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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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


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