From: Bin Wu
When we test the drive_mirror between different hosts by ndb devices,
we find that, during the cancel phase the qemu process crashes sometimes.
By checking the crash core file, we find the stack as follows, which means
a coroutine re-enter error occurs:
(gdb) bt
#0 0x7fdfc744d
From: Bin Wu
When a coroutine holds a lock, other coroutines who want to get
the lock must wait on a co_queue by adding themselves to the
CoQueue. However, if a waiting coroutine is woken up with the
lock still be holding by other coroutine, this waiting coroutine
will add itself to the co_queue
