Re: [Qemu-devel] [PATCH v2] mirror: follow AioContext change gracefully

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sun, Jun 12, 2016 at 02:51:04PM +0800, Fam Zheng wrote:
> @@ -119,7 +121,7 @@ static void mirror_iteration_done(MirrorOp *op, int ret)
>      qemu_iovec_destroy(&op->qiov);
>      g_free(op);
>  
> -    if (s->waiting_for_io) {
> +    if (s->waiting_for_io && !s->quiesce_requested) {
>          qemu_coroutine_enter(s->common.co, NULL);
>      }

Is it necessary to interact with s->waiting_for_io?  The coroutine
should reach a quiescent point later on anyway so it would be simpler to
leave this unchanged.

> +static void mirror_detach_aio_context(void *opaque)
> +{
> +    MirrorBlockJob *s = opaque;
> +
> +    /* Complete pending write requests */
> +    assert(!s->quiesce_requested);
> +    s->quiesce_requested = true;
> +    while (s->quiesce_requested || s->in_flight) {
> +        aio_poll(blk_get_aio_context(s->common.blk), true);
> +    }
> +}

Adding synchronous aio_poll() loops will bite us in the future.  For
example, if a guest resets the virtio device the vcpu will be hung until
I/O completes and sleeps finish.  This flaw in QEMU already exists today
and won't be fixed any time soon, so I guess this approach is okay...

Stefan

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