[PATCH 0/4] RFC/WIP: Fix scsi devices plug/unplug races w.r.t virtio-scsi iothread

[Maxim Levitsky]

Hi!

This is a patch series that is a result of my discussion with Paulo on
how to correctly fix the root cause of the BZ #1812399.

The root cause of this bug is the fact that IO thread is running mostly
unlocked versus main thread on which device hotplug is done.

qdev_device_add first creates the device object, then places it on the bus,
and only then realizes it.

However some drivers and currently only virtio-scsi enumerate its child bus
devices on each request that is received from the guest and that can happen on 
the IO
thread.

Thus we have a window when new device is on the bus but not realized and can be 
accessed
by the virtio-scsi driver in that state.

Fix that by doing two things:

1. Add partial RCU protection to the list of a bus's child devices.
This allows the scsi IO thread to safely enumerate the child devices
while it races with the hotplug placing the device on the bus.

2. Make the virtio-scsi driver check .realized property of the scsi device
and avoid touching the device if it isn't

I don't think that this is very pretty way to solve this, we discussed this
with Paulo and it kind of looks like the lesser evil. I am open to your 
thoughts about this.

Note that this patch series doesn't pass some unit tests and in particular 
qtest 'drive_del-test'
I did some light debug of this test and I see that the reason for this is that 
now child device deletion
can be delayed due to RCU. This is also something I would like to discuss in 
this RFC.

Note also that I might have some code style errors and bugs in this since I 
haven't
tested the code in depth yet, because I am not yet sure that this is the right 
way
to fix that bug

Also note that in the particular bug report the issue wasn't a race but rather 
due
to combination of things, the .realize code in the middle managed to trigger IO 
on the virtqueue
which caused the virtio-scsi driver to access the half realized device. However
since this can happen as well with real IO thread, this patch series was done,
which fixes this as well.

Best regards,
        Maxim Levitsky

Maxim Levitsky (4):
  scsi/scsi_bus: switch search direction in scsi_device_find
  device-core: use RCU for list of childs of a bus
  device-core: use atomic_set on .realized property
  virtio-scsi: don't touch scsi devices that are not yet realized

 hw/core/bus.c                  | 43 ++++++++++++++++++++----------
 hw/core/qdev.c                 | 48 ++++++++++++++++++++++------------
 hw/scsi/scsi-bus.c             | 27 ++++++++++++++++---
 hw/scsi/virtio-scsi.c          | 24 +++++++++++++++--
 include/hw/qdev-core.h         |  3 +++
 include/hw/virtio/virtio-bus.h |  7 +++--
 6 files changed, 114 insertions(+), 38 deletions(-)

-- 
2.17.2