On Fri,  4 May 2012 20:22:04 +0800, Asias He [off-list ref] wrote:

If we reset the virtio-blk device before the requests already dispatched
to the virtio-blk driver from the block layer are finised, we will stuck
in blk_cleanup_queue() and the remove will fail.

blk_cleanup_queue() calls blk_drain_queue() to drain all requests queued
before DEAD marking. However it will never success if the device is
already stopped. We'll have q->in_flight[] > 0, so the drain will not
finish.

How to reproduce the race:
1. hot-plug a virtio-blk device
2. keep reading/writing the device in guest
3. hot-unplug while the device is busy serving I/O

Test:
~1000 rounds of hot-plug/hot-unplug test passed with this patch.

Changes in v3:
- Drop blk_abort_queue and blk_abort_request
- Use __blk_end_request_all to complete request dispatched to driver

OK, replaced v2 with this one.

Thanks,
Rusty.