PROBLEM: md/raid5 bug by commit 415e72d03

[Qin Dehua]

By bisecting, commit 415e72d03(md/raid5: Allow recovered part of
partially recovered devices to be in-sync) was found as the cause of
follow problem:
	* md1_raid5 or md1_raid6 process will hang(99% CPU usage) after
repeatedly remove disk from then re-add disk to raid5 or raid6（there
is also a dd process write to the raid continuously).
	
Hardware platform is IOP 341 XScale processor(did not test on other
platforms）. The problem can be reproduced with this script:

{ while true; do dd if=/dev/zero of=/dev/md1 bs=1M count=90000 >
/dev/null 2>/dev/null;done; } &

while true;do
	mdadm /dev/md1 -f /dev/sda /dev/sdb
	sleep 1
	mdadm /dev/md1 -r /dev/sda /dev/sdb
	sleep 1
	mdadm /dev/md1 -a /dev/sda /dev/sdb
	sleep 6
done

Regards,
Qin Dehua
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