Re: [Qemu-devel] [PATCH 0/3] v4 Decouple block device removal from device removal

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

On Mon, Nov 08, 2010 at 11:32:01AM +0100, Markus Armbruster wrote:
> Ryan Harper <ryanh@us.ibm.com> writes:
> 
> > * Markus Armbruster <armbru@redhat.com> [2010-11-06 04:19]:
> >> Ryan Harper <ryanh@us.ibm.com> writes:
> >> 
> >> > * Markus Armbruster <armbru@redhat.com> [2010-11-05 11:11]:
> >> >> Ryan Harper <ryanh@us.ibm.com> writes:
> >> >> 
> >> >> > * Markus Armbruster <armbru@redhat.com> [2010-11-05 08:28]:
> >> >> >> I'd be fine with any of these:
> >> >> >> 
> >> >> >> 1. A new command "device_disconnet ID" (or similar name) to disconnect
> >> >> >>    device ID from any host parts.  Nice touch: you don't have to know
> >> >> >>    about the device's host part(s) to disconnect it.  But it might be
> >> >> >>    more work than the other two.
> >> >> >
> >> >> > This is sort of what netdev_del() and drive_unplug() are today; we're
> >> >> > just saying sever the connection of this device id.   
> >> >> 
> >> >> No, I have netdev_del as (3).
> >> >> 
> >> >> All three options are "sort of" the same, just different commands with
> >> >> a common purpose.
> >> >> 
> >> >> > I'd like to rename drive_unplug() to blockdev_del() and call it done.  I
> >> >> > was looking at libvirt and the right call to netdev_del is already
> >> >> > in-place; I'd just need to re-spin my block patch to call blockdev_del()
> >> >> > after invoking device_del() to match what is done for net.
> >> >> 
> >> >> Unless I'm missing something, you can't just rename: your unplug does
> >> >> not delete the host part.
> >> >> 
> >> >> >> 2. New commands netdev_disconnect, drive_disconnect (or similar names)
> >> >> >>    to disconnect a host part from a guest device.  Like (1), except you
> >> >> >>    have to point to the other end of the connection to cut it.
> >> >> >
> >> >> > What's the advantage here? We need an additional piece of info (host
> >> >> > part) in addition to the device id?
> >> >> 
> >> >> That's a disadvantage.
> >> >> 
> >> >> Possible advantage: implementation could be slightly easier than (1),
> >> >> because you don't have to find the host parts.
> >> >> 
> >> >> >> 3. A new command "drive_del ID" similar to existing netdev_del.  This is
> >> >> >>    (2) fused with delete.  Conceptual wart: you can't disconnect and
> >> >> >>    keep the host part around.  Moreover, delete is slightly dangerous,
> >> >> >>    because it renders any guest device still using the host part
> >> >> >>    useless.
> >> >> >
> >> >> > Hrm, I thought that's what (1) is.
> >> >> 
> >> >> No.
> >> >> 
> >> >> With (1), the argument is a *device* ID, and we disconnect *all* host
> >> >> parts connected to this device (typically just one).
> >> >> 
> >> >> With (3), the argument is a netdev/drive ID, and disconnect *this* host
> >> >> part from the peer device.
> >> >> 
> >> >> >                                     Well, either (1) or (3); I'd like to
> >> >> > rename drive_unplug() to blockdev_del() since they're similar function
> >> >> > w.r.t removing access to the host resource.  And we can invoke them in
> >> >> > the same way from libvirt (after doing guest notification, remove
> >> >> > access).
> >> >> 
> >> >> I'd call it drive_del for now, to match drive_add.
> >> >
> >> > OK, drive_del() and as you mentioned, drive_unplug will take out the
> >> > block driver, but doesn't remove the dinfo object; that ends up dying
> >> > when we call the device destructor.  I think for symmetry we'll want
> >> > drive_del to remove the dinfo object as well.
> >> 
> >> Exactly.
> >> 
> >> a. bdrv_detach() to zap the pointer from bdrv to qdev
> >> b. zap the pointer from qdev to bdrv
> >> c. drive_uninit() to dispose of the host part
> >
> > a-c need to be done to match netdev_del symmetry?  How hard of a req is
> > this?
> 
> Without (c), it's not a delete.  And (c) without (b) leaves a dangling
> pointer.  (c) without (a) fails an assertion in bdrv_delete().
> 
> Aside: (b) should probably be folded into bdrv_detach().
> 
> >> Step b could be awkward with (3), because you don't know device details.
> >> I guess you have to search device properties for a drive property
> >> pointing to bdrv.  I like (1) because it puts that loop in the one place
> >> where it belongs: qdev core.  (3) duplicates it in every HOSTDEV_del.
> >> Except for netdev_del, which is special because of VLANs.
> >> 
> >> To avoid step b, you could try to keep the bdrv around in a special
> >> zombie state.  Still have to free the dinfo, but can't use
> >> drive_uninit() for that then.
> >> 
> >> If you think I'm overcomplicating this, feel free to prove me wrong with
> >> working code :)
> >
> > drive_unplug() works as-is today; so it does feel very combursome at
> > this point.  Other than the name change and agreement on how mgmt should
> > invoke the command, it's been a long ride to get here.
> 
> Sometimes it takes a tough man to make a tender chicken.

> > I'll take my best shot at trying to clean up the other
> > pointers and objects; though on one of my attempts when I took out the
> > dinfo() object that didn't go so well; going to have to audit who uses
> > dinfo and where and what they check before calling it to have a proper
> > cleanup that doesn't remove the whole device altogether.
> 
> Steps a, b, c are the result of my (admittedly quick) audit.
> 
> Here's how the various objects are connected to each other:
> 
>                contains
> drivelist    -----------> DriveInfo
>                                 |
>                                 | .bdrv
>                                 | .id == .bdrv->device_name
>                                 |
>                contains         V
> bdrv_states  -----------> BlockDriverState
>                              |   ^
>                        .peer |   |
>                              |   |                          host part
> -----------------------------|---|-----------------------------------
>                              |   |                         guest part
>                              |   | property "drive"
>                              v   |
>                           DeviceState
> 
> To disconnect host from guest part, you need to cut both pointers.  To
> delete the host part, you need to delete both objects, BlockDriverState
> and DriveInfo.


If we remove DriveInfo, how can management later detect that guest part
was deleted? If you want symmetry with netdev, it's possible to keep a
shell of BlockDriverState/DriveInfo around (solving dangling pointer
problems).


-- 
MST