virtio block device hangs after "virtio_blk virtio3: requests:id 0 is not a head!"

The virtual machine is running block layer workloads, interrupted by unclean reboots (echo b > /proc/sysrq-trigger). Kernel version is 3.14.

Sometimes, I get this message on boot:

"virtio_blk virtio3: requests:id 0 is not a head!"

Then, I/O to the virtio block devices just hangs.

Unfortunately I don't have a test case and this is kind of hard to reproduce, but it seems related to having I/O in flight when the kernel is forced to reboot.