On Wed, Jan 08, 2014 at 07:39:26PM +0800, majianpeng wrote:

Because after kthread_create, the state of kthread is TASK_UNINTERRUPTIBLE.
If we don't wake up dc->writeback_thread, the thread will be hung-task.
For those reasons, we don't wake up writeback_thread:
A:only create backed device
B:create backed device and cache device, but don't write at once
C:write without writeback poliy

So like the way of create gc_thread, after creating writeback_thread, i
set the state is TASK_INTERRUPTIBLE.

I've seen this issue, but this isn't even where the sleep happens,
there's no way this could be fixing it..