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

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Date:   Wed, 8 Mar 2017 15:02:50 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     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 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.

-boris