text: /*	$NetBSD: union_vnops.c,v 1.36 1997/10/10 02:01:08 fvdl Exp $	*/
  
  /*
   * Copyright (c) 1992, 1993, 1994 The Regents of the University of California.
   * Copyright (c) 1992, 1993, 1994 Jan-Simon Pendry.
   * All rights reserved.
   *
   * This code is derived from software contributed to Berkeley by
   * Jan-Simon Pendry.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   * 3. All advertising materials mentioning features or use of this software
   *    must display the following acknowledgement:
   *	This product includes software developed by the University of
   *	California, Berkeley and its contributors.
   * 4. Neither the name of the University nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   * SUCH DAMAGE.
   *
   *	@(#)union_vnops.c	8.22 (Berkeley) 12/10/94
   */
  
  #include <sys/param.h>
  #include <sys/systm.h>
  #include <sys/proc.h>
  #include <sys/file.h>
  #include <sys/time.h>
  #include <sys/stat.h>
  #include <sys/types.h>
  #include <sys/vnode.h>
  #include <sys/mount.h>
  #include <sys/namei.h>
  #include <sys/malloc.h>
  #include <sys/buf.h>
  #include <sys/queue.h>
  #include <miscfs/union/union.h>
  
  /*
   * Global vfs data structures
   */
  
  int union_lookup	__P((void *));
  int union_create	__P((void *));
  int union_whiteout	__P((void *));
  int union_mknod		__P((void *));
  int union_open		__P((void *));
  int union_close		__P((void *));
  int union_access	__P((void *));
  int union_getattr	__P((void *));
  int union_setattr	__P((void *));
  int union_read		__P((void *));
  int union_write		__P((void *));
  int union_lease		__P((void *));
  int union_ioctl		__P((void *));
  int union_poll		__P((void *));
  int union_mmap		__P((void *));
  int union_fsync		__P((void *));
  int union_seek		__P((void *));
  int union_remove	__P((void *));
  int union_link		__P((void *));
  int union_rename	__P((void *));
  int union_mkdir		__P((void *));
  int union_rmdir		__P((void *));
  int union_symlink	__P((void *));
  int union_readdir	__P((void *));
  int union_readlink	__P((void *));
  int union_abortop	__P((void *));
  int union_inactive	__P((void *));
  int union_reclaim	__P((void *));
  int union_lock		__P((void *));
  int union_unlock	__P((void *));
  int union_bmap		__P((void *));
  int union_print		__P((void *));
  int union_islocked	__P((void *));
  int union_pathconf	__P((void *));
  int union_advlock	__P((void *));
  int union_strategy	__P((void *));
  
