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[Qemu-devel] [PATCH v2 00/11] Fix transactional snapshot with virtio-blk

From: Fam Zheng
Subject: [Qemu-devel] [PATCH v2 00/11] Fix transactional snapshot with virtio-blk dataplane and NBD export
Date: Wed, 13 May 2015 17:28:08 +0000

Changes from RFC:
  - Add op blocker listener in nbd server.
  - Add other transaction types.
  - Only notify listeners when changing from/to empty. (Paolo)

Reported by Paolo.

Unlike the iohandler in main loop, iothreads currently process the event
notifier used by virtio-blk ioeventfd in nested aio_poll. This is dangerous
without proper protection, because guest requests could sneak to block layer
where they mustn't.

For example, a QMP transaction may involve multiple bdrv_drain_all() in
handling the list of AioContext it works on. If an aio_poll in one of the
bdrv_drain_all() happens to process a guest VQ kick, and dispatches the
ioeventfd event to virtio-blk, a new guest write is then submitted, and voila,
the transaction semantics is violated.

This series avoids this problem by disabling virtio-blk handlers during
bdrv_drain_all() and transactions.

- Patches 1~3 add the block layer op blocker change notifier code.
- Patches 4,5 secure virtio-blk dataplane.
- Patch 6 secures nbd export.
- Patch 7~10 protect each transaction type from being voilated by new IO
  generated in nested aio_poll.
- Patch 11 protects bdrv_drain and bdrv_drain_all.


virtio-scsi-dataplane will be a bit more complicated, but still doable.  It
would probably need one more interface abstraction between scsi-disk, scsi-bus
and virtio-scsi.

Although other devices don't have a pause/resume callback yet, the
blk_check_request, which returns -EBUSY if "device io" op blocker is set, could
hopefully cover most cases already.

Timers and block jobs also generate IO, but it should be fine as long as they
don't change guest visible data, which is true AFAICT. Besides, bdrv_drain_all
already pauses block jobs.

Fam Zheng (11):
  block: Add op blocker type "device IO"
  block: Add op blocker notifier list
  block-backend: Add blk_op_blocker_add_notifier
  virtio-blk: Move complete_request to 'ops' structure
  virtio-blk: Don't handle output when there is "device IO" op blocker
  nbd-server: Clear "can_read" when "device io" blocker is set
  blockdev: Block device IO during internal snapshot transaction
  blockdev: Block device IO during external snapshot transaction
  blockdev: Block device IO during drive-backup transaction
  blockdev: Block device IO during blockdev-backup transaction
  block: Block "device IO" during bdrv_drain and bdrv_drain_all

 block.c                         | 28 +++++++++++++++++
 block/block-backend.c           | 10 ++++++
 block/io.c                      | 12 +++++++
 blockdev.c                      | 49 ++++++++++++++++++++++++-----
 hw/block/dataplane/virtio-blk.c | 36 ++++++++++++++++++---
 hw/block/virtio-blk.c           | 69 +++++++++++++++++++++++++++++++++++++++--
 include/block/block.h           |  9 ++++++
 include/block/block_int.h       |  3 ++
 include/hw/virtio/virtio-blk.h  | 17 ++++++++--
 include/sysemu/block-backend.h  |  2 ++
 nbd.c                           | 18 +++++++++++
 11 files changed, 235 insertions(+), 18 deletions(-)


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