[Qemu-block] QEMU 1.1.2: block IO throttle might occasionally freeze running process's IO to zero

[Bob Chen]

Test case:

1. QEMU 1.1.2

2. Run fio inside the vm, give it some pressure. Watch the realtime throughput

3. block_set_io_throttle drive_2 100000000 0 0 2000 0 0         # throttle bps and iops, any value

4. Observed that the IO is very likely to freeze to zero. The fio process stuck!

5. Kill the former fio process, start a new one. The IO turns back to normal

Didn't reproduce it with QEMU 2.5.

Actually I'm not wishfully thinking the community would help fix this bug on such an ancient version. Just hope someone can tell me what is the root cause. Then I have to evaluate whether I should move to higher version QEMU, or fix this bug on 1.1.2 in-place(if it is a small one).