test4: loop deadlock

[Fruhwirth Clemens]

Block-backed loop devices can cause a deadlock.

How to reproduce:

You need ~100mb partition.
Boot with mem=32m (otherwise the cache will be to big in the most cases) 

swapoff -a
losetup /dev/loop0 /dev/hdXX
for i in `seq 1 10`; do
 dd if=/dev/zero of=/dev/loop0 &
done

Now the box "should" freeze. This doesn't happen with filebased read/write
or blockbased read. Jari's loop survives this stress test without problems.
So probably we should continue with merging his loop changes. Andries, are
you still interested in that?

(As long term solution for the indisputable ugliness of loop.c: just throw
away block-backend support and move the cryptostuff to device mapper:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105967481007242&w=2 
The patch is working great, btw, much more stable than loop+cryptoloop.
I'd really like to see it merged)

Regards, Clemens

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