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

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Date:	Thu, 28 Apr 2016 23:06:23 +0800
From:	Chao Yu <chao@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chao Yu <yuchao0@...wei.com>, Jaegeuk Kim <jaegeuk@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when
 waiting on all pages writeback

On 2016/4/28 22:37, Peter Zijlstra wrote:
> On Thu, Apr 28, 2016 at 10:30:55PM +0800, Chao Yu wrote:
>> 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@...wei.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?
> 
> Yes, preemption ignores task_struct::state.

Got it. Thanks very much for your help! :)

>