  int (**union_vnodeop_p) __P((void *));
  struct vnodeopv_entry_desc union_vnodeop_entries[] = {
  	{ &vop_default_desc, vn_default_error },
  	{ &vop_lookup_desc, union_lookup },		/* lookup */
  	{ &vop_create_desc, union_create },		/* create */
  	{ &vop_whiteout_desc, union_whiteout },		/* whiteout */
  	{ &vop_mknod_desc, union_mknod },		/* mknod */
  	{ &vop_open_desc, union_open },			/* open */
  	{ &vop_close_desc, union_close },		/* close */
  	{ &vop_access_desc, union_access },		/* access */
  	{ &vop_getattr_desc, union_getattr },		/* getattr */
  	{ &vop_setattr_desc, union_setattr },		/* setattr */
  	{ &vop_read_desc, union_read },			/* read */
  	{ &vop_write_desc, union_write },		/* write */
  	{ &vop_lease_desc, union_lease },		/* lease */
  	{ &vop_ioctl_desc, union_ioctl },		/* ioctl */
  	{ &vop_poll_desc, union_poll },			/* poll */
  	{ &vop_mmap_desc, union_mmap },			/* mmap */
  	{ &vop_fsync_desc, union_fsync },		/* fsync */
  	{ &vop_seek_desc, union_seek },			/* seek */
  	{ &vop_remove_desc, union_remove },		/* remove */
  	{ &vop_link_desc, union_link },			/* link */
  	{ &vop_rename_desc, union_rename },		/* rename */
  	{ &vop_mkdir_desc, union_mkdir },		/* mkdir */
  	{ &vop_rmdir_desc, union_rmdir },		/* rmdir */
  	{ &vop_symlink_desc, union_symlink },		/* symlink */
  	{ &vop_readdir_desc, union_readdir },		/* readdir */
  	{ &vop_readlink_desc, union_readlink },		/* readlink */
  	{ &vop_abortop_desc, union_abortop },		/* abortop */
  	{ &vop_inactive_desc, union_inactive },		/* inactive */
  	{ &vop_reclaim_desc, union_reclaim },		/* reclaim */
  	{ &vop_lock_desc, union_lock },			/* lock */
  	{ &vop_unlock_desc, union_unlock },		/* unlock */
  	{ &vop_bmap_desc, union_bmap },			/* bmap */
  	{ &vop_strategy_desc, union_strategy },		/* strategy */
  	{ &vop_print_desc, union_print },		/* print */
  	{ &vop_islocked_desc, union_islocked },		/* islocked */
  	{ &vop_pathconf_desc, union_pathconf },		/* pathconf */
  	{ &vop_advlock_desc, union_advlock },		/* advlock */
  #ifdef notdef
  	{ &vop_blkatoff_desc, union_blkatoff },		/* blkatoff */
  	{ &vop_valloc_desc, union_valloc },		/* valloc */
  	{ &vop_vfree_desc, union_vfree },		/* vfree */
  	{ &vop_truncate_desc, union_truncate },		/* truncate */
  	{ &vop_update_desc, union_update },		/* update */
  	{ &vop_bwrite_desc, union_bwrite },		/* bwrite */
  #endif
  	{ (struct vnodeop_desc*)NULL, (int(*) __P((void *)))NULL }
  };
  struct vnodeopv_desc union_vnodeop_opv_desc =
  	{ &union_vnodeop_p, union_vnodeop_entries };
  
  #define FIXUP(un) { \
  	if (((un)->un_flags & UN_ULOCK) == 0) { \
  		union_fixup(un); \
  	} \
  }
  static void union_fixup __P((struct union_node *));
  static int union_lookup1 __P((struct vnode *, struct vnode **,
  			      struct vnode **, struct componentname *));
  
  
  static void
  union_fixup(un)
  	struct union_node *un;
  {
  
  	VOP_LOCK(un->un_uppervp);
  	un->un_flags |= UN_ULOCK;
  }
  
  static int
  union_lookup1(udvp, dvpp, vpp, cnp)
  	struct vnode *udvp;
  	struct vnode **dvpp;
  	struct vnode **vpp;
  	struct componentname *cnp;
  {
  	int error;
  	struct vnode *tdvp;
  	struct vnode *dvp;
  	struct mount *mp;
  
  	dvp = *dvpp;
  
  	/*
  	 * If stepping up the directory tree, check for going
  	 * back across the mount point, in which case do what
  	 * lookup would do by stepping back down the mount
  	 * hierarchy.
  	 */
  	if (cnp->cn_flags & ISDOTDOT) {
  		while ((dvp != udvp) && (dvp->v_flag & VROOT)) {
  			/*
  			 * Don't do the NOCROSSMOUNT check
  			 * at this level.  By definition,
  			 * union fs deals with namespaces, not
  			 * filesystems.
  			 */
  			tdvp = dvp;
  			*dvpp = dvp = dvp->v_mount->mnt_vnodecovered;
  			vput(tdvp);
  			VREF(dvp);
  			VOP_LOCK(dvp);
  		}
  	}
  
          error = VOP_LOOKUP(dvp, &tdvp, cnp);
  	if (error)
  		return (error);
  
  	/*
  	 * The parent directory will have been unlocked, unless lookup
  	 * found the last component.  In which case, re-lock the node
  	 * here to allow it to be unlocked again (phew) in union_lookup.
  	 */
  	if (dvp != tdvp && !(cnp->cn_flags & ISLASTCN))
  		VOP_LOCK(dvp);
  
  	dvp = tdvp;
  
