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

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Date:   Wed, 8 Mar 2017 08:58:12 -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


>>> +}
>>> +
>>> +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()?


>
>
>> BTW, where is xen_9pfs_queued()
>> defined? I couldn't find it. Same for xen_9pfs_mask() and
>> xen_9pfs_write_packet().
> They are provided by the new ring macros, see
> include/xen/interface/io/ring.h (the first patch).

Oh, right. I was searching for the string literally.

-boris