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I wanted to add another disk to a RAID6 array I have. So I ran
$ mdadm --add /dev/md127 /dev/sdj1
$ mdadm --grow --raid-devices=8 --backup-file=/boot/grow_md127.bak  /dev/md127

This appeared to work right, but looking at /proc/mdstat, it says

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md127 : active raid6 sdd1[8] sdj[6] sdg[0] sdk[4] sdh[1] sdi[2] sdc[5] sda1[7]
      14650675200 blocks super 1.2 level 6, 512k chunk, algorithm 2
[8/8] [UUUUUUUU]
      [>....................]  reshape =  0.0% (1/2930135040)
finish=445893299483.7min speed=0K/sec

unused devices: <none>

That is, it's stuck. And it's been that way since (about 36h now)

Looking at some logs, I found this in messages:

Jan 28 20:24:27 ooo systemd: Created slice
system-mdadm\x2dgrow\x2dcontinue.slice.
Jan 28 20:24:27 ooo audit: SERVICE_START pid=1 uid=0 auid=4294967295
ses=4294967295 subj=system_u:system_r:init_t:s0
msg='unit=mdadm-grow-continue@md127 comm="systemd"
exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=?
 res=success'
Jan 28 20:24:27 ooo systemd: Starting system-mdadm\x2dgrow\x2dcontinue.slice.
Jan 28 20:24:28 ooo audit: AVC avc:  denied  { write } for  pid=11103
comm="mdadm" name="grow_md127.bak" dev="sdf1" ino=426
scontext=system_u:system_r:mdadm_t:s0
tcontext=unconfined_u:object_r:boot_t:s0 tclass=file permissive=0
Jan 28 20:24:28 ooo audit: SYSCALL arch=c000003e syscall=2 success=no
exit=-13 a0=ec1fc0 a1=242 a2=180 a3=7800 items=0 ppid=1 pid=11103
auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0
fsgid=0 tty=(none) ses=4294
967295 comm="mdadm" exe="/usr/sbin/mdadm"
subj=system_u:system_r:mdadm_t:s0 key=(null)
Jan 28 20:24:28 ooo systemd: mdadm-grow-continue: Main
process exited, code=exited, status=1/FAILURE
Jan 28 20:24:28 ooo systemd: mdadm-grow-continue: Unit
entered failed state.
Jan 28 20:24:28 ooo audit: SERVICE_STOP pid=1 uid=0 auid=4294967295
ses=4294967295 subj=system_u:system_r:init_t:s0
msg='unit=mdadm-grow-continue@md127 comm="systemd"
exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=?
res=failed'
Jan 28 20:24:28 ooo systemd: mdadm-grow-continue: Failed
with result 'exit-code'.
Jan 28 20:24:32 ooo setroubleshoot: SELinux is preventing
/usr/sbin/mdadm from write access on the file grow_md127.bak. For
complete SELinux messages. run sealert -l
43815f80-8b00-40d9-86a3-4a6a432f3e05
Jan 28 20:24:32 ooo python3: SELinux is preventing /usr/sbin/mdadm
from write access on the file grow_md127.bak.#012#012*****  Plugin
kernel_modules (91.4 confidence) suggests
********************#012#012If you do not think m
dadm should try write access on grow_md127.bak.#012Then you may be
under attack by a hacker, since confined applications should not need
this access.#012Do#012contact your security administrator and report
this issue.#012#012**
***  Plugin catchall (9.59 confidence) suggests
**************************#012#012If you believe that mdadm should be
allowed write access on the grow_md127.bak file by default.#012Then
you should report this as a bug.#012You
 can generate a local policy module to allow this
access.#012Do#012allow this access for now by executing:#012# grep
mdadm /var/log/audit/audit.log | audit2allow -M mypol#012# semodule -i
mypol.pp#012

So it seems selinux is preventing writes to the backup file I specified.
(I put it in /boot, since that's the only file system I have that's
not on the array.)
Interestingly, the file exists:

$ ls -l /boot/grow_md127.bak
-rw-------. 1 root root 15732736 Jan 28 20:24 /boot/grow_md127.bak
$

I think the root cause here is that my invocation of mdadm was
unconfined, because I was running it interactively.
So it could create the backup file. But apparently, it fires something like an at job to finish up.
And the background version starts via systemd, and runs as mdadm_t, which
can't read that file (because it's in /boot). And poof.

I'm not sure how to fix this. It needs access to that file, and it's user specified on the first invocation.

The workaround in my case was to do the suggested audit2allow thing from the log
and with that added rule installed:
$ mdadm --grow --continue --backup-file=/boot/grow_md127.bak /dev/md127

The system is an uptodate Fedora 23, x86_64, with kernel
4.3.3-303.fc23.x86_64 and mdadm-3.3.4-2.fc23.x86_64, selinux-policy-3.13.1-158.2.fc23.noarch.


Reproducible: Didn't try

Steps to Reproduce:
1. Add a disk 
2. $ mdadm --add /dev/mdX /dev/sdY
3. $ mdadm --grow --raid-devices=<new number> --backup-file=/boot/grow_md127.bak  /dev/mdX
4. Observe that no progress is made in /proc/mdstat
Actual Results:  
No progress on the reshape.

Expected Results:  
Reshape should complete.

This thread on linux-raid http://www.spinics.net/lists/raid/msg51400.html has background.