  	/*
  	 * Lastly check if the current node is a mount point in
  	 * which case walk up the mount hierarchy making sure not to
  	 * bump into the root of the mount tree (ie. dvp != udvp).
  	 */
  	while (dvp != udvp && (dvp->v_type == VDIR) &&
  	       (mp = dvp->v_mountedhere)) {
  
  		if (mp->mnt_flag & MNT_MLOCK) {
  			mp->mnt_flag |= MNT_MWAIT;
  			sleep((caddr_t) mp, PVFS);
  			continue;
  		}
  
  		if ((error = VFS_ROOT(mp, &tdvp)) != 0) {
  			vput(dvp);
  			return (error);
  		}
  
  		vput(dvp);
  		dvp = tdvp;
  	}
  
  	*vpp = dvp;
  	return (0);
  }
  
  int
  union_lookup(v)
  	void *v;
  {
  	struct vop_lookup_args /* {
  		struct vnodeop_desc *a_desc;
  		struct vnode *a_dvp;
  		struct vnode **a_vpp;
  		struct componentname *a_cnp;
  	} */ *ap = v;
  	int error;
  	int uerror, lerror;
  	struct vnode *uppervp, *lowervp;
  	struct vnode *upperdvp, *lowerdvp;
  	struct vnode *dvp = ap->a_dvp;
  	struct union_node *dun = VTOUNION(dvp);
  	struct componentname *cnp = ap->a_cnp;
  	int lockparent = cnp->cn_flags & LOCKPARENT;
  	struct union_mount *um = MOUNTTOUNIONMOUNT(dvp->v_mount);
  	struct ucred *saved_cred = NULL;
  	int iswhiteout;
  	struct vattr va;
  
  #ifdef notyet
  	if (cnp->cn_namelen == 3 &&
  			cnp->cn_nameptr[2] == '.' &&
  			cnp->cn_nameptr[1] == '.' &&
  			cnp->cn_nameptr[0] == '.') {
  		dvp = *ap->a_vpp = LOWERVP(ap->a_dvp);
  		if (dvp == NULLVP)
  			return (ENOENT);
  		VREF(dvp);
  		VOP_LOCK(dvp);
  		if (!lockparent || !(cnp->cn_flags & ISLASTCN))
  			VOP_UNLOCK(ap->a_dvp);
  		return (0);
  	}
  #endif
  
  	cnp->cn_flags |= LOCKPARENT;
  
  	upperdvp = dun->un_uppervp;
  	lowerdvp = dun->un_lowervp;
  	uppervp = NULLVP;
  	lowervp = NULLVP;
  	iswhiteout = 0;
  
  	/*
  	 * do the lookup in the upper level.
  	 * if that level comsumes additional pathnames,
  	 * then assume that something special is going
  	 * on and just return that vnode.
  	 */
  	if (upperdvp != NULLVP) {
  		FIXUP(dun);
  		/*
  		 * If we're doing `..' in the underlying filesystem,
  		 * we must drop our lock on the union node before
  		 * going up the tree in the lower file system--if we block
  		 * on the lowervp lock, and that's held by someone else
  		 * coming down the tree and who's waiting for our lock,
  		 * we would be hosed.
  		 */
  		if (cnp->cn_flags & ISDOTDOT) {
  			/* retain lock on underlying VP: */
  			dun->un_flags |= UN_KLOCK;
  			VOP_UNLOCK(dvp);
  		}
  		uerror = union_lookup1(um->um_uppervp, &upperdvp,
  				       &uppervp, cnp);
  		if (cnp->cn_flags & ISDOTDOT) {
  			if (dun->un_uppervp == upperdvp) {
  				/*
  				 * we got the underlying bugger back locked...
  				 * now take back the union node lock.  Since we
  				 * hold the uppervp lock, we can diddle union
  				 * locking flags at will. :)
  				 */
  				dun->un_flags |= UN_ULOCK;
  			}
  			/*
  			 * if upperdvp got swapped out, it means we did
  			 * some mount point magic, and we do not have
  			 * dun->un_uppervp locked currently--so we get it
  			 * locked here (don't set the UN_ULOCK flag).
  			 */
  			VOP_LOCK(dvp);
  		}
  
  		/*if (uppervp == upperdvp)
  		  dun->un_flags |= UN_KLOCK;*/
  
  		if (cnp->cn_consume != 0) {
  			*ap->a_vpp = uppervp;
  			if (!lockparent)
  				cnp->cn_flags &= ~LOCKPARENT;
  			return (uerror);
  		}
  		if (uerror == ENOENT || uerror == EJUSTRETURN) {
  			if (cnp->cn_flags & ISWHITEOUT) {
  				iswhiteout = 1;
  			} else if (lowerdvp != NULLVP) {
  				lerror = VOP_GETATTR(upperdvp, &va,
  						     cnp->cn_cred,
  						     cnp->cn_proc);
  				if (lerror == 0 && (va.va_flags & OPAQUE))
  					iswhiteout = 1;
  			}
  		}
  	} else {
  		uerror = ENOENT;
  	}
  
