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

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