Re: [Qemu-devel] [RFC PATCH 4/9] linux-aio: Implement .cancel_async

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Aug 21, 2014 at 07:56:51PM +0800, Fam Zheng wrote:
> @@ -110,6 +109,22 @@ static void qemu_laio_completion_cb(EventNotifier *e)
>      }
>  }
>  
> +static void laio_cancel_async(BlockDriverAIOCB *blockacb)
> +{
> +    struct qemu_laiocb *laiocb = (struct qemu_laiocb *)blockacb;
> +    struct io_event event;
> +    int ret;
> +
> +    ret = io_cancel(laiocb->ctx->ctx, &laiocb->iocb, &event);
> +    laiocb->ret = -ECANCELED;
> +    if (!ret) {
> +        /* iocb is not cancelled, cb will be called by the event loop later */
> +        return;
> +    }

No callback will be invoked if io_cancel(2) every cancels the request
immediately.

The current kernel implementation always returns -EINPROGRESS or some of
other error value.  But some day it might return 0 and this would leak
the request!

> +
> +    laiocb->common.cb(laiocb->common.opaque, laiocb->ret);
> +}

It would be cleaner to reuse laio_cancel_async() from laio_cancel() to
avoid code duplication.  For example, there is a useful comment in
laio_cancel() explaining that io_cancel(2) doesn't cancel I/O in
practice on 2.6.31 era kernels.

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