  	/*
  	 * in a similar way to the upper layer, do the lookup
  	 * in the lower layer.   this time, if there is some
  	 * component magic going on, then vput whatever we got
  	 * back from the upper layer and return the lower vnode
  	 * instead.
  	 */
  	if (lowerdvp != NULLVP && !iswhiteout) {
  		int nameiop;
  
  		VOP_LOCK(lowerdvp);
  
  		/*
  		 * Only do a LOOKUP on the bottom node, since
  		 * we won't be making changes to it anyway.
  		 */
  		nameiop = cnp->cn_nameiop;
  		cnp->cn_nameiop = LOOKUP;
  		if (um->um_op == UNMNT_BELOW) {
  			saved_cred = cnp->cn_cred;
  			cnp->cn_cred = um->um_cred;
  		}
  		/*
  		 * we shouldn't have to worry about locking interactions
  		 * between the lower layer and our union layer (w.r.t.
  		 * `..' processing) because we don't futz with lowervp
  		 * locks in the union-node instantiation code path.
  		 */
  		lerror = union_lookup1(um->um_lowervp, &lowerdvp,
  				&lowervp, cnp);
  		if (um->um_op == UNMNT_BELOW)
  			cnp->cn_cred = saved_cred;
  		cnp->cn_nameiop = nameiop;
  
  		if (lowervp != lowerdvp)
  			VOP_UNLOCK(lowerdvp);
  
  		if (cnp->cn_consume != 0) {
  			if (uppervp != NULLVP) {
  				if (uppervp == upperdvp)
  					vrele(uppervp);
  				else
  					vput(uppervp);
  				uppervp = NULLVP;
  			}
  			*ap->a_vpp = lowervp;
  			if (!lockparent)
  				cnp->cn_flags &= ~LOCKPARENT;
  			return (lerror);
  		}
  	} else {
  		lerror = ENOENT;
  		if ((cnp->cn_flags & ISDOTDOT) && dun->un_pvp != NULLVP) {
  			lowervp = LOWERVP(dun->un_pvp);
  			if (lowervp != NULLVP) {
  				VREF(lowervp);
  				VOP_LOCK(lowervp);
  				lerror = 0;
  			}
  		}
  	}
  
  	if (!lockparent)
  		cnp->cn_flags &= ~LOCKPARENT;
  
  	/*
  	 * at this point, we have uerror and lerror indicating
  	 * possible errors with the lookups in the upper and lower
  	 * layers.  additionally, uppervp and lowervp are (locked)
  	 * references to existing vnodes in the upper and lower layers.
  	 *
  	 * there are now three cases to consider.
  	 * 1. if both layers returned an error, then return whatever
  	 *    error the upper layer generated.
  	 *
  	 * 2. if the top layer failed and the bottom layer succeeded
  	 *    then two subcases occur.
  	 *    a.  the bottom vnode is not a directory, in which
  	 *	  case just return a new union vnode referencing
  	 *	  an empty top layer and the existing bottom layer.
  	 *    b.  the bottom vnode is a directory, in which case
  	 *	  create a new directory in the top-level and
  	 *	  continue as in case 3.
  	 *
  	 * 3. if the top layer succeeded then return a new union
  	 *    vnode referencing whatever the new top layer and
  	 *    whatever the bottom layer returned.
  	 */
  
  	*ap->a_vpp = NULLVP;
  
