mdraid10 regression in 2.6.27.4 (possibly earlier)

[Peter Rabbitson <rabbit+list@rabbit.us>]

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Hi,

Some weeks ago I upgraded from 2.6.23 to 2.6.27.4. After a failed hard
drive I realized that re-adding drives to a degraded raid10 no longer
works (it adds the drive as a spare and never starts a resync). Booting
back into the old .23 kernel allowed me to complete and resync the array
as usual. Attached find a test case reliably failing on vanilla 2.6.27.4
with no patches.

Thank you



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#!/bin/bash

set -e
[ -e /dev/loop1 ] || modprobe loop

for i in 1 2 3 4; do
    dd if=/dev/zero of=blkloop_$i bs=10M count=1
    losetup /dev/loop$i blkloop_$i
done

mdadm -C /dev/md7 -n 4 -l 10 -p f3 /dev/loop1 /dev/loop2 /dev/loop3 /dev/loop4

# wait for sync
sleep 2

mdadm -f /dev/md7 /dev/loop1
mdadm -r /dev/md7 /dev/loop1

mdadm -a /dev/md7 /dev/loop1
for i in 1 2 3 4; do
    cat /proc/mdstat
    sleep 2
done


mdadm -S /dev/md7
for i in 1 2 3 4; do
    losetup -d /dev/loop$i
    rm blkloop_$i
done