Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion

From	Jaegeuk Kim <jaegeuk@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion
Date	2017-09-15 05:50 +0200
Message-ID	<upPND-5K6-3@gated-at.bofh.it> (permalink)
References	(5 earlier) <uprih-6FS-3@gated-at.bofh.it> <upHdo-aN-23@gated-at.bofh.it> <upHQ5-Dd-5@gated-at.bofh.it> <upMwp-3AN-7@gated-at.bofh.it> <upOeT-4Qy-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 09/15, Al Viro wrote:
> On Thu, Sep 14, 2017 at 05:19:39PM -0700, Jaegeuk Kim wrote:
> > Instead, I put more traces in the reboot procedure, and got a clue to suspect
> > the below flow.
> > 
> > delayed_fput()                 init
> >                                - umount
> >  - mntput()
> >  - mntput_no_expire()            - mntput_no_expire()
> >                                  - mnt_add_count(-1);
> >                                  - mnt_get_count() return;
> >                                  - return 0;
> >  - mnt_add_count(-1);
> >  - delayed_mntput_work
> >                                - device_shutdown
> >  - ext4_put_super()
> >  - EIO
> > 
> > Does this make any sense?
> 
> Which filesystem it is?  With root I would've expected remount ro done
> by sys_umount(); with anything else...  How has it managed to avoid
> -EBUSY?  If it was umount -l (IOW, MNT_DETACH), I can see that happening,
> but...  How would flushing prevent the scenario when the same opened
> file had remained open until after the umount(2) return?

It's ext4, and we use umount(0) and retry it several times if -EBUSY happens.
But, I don't see -EBUSY error in the log.

> In other words, where has that fput() come from and how had it managed
> to get past the umount(2)?

Huge number of fput() were called by system drivers when init kills all the
processes before umount(2). So, most of fput() were added in delayed_fput_list.
Then, it seems there is a race between delayed_fput() and umount(). Anyway,
even after umount returns zero, it seems ext4's superblock is still alive
and waiting for delayed_fput() which will finally call put_super.

Thread

[PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-13 22:20 +0200
  Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-14 01:10 +0200
    Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-14 01:40 +0200
      Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-14 01:50 +0200
        Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-14 03:20 +0200
          Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-14 03:40 +0200
            Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-14 20:40 +0200
              Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-14 21:20 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-15 02:20 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-15 04:10 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-15 05:50 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-15 06:30 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-15 20:50 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Theodore Ts'o <tytso@mit.edu> - 2017-09-16 00:20 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-16 01:30 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-16 01:50 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-19 18:00 +0200
                Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion Amir Goldstein <amir73il@gmail.com> - 2017-09-16 09:20 +0200
  Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-20 19:40 +0200
    Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-20 20:40 +0200
      Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-21 02:40 +0200
        Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Al Viro <viro@ZenIV.linux.org.uk> - 2017-09-21 04:50 +0200
          Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-21 07:10 +0200
            Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Theodore Ts'o <tytso@mit.edu> - 2017-09-21 17:00 +0200
              Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount  completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-21 19:20 +0200
    Re: [PATCH v3] vfs: introduce UMOUNT_WAIT to wait for  delayed_fput/mntput completion Jaegeuk Kim <jaegeuk@kernel.org> - 2017-09-21 20:30 +0200

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