On Thu, Jul 25, 2019 at 09:32:13AM -0700, Linus Torvalds wrote:
> On Thu, Jul 25, 2019 at 6:26 AM Cyril Hrubis <chrubis@suse.cz> wrote:
> >
> > This looks like mkfs.vfat got EBUSY after the loop device was
> > succesfully umounted.
> 
> Hmm. Smells like the RCU-delaying got triggered again.
> 
> We have that "synchronize_rcu_expedited()" in namespace_unlock(),
> which is so that everything should be done by the time we return to
> user space.
> 
> Al, maybe that RCU synchronization should be after the mntput()s?

There are several interesting issues in there, but synchronize_rcu()
should be between zeroing ->mnt_ns and dropping the final refs.
I'm digging through that crap right now; at least one bug is dealt
with by #fixes, but there's more, unfortunately.