panic on 4.20 server exporting xfs filesystem

[Christoph Hellwig]

FYI, I've now managed to reproduce the issue.  I haven't had time
to dig deeper, but it smells a lot like a callback path issue.

Can you send the recursion fix to Linus ASAP and I'll send you a patch
to turn the nopnfs option into a pnfs one, so we're at least doing
fine for 4.0 in any case.  I hope to debug this and may even have
a real fix soon, but so far I don't know how long it will take.