  	/* case 1. */
  	if ((uerror != 0) && (lerror != 0)) {
  		return (uerror);
  	}
  
  	/* case 2. */
  	if (uerror != 0 /* && (lerror == 0) */ ) {
  		if (lowervp->v_type == VDIR) { /* case 2b. */
  			/*
  			 * We may be racing another process to make the
  			 * upper-level shadow directory.  Be careful with
  			 * locks/etc!
  			 */
  			dun->un_flags &= ~UN_ULOCK;
  			VOP_UNLOCK(upperdvp);
  			uerror = union_mkshadow(um, upperdvp, cnp, &uppervp);
  			VOP_LOCK(upperdvp);
  			dun->un_flags |= UN_ULOCK;
  
  			if (uerror) {
  				if (lowervp != NULLVP) {
  					vput(lowervp);
  					lowervp = NULLVP;
  				}
  				return (uerror);
  			}
  		}
  	}
  
  	if (lowervp != NULLVP)
  		VOP_UNLOCK(lowervp);
  
  	error = union_allocvp(ap->a_vpp, dvp->v_mount, dvp, upperdvp, cnp,
  			      uppervp, lowervp, 1);
  
  	if (error) {
  		if (uppervp != NULLVP)
  			vput(uppervp);
  		if (lowervp != NULLVP)
  			vrele(lowervp);
  	} else {
  		if (*ap->a_vpp != dvp)
  			if (!lockparent || !(cnp->cn_flags & ISLASTCN))
  				VOP_UNLOCK(dvp);
  		if (cnp->cn_namelen == 1 &&
  		    cnp->cn_nameptr[0] == '.' &&
  		    *ap->a_vpp != dvp) {
  		    panic("union_lookup returning . (%p) not same as startdir (%p)",
  			  ap->a_vpp, dvp);
  		}
  
  	}
  
  	return (error);
  }
  
  int
  union_create(v)
  	void *v;
  {
  	struct vop_create_args /* {
  		struct vnode *a_dvp;
  		struct vnode **a_vpp;
  		struct componentname *a_cnp;
  		struct vattr *a_vap;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_dvp);
  	struct vnode *dvp;
  
  	if ((dvp = un->un_uppervp) != NULLVP) {
  		int error;
  		struct vnode *vp;
  		struct mount *mp;
  
  		FIXUP(un);
  
  		VREF(dvp);
  		un->un_flags |= UN_KLOCK;
  		mp = ap->a_dvp->v_mount;
  		vput(ap->a_dvp);
  		error = VOP_CREATE(dvp, &vp, ap->a_cnp, ap->a_vap);
  		if (error)
  			return (error);
  
  		error = union_allocvp(
  				ap->a_vpp,
  				mp,
  				NULLVP,
  				NULLVP,
  				ap->a_cnp,
  				vp,
  				NULLVP,
  				1);
  		if (error)
  			vput(vp);
  		return (error);
  	}
  
  	vput(ap->a_dvp);
  	return (EROFS);
  }
  
  int
  union_whiteout(v)
  	void *v;
  {
  	struct vop_whiteout_args /* {
  		struct vnode *a_dvp;
  		struct componentname *a_cnp;
  		int a_flags;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_dvp);
  
  	if (un->un_uppervp == NULLVP)
  		return (EOPNOTSUPP);
  
  	FIXUP(un);
  	return (VOP_WHITEOUT(un->un_uppervp, ap->a_cnp, ap->a_flags));
  }
  
  int
  union_mknod(v)
  	void *v;
  {
  	struct vop_mknod_args /* {
  		struct vnode *a_dvp;
  		struct vnode **a_vpp;
  		struct componentname *a_cnp;
  		struct vattr *a_vap;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_dvp);
  	struct vnode *dvp;
  
  	if ((dvp = un->un_uppervp) != NULLVP) {
  		int error;
  		struct vnode *vp;
  		struct mount *mp;
  
  		FIXUP(un);
  
  		VREF(dvp);
  		un->un_flags |= UN_KLOCK;
  		mp = ap->a_dvp->v_mount;
  		vput(ap->a_dvp);
  		error = VOP_MKNOD(dvp, &vp, ap->a_cnp, ap->a_vap);
  		if (error)
  			return (error);
  
  		if (vp != NULLVP) {
  			error = union_allocvp(
  					ap->a_vpp,
  					mp,
  					NULLVP,
  					NULLVP,
  					ap->a_cnp,
  					vp,
  					NULLVP,
  					1);
  			if (error)
  				vput(vp);
  		}
  		return (error);
  	}
  
  	vput(ap->a_dvp);
  	return (EROFS);
  }
  
  int
  union_open(v)
  	void *v;
  {
  	struct vop_open_args /* {
  		struct vnodeop_desc *a_desc;
  		struct vnode *a_vp;
  		int a_mode;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  	struct vnode *tvp;
  	int mode = ap->a_mode;
  	struct ucred *cred = ap->a_cred;
  	struct proc *p = ap->a_p;
  	int error;
  
