linux-kernel - task kworker / kloopd blocked for more than 120 seconds

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Message-ID: <20150528113759.03088f57@leda.localdomain>
Date:	Thu, 28 May 2015 11:37:59 +0200
From:	Christian Hesse <list@...rm.de>
To:	linux-kernel@...r.kernel.org
Subject: task kworker / kloopd blocked for more than 120 seconds

Hello everybody,

I am experiencing issues on a live system based on Arch Linux [0][1] with
Linux kernel 4.0.x when booted in RAM mode (parameter 'copytoram') [2].
Probably the problem exists some time longer, I am not sure when this first
happened and what versions are effected.

The boot sequence looks like this:

-> boot kernel
-> start initramfs
-> get the squashfs image file (there is no difference with loop mount,
   nbd or http)
-> root filesystem is set up with loop devices and device mapper
-> boot continues

The root filesystem is accessed as follows:

-> ext4 filesystem
  -> device mapper snapshot target
    -> /dev/loop1: r/o ext4 image
      -> mounted squashfs
        -> /dev/loop0: squashfs image
    -> /dev/loop2: r/w COW image (sparse file)

    
Assembling the root filesystem stack always succeeds, but sometimes boot hangs
with a lot of:

INFO: task kworker/u#:#:### blocked for more than 120 seconds

and a single:

INFO: task kloopd/### blocked for more than 120 seconds

Any idea what goes wrong? Looks like a race condition or deadlock.

[0] https://wiki.archlinux.org/index.php/Archiso
[1] https://projects.archlinux.org/archiso.git/
[2] https://projects.archlinux.org/archiso.git/tree/docs/README.bootparams
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