Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

[Chao Yu <chao@kernel.org>]

On 2016/4/28 22:03, Peter Zijlstra wrote:
> On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote:
>>> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
>>>> From: Chao Yu <yuchao0@huawei.com>
>>>>
>>>> The following condition can happen in a preemptible kernel, it may cause
>>>> checkpointer hunging.
>>>>
>>>> CPU0:					CPU1:
>>>>  - write_checkpoint
>>>>   - do_checkpoint
>>>>    - wait_on_all_pages_writeback
>>>> 					 - f2fs_write_end_io
>>>> 					  - wake_up
>>>> 					this is last writebacked page, but
>>>> 					no sleeper in sbi->cp_wait wait
>>>> 					queue, wake_up is not been called.
>>>>     - prepare_to_wait(TASK_UNINTERRUPTIBLE)
>>>>     Here, current task can been preempted,
>>>>     but there will be no waker since last
>>>>     write_end_io has bypassed wake_up. So
>>>>     current task will sleep forever.
> 
> But here, you should be verifying if you really should go sleep; as the
> code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just
> completed that very last one, this will break out.

You mean after being preempted with TASK_UNINTERRUPTIBLE status, that task still
has chance to be scheduled to check '!get_pages(, F2FS_WRITEBACK)', is that right?

> 
>>>>     - io_schedule_timeout
>>>