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Date: Sat, 20 Aug 2016 01:30:34 +0200
From: David Gauchard <gauchard@laas.fr>
To: linux-fsdevel@vger.kernel.org
Subject: unionfs-fuse freeze, starting with Linux 4.0
Message-ID: <20160819233034.GC22757@laas.fr>
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Sender: linux-fsdevel-owner@vger.kernel.org
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Hello,

I am sent here by the libfuse maintainer with the following issue:

[ https://github.com/libfuse/libfuse/issues/57 ]

Using unionfs-fuse, here is a simple script that freezes the caller when
accessing to the mounted directory.  After some tests, it appears that this
script is working with previous versions of the kernel's fuse-api (with any
version of unionfs or fuse even the latests).

On <v4 kernel, it works (kernel/fuse api up to 7.22).
On 4+ kernels (or kernel's fuse-api 7.23), it freezes (the script stops on
the "freezing?" message, a reboot is needed to sort this out)

The idea behind this script is mounting an existing directory above itself
with modifcations pushed to another one.  I'm aware I'm (was) exploiting
some sort of out-of-purpose side effect.

There is at least a bug (freezing), but it would be great to restore the
feature back.

Best

david


#!/bin/sh
set -x
while umount /tmp/test; do true; done
set -e
rm -rf /tmp/rw
mkdir -p /tmp/test /tmp/rootbind /tmp/rw
date > /tmp/test/a
mount -o bind / /tmp/rootbind
unionfs -o allow_other,use_ino,suid,dev,nonempty,cow /tmp/rw=RW:/tmp/rootbind/tmp/test=RO /tmp/test
echo freezing?
echo not freezing >> /tmp/test/a
cat /tmp/test/a
cat /tmp/rootbind/tmp/test/a