the vnode operations vector for active vnodes is unsafe because it
is not known whether deadfs or the original file system will be
called.
- Pass down LK_RETRY to the lock operation (hint for deadfs only).
- Change deadfs lock operation to return ENOENT if LK_RETRY is unset.
- Change all other lock operations to check for dead vnode once
the vnode is locked and unlock and return ENOENT in this case.
With these changes in place vnode lock operations will never succeed
after vclean() has marked the vnode as VI_XLOCK and before vclean()
has changed the operations vector.
Adresses PR kern/37706 (Forced unmount of file systems is unsafe)
Discussed on tech-kern.
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