Comment # 6 on bug 1231836 from Fabian Vogt

(In reply to Wenruo Qu from comment #5)
> The direct cause is, when the initial super RO flag mismatches, we got EBUSY
> and go btrfs_reconfigure_for_mount(), which flips our RO flag and retry.
> 
> But since the background process is also re-mounting which can flips the RO
> flag to a different one, we got -EBUSY again because the newly flipped RO
> flag conflicts with the newly remounted flag.

Heh, so I got a lucky guess!

> This involves a lot of VFS calls which can be a little complex, but I'll try
> if we can do a mutex or something like that to avoid remount/mount to race
> on the same btrfs.

I saw your patch on the ML, which went into a different direction with more
brute force...

Your approach of just retrying until no more -EBUSY probably works, but I
wonder whether this retry loop is a good idea. In two cases it might retry
indefinitely:

a) -EBUSY might be returned for other reasons (if not possible right now, maybe
in the future?)
b) Something flips ro/rw all the time (rather artificial, but technically
possible)