd_front_evtchnl *channel = dev_id;
> > +	struct xen_snd_front_info *front_info = channel->front_info;
> > +	struct xensnd_event_page *page = channel->u.evt.page;
> > +	u32 cons, prod;
> > +	unsigned long flags;
> > +
> > +	if (unlikely(channel->state != EVTCHNL_STATE_CONNECTED))
> > +		return IRQ_HANDLED;
> > +
> > +	spin_lock_irqsave(&front_info->io_lock, flags);
> > +
> > +	prod = page->in_prod;
> > +	/* ensure we see ring contents up to prod */
> > +	virt_rmb();
> > +	if (prod == page->in_cons)
> > +		goto out;
> > +
> > +	for (cons = page->in_cons; cons != prod; cons++) {
> Ditto.
Same as above
> thanks,
>
> Takashi
Thank you,
Oleksandr