Re: [Qemu-devel] Re: [PATCH 06/13] Threadlet: Add dequeue_work threadlet API

[Stefan Hajnoczi]

On Thu, Jan 6, 2011 at 10:43 AM, Arun R Bharadwaj
<address@hidden> wrote:
> * Stefan Hajnoczi <address@hidden> [2011-01-05 19:55:46]:
>
>> On Tue, Jan 04, 2011 at 10:57:39AM +0530, Arun R Bharadwaj wrote:
>> > @@ -574,33 +574,39 @@ static void paio_remove(struct qemu_paiocb *acb)
>> >      }
>> >  }
>> >
>> > -static void paio_cancel(BlockDriverAIOCB *blockacb)
>> > +/**
>> > + * dequeue_work: Cancel a task queued on the global queue.
>> > + * @work: Contains the information of the task that needs to be cancelled.
>> > + *
>> > + * Returns: 0 if the task is successfully cancelled.
>> > + *          1 otherwise.
>> > + */
>> > +static int dequeue_work(ThreadletWork *work)
>> >  {
>> > -    struct qemu_paiocb *acb = (struct qemu_paiocb *)blockacb;
>> > -    int active = 0;
>> > +    int ret = 1;
>> >
>> >      qemu_mutex_lock(&globalqueue.lock);
>> > -    if (!acb->active) {
>> > -        QTAILQ_REMOVE(&globalqueue.request_list, &acb->work, node);
>> > -        acb->ret = -ECANCELED;
>> > -    } else if (acb->ret == -EINPROGRESS) {
>> > -        active = 1;
>> > -    }
>> > +    QTAILQ_REMOVE(&globalqueue.request_list, work, node);
>> > +    ret = 0;
>> >      qemu_mutex_unlock(&globalqueue.lock);
>> >
>> > -    qemu_mutex_lock(&aiocb_mutex);
>> > -    if (!active) {
>> > -        acb->ret = -ECANCELED;
>> > -    } else {
>> > -        while (acb->ret == -EINPROGRESS) {
>> > -            /*
>> > -             * fail safe: if the aio could not be canceled,
>> > -             * we wait for it
>> > -             */
>> > -            qemu_cond_wait(&aiocb_completion, &aiocb_mutex);
>> > +    return ret;
>>
>> It always succeeds?  Why bother with the ret local variable?
>>
>
> Yes, I'll remove this.
>
>> > +}
>> > +
>> > +static void paio_cancel(BlockDriverAIOCB *blockacb)
>> > +{
>> > +    struct qemu_paiocb *acb = (struct qemu_paiocb *)blockacb;
>> > +    if (!acb->active) {
>> > +        if (dequeue_work(&acb->work) != 0) {
>> > +            /* Wait for running work item to complete */
>> > +            qemu_mutex_lock(&aiocb_mutex);
>> > +            while (acb->ret == -EINPROGRESS) {
>> > +                qemu_cond_wait(&aiocb_completion, &aiocb_mutex);
>> > +            }
>> > +            qemu_mutex_unlock(&aiocb_mutex);
>> >          }
>> >      }
>> > -    qemu_mutex_unlock(&aiocb_mutex);
>> > +
>> >      paio_remove(acb);
>>
>> I'm not convinced this function works.  If the request is active in a
>> worker thread and paio_cancel() is called then we invoke paio_remove().
>>
>
> True. So can we do this: Since we have a patch which separately
> removes the active field [PATCH 7/13], can we fold patch 7 and this
> patch into a single patch? So that way we can maintain the
> correctness, because we are actually waiting for the active work to
> complete by doing a while (acb->ret == -EINPROGRESS)

Sounds good.  IIRC the paio_cancel() code in the last version of the
patch looked correct, so it probably *is* just a case of not splitting
this part up into separate patches.

Stefan