On Thu, May 07, 2015 at 02:43:43PM +0100, Stefan Hajnoczi wrote:
On Wed, May 06, 2015 at 07:23:32PM +0800, Fam Zheng wrote:
Reported by Paolo.
Unlike the iohandler in main loop, iothreads currently process the event
notifier used as virtio-blk ioeventfd in all 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 by dispatching the
ioeventfd event, 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.
Notes:
If the general approach is right, other transaction types could get the
blockers similarly, in next revision. And some related bdrv_drain_all()
could
also be changed to bdrv_drain().
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.
Fam Zheng (7):
block: Add op blocker type device IO
block: Block device IO during bdrv_drain and bdrv_drain_all
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
blockdev: Add device IO op blocker during snapshot transaction
block.c | 20
block/block-backend.c | 10 ++
block/io.c | 12 +++
blockdev.c | 7 +
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 ++
10 files changed, 174 insertions(+), 11 deletions(-)
Please also add a listener to nbd.c so NBD exports cannot submit I/O
during bdrv_drain_all().
qmp_transaction probably also needs this.
Stefan
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