[Bug 279138] NFS and NFSUPG and BUFWAIT

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279138

--- Comment #3 from Konstantin Belousov <kib@FreeBSD.org> ---
The first LoR looks as a false positive, mostly.  It is readdirplus doing
scan of the directory, and then querying the attributes of each found entry'
node, except dotdot.  So the lock order is parent vnode -> (it's buffers) ->
child vnode, as expected by VFS.

I said "mostly" because server can do the directory move like from A->B to
B->A while client is not aware, and I am not completely sure that our client-
side invalidations would avoid tricking in this situation.  But this is very
unlikely.

For the second LoR, nfsupg->bufwait, the reported order is right.  I wonder
where the reversed order was recorded.

That said, to diagnose the problem, you need to gather the information listed
in the developer's handbook, debugging kernels, debugging deadlocks:
https://docs.freebsd.org/en/books/developers-handbook/kerneldebug/#kerneldebug-deadlocks

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