Re: [syzbot] WARNING in __init_work

[Christoph Hellwig]

On Mon, Sep 20, 2021 at 05:38:59AM -0700, Paul E. McKenney wrote:
> > Well, the block code already does a bdi_unregister in del_gendisk.
> > So if we end up freeing the whole device bdev with a registered bdi
> > something is badly going wrong.  Unfortunately the log in this report
> > isn't much help on how we got there.  IIRC syzbot will eventually spew
> > out a reproducer, so it might be worth to wait for that.
> 
> If it does turn out that you need to block in an RCU callback,
> queue_rcu_work() can be helpful.  This schedules a workqueue from the RCU
> callback, allowing the function passed to the preceding INIT_RCU_WORK()
> to block.

In this case we really should not block here.  The problem is that
we are hitting the strange bdi auto-unregister misfeature due to a bug
elsewhere.  Which reminds that I have a patch series to remove this
auto unregistration which I need to bring bag once this is fixed.

That being said queue_rcu_work would have been really useful in a few
places I touched in that past.