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

[Kevin Wolf <kwolf@redhat.com>]

Am 15.08.2012 07:21, schrieb 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.

What does "crash" really mean? IO errors certainly shouldn't cause a
kernel to crash?

> 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.

You would have to take the global qemu mutex at least. I agree it's not
a good thing to do.

Kevin