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SubjectRe: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback
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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@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?
>
> Yes, preemption ignores task_struct::state.

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

>

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