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 16:03:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chao Yu <yuchao0@...wei.com>
Cc:	Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...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 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.

> >>     - io_schedule_timeout
> >