md raid6: slow resync if fs is mounted rw

Dear list,

I am seeing a strange performance issue on a 4 x 4TB md raid6 on kernel
3.12. Initial resync goes nice and fast at 130MB/s:

# cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md1 : active raid6 sda3[0] sdd3[3] sdc3[2] sdb3[1]
      7751269376 blocks super 1.2 level 6, 512k chunk, algorithm 2 [4/4] [UUUU]
      [=>...................]  resync =  7.5% (293925504/3875634688) finish=458.4min speed=130210K/sec

md0 : active raid1 sda2[0] sdd2[3] sdc2[2] sdb2[1]
      29280128 blocks super 1.2 [4/4] [UUUU]

unused devices: <none>

...unless the ext4 filesystem on the array is mounted read-write!
Then, speed plummets to 1 - 7MB/s. The filesystem is empty and totally
idle; a simple "mount -o remount,ro" restores the speed to 130MB/s.

I dumped the /sys/block/md1/md tree with the filesystem mounted ro and
rw, the diff looks like this:

So, stripe_cache_active is zero if the fs is mounted rw. Hmm.

Interestingly, if I increase sync_speed_min to 100MB/s, I do get 100MB/s.

Any idea what is going on?

Best regards,
Jakob Unterwurzacher