linux-kernel - Loop device partition scanning is unreliable

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Date:	Thu, 19 Jul 2012 09:42:40 -0600
From:	Daniel Drake <dsd@...top.org>
To:	kzak@...hat.com, kay.sievers@...y.org
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Loop device partition scanning is unreliable

Hi,

I'm having trouble with the loop device partition scanning code.

If I create a blank file, put a partition table on it with fdisk, and
then immediately turn it into a partitioned loop device, the
partitions dont always show up.

Here is a script to test this:
http://dev.laptop.org/~dsd/20120719/loop-partition.sh

I have reproduced this on 5 systems, a mixture of 32 and 64 bit. It
doesn't seem to matter if the underlying filesystem is ext4 or tmpfs.
I've reproduced it on 3.3, 3.4.5 and 3.5-rc7.

On some systems it seems to always fail within 8 loops. On others it
takes more time (100+ loops). I think it crashes more reliable when
the system is under load - I'm testing with stress
(http://weather.ou.edu/~apw/projects/stress/): stress -c 6 -m 6 -d 1

Any thoughts?

Thanks,
Daniel
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