59409cb90f
This is aimed at preventing stacked filesystems like nullfs and unionfs from "losing" their lower mounts due to forced unmount. Otherwise, VFS operations that are passed through to the lower filesystem(s) may crash or otherwise cause unpredictable behavior. Introduce two new functions: vfs_pin_from_vp() and vfs_unpin(). which are intended to be called on the lower mount(s) when the stacked filesystem is mounted and unmounted, respectively. Much as registration in the mnt_uppers list previously did, pinning will prevent even forced unmount of the lower FS and will allow the stacked FS to freely operate on the lower mount either by direct use of the struct mount* or indirect use through a properly-referenced vnode's v_mount field. vfs_pin_from_vp() is modeled after vfs_ref_from_vp() in that it uses the mount interlock coupled with re-checking vp->v_mount to ensure that it will fail in the face of a pending unmount request, even if the concurrent unmount fully completes. Adopt these new functions in both nullfs and unionfs. Reviewed By: kib, markj Differential Revision: https://reviews.freebsd.org/D30401
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