Re: [Qemu-devel] [PATCH v6 2/2] block: Support GlusterFS as a QEMU block backend

[Bharata B Rao]

On Tue, Aug 14, 2012 at 10:29:26AM +0200, Kevin Wolf wrote:
> >>> +static void gluster_finish_aiocb(struct glfs_fd *fd, ssize_t ret, void 
> >>> *arg)
> >>> +{
> >>> +    GlusterAIOCB *acb = (GlusterAIOCB *)arg;
> >>> +    BDRVGlusterState *s = acb->common.bs->opaque;
> >>> +
> >>> +    acb->ret = ret;
> >>> +    if (qemu_gluster_send_pipe(s, acb) < 0) {
> >>> +        /*
> >>> +         * Gluster AIO callback thread failed to notify the waiting
> >>> +         * QEMU thread about IO completion. Nothing much can be done
> >>> +         * here but to abruptly abort.
> >>> +         *
> >>> +         * FIXME: Check if the read side of the fd handler can somehow
> >>> +         * be notified of this failure paving the way for a graceful 
> >>> exit.
> >>> +         */
> >>> +        error_report("Gluster failed to notify QEMU about IO 
> >>> completion");
> >>> +        abort();
> >>
> >> In the extreme case you may choose to make this disk inaccessible
> >> (something like bs->drv = NULL), but abort() kills the whole VM and
> >> should only be called when there is a bug.
> > 
> > There have been concerns raised about this earlier too. I settled for this
> > since I couldn't see a better way out and I could see the precedence
> > for this in posix-aio-compat.c
> > 
> > So I could just do the necessary cleanup, set bs->drv to NULL and return 
> > from
> > here ? But how do I wake up the QEMU thread that is waiting on the read side
> > of the pipe ? W/o that, the QEMU thread that waits on the read side of the
> > pipe is still hung.
> 
> There is no other thread. But you're right, you should probably
> unregister the aio_fd_handler and any other pending callbacks.

As I clarified in the other mail, this (gluster_finish_aiocb) is called
from gluster thread context and hence QEMU thread that raised the original
read/write request is still blocked on qemu_aio_wait().

I tried the following cleanup instead of abrupt abort:

close(read_fd); /* This will wake up the QEMU thread blocked on 
select(read_fd...) */
close(write_fd);
qemu_aio_set_fd_handler(read_fd, NULL, NULL, NULL, NULL);
qemu_aio_release(acb);
s->qemu_aio_count--;
bs->drv = NULL;

I tested this by manually injecting faults into qemu_gluster_send_pipe().
With the above cleanup, the guest kernel crashes with IO errors.

Is there anything else that I need to do or do differently to retain the
VM running w/o disk access ?

I thought of completing the aio callback by doing
acb->common.cb(acb->common.opaque, -EIO);
but that would do a coroutine enter from gluster thread, which I don't think
should be done.

Regards,
Bharata.