md: Don't ignore read-only array in md_check_recovery()

In the Linux kernel, the following vulnerability has been resolved:

md: Don't ignore read-only array in md_check_recovery()

Usually if the array is not read-write, md_check_recovery() won't

register new sync_thread in the first place. And if the array is

read-write and sync_thread is registered, md_set_readonly() will

unregister sync_thread before setting the array read-only. md/raid

follow this behavior hence there is no problem.

After commit f52f5c71f3d4 ("md: fix stopping sync thread"), following

hang can be triggered by test shell/integrity-caching.sh:

1) array is read-only. dm-raid update super block:

rs_update_sbs

ro = mddev->ro

mddev->ro = 0

-> set array read-write

md_update_sb

2) register new sync thread concurrently.

3) dm-raid set array back to read-only:

rs_update_sbs

mddev->ro = ro

4) stop the array:

raid_dtr

md_stop

stop_sync_thread

set_bit(MD_RECOVERY_INTR, &mddev->recovery);

md_wakeup_thread_directly(mddev->sync_thread);

wait_event(..., !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))

5) sync thread done:

md_do_sync

set_bit(MD_RECOVERY_DONE, &mddev->recovery);

md_wakeup_thread(mddev->thread);

6) daemon thread can't unregister sync thread:

md_check_recovery

if (!md_is_rdwr(mddev) &&

!test_bit(MD_RECOVERY_NEEDED, &mddev->recovery))

return;

-> -> MD_RECOVERY_RUNNING can't be cleared, hence step 4 hang;

The root cause is that dm-raid manipulate 'mddev->ro' by itself,

however, dm-raid really should stop sync thread before setting the

array read-only. Unfortunately, I need to read more code before I

can refacter the handler of 'mddev->ro' in dm-raid, hence let's fix

the problem the easy way for now to prevent dm-raid regression.