linux-kernel - Re: [PATCH 5/7] xen/9pfs: send requests to the backend

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Date:   Wed, 8 Mar 2017 12:56:17 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano@...reto.com>, jgross@...e.com,
        Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>,
        v9fs-developer@...ts.sourceforge.net
Subject: Re: [PATCH 5/7] xen/9pfs: send requests to the backend

On Wed, 8 Mar 2017, Boris Ostrovsky wrote:
> On 03/08/2017 02:33 PM, Stefano Stabellini wrote:
> > On Wed, 8 Mar 2017, Boris Ostrovsky wrote:
> >>>>> +}
> >>>>> +
> >>>>> +static int p9_xen_write_todo(struct xen_9pfs_dataring *ring, RING_IDX size)
> >>>>> +{
> >>>>> +	RING_IDX cons, prod;
> >>>>> +
> >>>>> +	cons = ring->intf->out_cons;
> >>>>> +	prod = ring->intf->out_prod;
> >>>>> +	mb();
> >>>>> +
> >>>>> +	if (XEN_9PFS_RING_SIZE - xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE) >= size)
> >>>>> +		return 1;
> >>>>> +	else
> >>>>> +		return 0;
> >>>>>  }
> >>>>>  
> >>>>>  static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
> >>>>>  {
> >>>>> +	struct xen_9pfs_front_priv *priv = NULL;
> >>>>> +	RING_IDX cons, prod, masked_cons, masked_prod;
> >>>>> +	unsigned long flags;
> >>>>> +	uint32_t size = p9_req->tc->size;
> >>>>> +	struct xen_9pfs_dataring *ring;
> >>>>> +	int num;
> >>>>> +
> >>>>> +	list_for_each_entry(priv, &xen_9pfs_devs, list) {
> >>>>> +		if (priv->client == client)
> >>>>> +			break;
> >>>>> +	}
> >>>>> +	if (priv == NULL || priv->client != client)
> >>>>> +		return -EINVAL;
> >>>>> +
> >>>>> +	num = p9_req->tc->tag % priv->num_rings;
> >>>>> +	ring = &priv->rings[num];
> >>>>> +
> >>>>> +again:
> >>>>> +	while (wait_event_interruptible(ring->wq,
> >>>>> +				p9_xen_write_todo(ring, size) > 0) != 0);
> >>>>> +
> >>>>> +	spin_lock_irqsave(&ring->lock, flags);
> >>>>> +	cons = ring->intf->out_cons;
> >>>>> +	prod = ring->intf->out_prod;
> >>>>> +	mb();
> >>>>> +
> >>>>> +	if (XEN_9PFS_RING_SIZE - xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE) < size) {
> >>>> This looks like p9_xen_write_todo().
> >>> p9_xen_write_todo is just a wrapper around xen_9pfs_queued to provide
> >>> a return value that works well with wait_event_interruptible.
> >>>
> >>> I would prefer not to call p9_xen_write_todo here, because it's simpler
> >>> if we don't read prod and cons twice.
> >> I was referring to the whole code fragment after spin_lock_irqsave(),
> >> not just the last line. Isn't it exactly !p9_xen_write_todo()?
> > Yes, it is true they are almost the same. The difference, and the reason
> > for p9_xen_write_todo to exist, is that p9_xen_write_todo is called in
> > the wait_event_interruptible loop, as such it needs to read prod and
> > cons every time. On the other end, here we want to read them once. Does
> > it make sense?
> 
> 
> I am clearly being particularly dense here but what I was thinking was:
> 
> again:
> 	while (wait_event_interruptible(ring->wq,
> 				p9_xen_write_todo(ring, size) > 0) != 0);
> 
> 	spin_lock_irqsave(&ring->lock, flags);
> 	if (!p9_xen_write_todo(ring, size)) {
> 		spin_unlock_irqrestore(&ring->lock, flags);
> 		goto again;
> 	}
> 
> There is no extra read of prod/cons.

Yes, there are: just after this if statement we would have to read them
again to calculate masked_prod and masked_cons.