[Bug 214326] nullfs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214326

            Bug ID: 214326
           Summary: nullfs
           Product: Base System
           Version: 10.3-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: root at linkage.white-void.net

Created attachment 176786
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=176786&action=edit
An experimental patch for unionfs internal cache(Nov 8, 2016)

** This report may be related to, or duplicate of Bug #186360 **

With unionfs+nullfs, I saw a strange behaviour.

# mkdir mountfrom1 mountfrom2 mountto
# mkdir mountfrom1/a
# touch mountfrom2/b
# mount -t unionfs mountfrom1 mountto
# mount -t nullfs mountfrom2 mountto/a
# ls mountto/a/b
ls: mountto/a/b: No such file or directory
# ls mountto/a
b
# ls mountto/a/b
mountto/a/b

I digged into unionfs module, and found 'unionfs_lookup' is returning different
vnode for same single object 'a', for mount and later "ls" command.
It is caused by a bug in unionfs's internal cache (I think), and I solved it
with an attached patch.

Well, my patch omits a check for 'ISLASTCN', but I don't know why the check is
needed.

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