Re: [PATCH v2 2/6] vhost-user-blk: Don't reconnect during initialisation

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 04, 2021 at 11:27:12AM +0200, Kevin Wolf wrote:
> Am 04.05.2021 um 10:59 hat Michael S. Tsirkin geschrieben:
> > On Thu, Apr 29, 2021 at 07:13:12PM +0200, Kevin Wolf wrote:
> > > This is a partial revert of commits 77542d43149 and bc79c87bcde.
> > > 
> > > Usually, an error during initialisation means that the configuration was
> > > wrong. Reconnecting won't make the error go away, but just turn the
> > > error condition into an endless loop. Avoid this and return errors
> > > again.
> > 
> > So there are several possible reasons for an error:
> > 
> > 1. remote restarted - we would like to reconnect,
> >    this was the original use-case for reconnect.
> > 
> >    I am not very happy that we are killing this usecase.
> 
> This patch is killing it only during initialisation, where it's quite
> unlikely compared to other cases and where the current implementation is
> rather broken. So reverting the broken feature and going back to a
> simpler correct state feels like a good idea to me.
> 
> The idea is to add the "retry during initialisation" feature back on top
> of this, but it requires some more changes in the error paths so that we
> can actually distinguish different kinds of errors and don't retry when
> we already know that it can't succeed.

Okay ... let's make all this explicit in the commit log though, ok?

> > 2. qemu detected an error and closed the connection
> >    looks like we try to handle that by reconnect,
> >    this is something we should address.
> 
> Yes, if qemu produces the error locally, retrying is useless.
> 
> > 3. remote failed due to a bad command from qemu.
> >    this usecase isn't well supported at the moment.
> > 
> >    How about supporting it on the remote side? I think that if the
> >    data is well-formed just has a configuration remote can not support
> >    then instead of closing the connection, remote can wait for
> >    commands with need_reply set, and respond with an error. Or at
> >    least do it if VHOST_USER_PROTOCOL_F_REPLY_ACK has been negotiated.
> >    If VHOST_USER_SET_VRING_ERR is used then signalling that fd might
> >    also be reasonable.
> > 
> >    OTOH if qemu is buggy and sends malformed data and remote detects
> >    that then hacing qemu retry forever is ok, might actually be
> >    benefitial for debugging.
> 
> I haven't really checked this case yet, it seems to be less common.
> Explicitly communicating an error is certainly better than just cutting
> the connection. But as you say, it means QEMU is buggy, so blindly
> retrying in this case is kind of acceptable.
> 
> Raphael suggested that we could limit the number of retries during
> initialisation so that it wouldn't result in a hang at least.

not sure how do I feel about random limits ... how would we
set the limit?


> > > Additionally, calling vhost_user_blk_disconnect() from the chardev event
> > > handler could result in use-after-free because none of the
> > > initialisation code expects that the device could just go away in the
> > > middle. So removing the call fixes crashes in several places.
> > > For example, using a num-queues setting that is incompatible with the
> > > backend would result in a crash like this (dereferencing dev->opaque,
> > > which is already NULL):
> > > 
> > >  #0  0x0000555555d0a4bd in vhost_user_read_cb (source=0x5555568f4690, condition=(G_IO_IN | G_IO_HUP), opaque=0x7fffffffcbf0) at ../hw/virtio/vhost-user.c:313
> > >  #1  0x0000555555d950d3 in qio_channel_fd_source_dispatch (source=0x555557c3f750, callback=0x555555d0a478 <vhost_user_read_cb>, user_data=0x7fffffffcbf0) at ../io/channel-watch.c:84
> > >  #2  0x00007ffff7b32a9f in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
> > >  #3  0x00007ffff7b84a98 in g_main_context_iterate.constprop () at /lib64/libglib-2.0.so.0
> > >  #4  0x00007ffff7b32163 in g_main_loop_run () at /lib64/libglib-2.0.so.0
> > >  #5  0x0000555555d0a724 in vhost_user_read (dev=0x555557bc62f8, msg=0x7fffffffcc50) at ../hw/virtio/vhost-user.c:402
> > >  #6  0x0000555555d0ee6b in vhost_user_get_config (dev=0x555557bc62f8, config=0x555557bc62ac "", config_len=60) at ../hw/virtio/vhost-user.c:2133
> > >  #7  0x0000555555d56d46 in vhost_dev_get_config (hdev=0x555557bc62f8, config=0x555557bc62ac "", config_len=60) at ../hw/virtio/vhost.c:1566
> > >  #8  0x0000555555cdd150 in vhost_user_blk_device_realize (dev=0x555557bc60b0, errp=0x7fffffffcf90) at ../hw/block/vhost-user-blk.c:510
> > >  #9  0x0000555555d08f6d in virtio_device_realize (dev=0x555557bc60b0, errp=0x7fffffffcff0) at ../hw/virtio/virtio.c:3660
> > 
> > Right. So that's definitely something to fix.
> > 
> > > 
> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> 
> Kevin