Description of problem:
Testing BZ1320211 found below error output

mdadm: failed to read /proc/mdstat while unblocking container

Version-Release number of selected component (if applicable):
mdadm-3.4-2.el6
2.6.32-634.el6.x86_64

How reproducible:
70%

Steps to Reproduce:
mdadm -CR /dev/md/imsm0 -e imsm -n3 /dev/sd[bcd]
mdadm -CR /dev/md/vol0 -l0 -n2 /dev/sd[bc]
cat /proc/mdstat 
mdadm --wait /dev/md/vol0
mdadm -D /dev/md126 
export MDADM_EXPERIMENTAL=1
mdadm -G /dev/md/imsm0 -n3
sleep 0.5
mdadm -Ss


Actual results:


Expected results:


Additional info:
This issue also can be reproduced with upstream mdadm

+ mdadm -CR /dev/md/imsm0 -e imsm -n3 /dev/sdb /dev/sdc /dev/sdd
mdadm: /dev/sdb appears to contain an ext2fs file system
       size=262144K  mtime=Mon Nov  9 10:42:05 2015
mdadm: /dev/sdb appears to be part of a raid array:
       level=container devices=0 ctime=Thu Jan  1 08:00:00 1970
mdadm: /dev/sdc appears to be part of a raid array:
       level=container devices=0 ctime=Thu Jan  1 08:00:00 1970
mdadm: /dev/sdd appears to be part of a raid array:
       level=container devices=0 ctime=Thu Jan  1 08:00:00 1970
mdadm: container /dev/md/imsm0 prepared.
+ mdadm -CR /dev/md/vol0 -l0 -n2 /dev/sdb /dev/sdc
mdadm: /dev/sdb appears to contain an ext2fs file system
       size=262144K  mtime=Mon Nov  9 10:42:05 2015
mdadm: /dev/sdb appears to be part of a raid array:
       level=container devices=0 ctime=Thu Jan  1 08:00:00 1970
mdadm: partition table exists on /dev/sdb but will be lost or
       meaningless after creating array
mdadm: /dev/sdc appears to be part of a raid array:
       level=container devices=0 ctime=Thu Jan  1 08:00:00 1970
mdadm: partition table exists on /dev/sdc but will be lost or
       meaningless after creating array
mdadm: Creating array inside imsm container md127
mdadm: array /dev/md/vol0 started.
+ cat /proc/mdstat
Personalities : [raid0] [raid6] [raid5] [raid4] 
md126 : active raid0 sdc[1] sdb[0]
      1953518592 blocks super external:/md127/0 128k chunks
      
md127 : inactive sdd[2](S) sdc[1](S) sdb[0](S)
      3315 blocks super external:imsm
       
unused devices: <none>
+ mdadm --wait /dev/md/vol0
+ mdadm -D /dev/md126
/dev/md126:
      Container : /dev/md/imsm0, member 0
     Raid Level : raid0
     Array Size : 1953518592 (1863.02 GiB 2000.40 GB)
   Raid Devices : 2
  Total Devices : 2

          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

     Chunk Size : 128K


           UUID : ecec1252:2e0e4751:bd5f90b5:879242a8
    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       1       8       32        1      active sync   /dev/sdc
+ export MDADM_EXPERIMENTAL=1
+ MDADM_EXPERIMENTAL=1
+ mdadm -G /dev/md/imsm0 -n3
mdadm: multi-array reshape continues in background
+ sleep 0.5
mdadm: level of /dev/md/vol0 changed to raid4
mdadm: Need to backup 768K of critical section..
+ mdadm -Ss
[root@dhcp-12-163 ~]# mdadm: failed to read /proc/mdstat while unblocking container