  	/*
  	 * If there is an existing upper vp then simply open that.
  	 */
  	tvp = un->un_uppervp;
  	if (tvp == NULLVP) {
  		/*
  		 * If the lower vnode is being opened for writing, then
  		 * copy the file contents to the upper vnode and open that,
  		 * otherwise can simply open the lower vnode.
  		 */
  		tvp = un->un_lowervp;
  		if ((ap->a_mode & FWRITE) && (tvp->v_type == VREG)) {
  			error = union_copyup(un, (mode&O_TRUNC) == 0, cred, p);
  			if (error == 0)
  				error = VOP_OPEN(un->un_uppervp, mode, cred, p);
  			return (error);
  		}
  
  		/*
  		 * Just open the lower vnode
  		 */
  		un->un_openl++;
  		VOP_LOCK(tvp);
  		error = VOP_OPEN(tvp, mode, cred, p);
  		VOP_UNLOCK(tvp);
  
  		return (error);
  	}
  
  	FIXUP(un);
  
  	error = VOP_OPEN(tvp, mode, cred, p);
  
  	return (error);
  }
  
  int
  union_close(v)
  	void *v;
  {
  	struct vop_close_args /* {
  		struct vnode *a_vp;
  		int  a_fflag;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  	struct vnode *vp;
  
  	if ((vp = un->un_uppervp) == NULLVP) {
  #ifdef UNION_DIAGNOSTIC
  		if (un->un_openl <= 0)
  			panic("union: un_openl cnt");
  #endif
  		--un->un_openl;
  		vp = un->un_lowervp;
  	}
  
  #ifdef	DIAGNOSTIC
  	/*
  	 * We should never encounter a vnode with both upper and
  	 * lower vnodes NULL.
  	 */
  	if (vp == NULLVP) {
  		vprint("empty union vnode", vp);
  		panic("union_close empty vnode");
  	}
  #endif
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_close), ap));
  }
  
  /*
   * Check access permission on the union vnode.
   * The access check being enforced is to check
   * against both the underlying vnode, and any
   * copied vnode.  This ensures that no additional
   * file permissions are given away simply because
   * the user caused an implicit file copy.
   */
  int
  union_access(v)
  	void *v;
  {
  	struct vop_access_args /* {
  		struct vnodeop_desc *a_desc;
  		struct vnode *a_vp;
  		int a_mode;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  	int error = EACCES;
  	struct vnode *vp;
  
  	if ((vp = un->un_uppervp) != NULLVP) {
  		FIXUP(un);
  		return (VOP_ACCESS(vp, ap->a_mode, ap->a_cred, ap->a_p));
  	}
  
  	if ((vp = un->un_lowervp) != NULLVP) {
  		VOP_LOCK(vp);
  		error = VOP_ACCESS(vp, ap->a_mode, ap->a_cred, ap->a_p);
  		if (error == 0) {
  			struct union_mount *um = MOUNTTOUNIONMOUNT(ap->a_vp->v_mount);
  
  			if (um->um_op == UNMNT_BELOW)
  				error = VOP_ACCESS(vp, ap->a_mode,
  						um->um_cred, ap->a_p);
  		}
  		VOP_UNLOCK(vp);
  		if (error)
  			return (error);
  	}
  
  	return (error);
  }
  
  /*
   * We handle getattr only to change the fsid and
   * track object sizes
   */
  int
  union_getattr(v)
  	void *v;
  {
  	struct vop_getattr_args /* {
  		struct vnode *a_vp;
  		struct vattr *a_vap;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	int error;
  	struct union_node *un = VTOUNION(ap->a_vp);
  	struct vnode *vp = un->un_uppervp;
  	struct vattr *vap;
  	struct vattr va;
  
  
  	/*
  	 * Some programs walk the filesystem hierarchy by counting
  	 * links to directories to avoid stat'ing all the time.
  	 * This means the link count on directories needs to be "correct".
  	 * The only way to do that is to call getattr on both layers
  	 * and fix up the link count.  The link count will not necessarily
  	 * be accurate but will be large enough to defeat the tree walkers.
  	 */
  
  	vap = ap->a_vap;
  
  	if ((vp = un->un_uppervp) != NULLVP) {
  		/*
  		 * It's not clear whether VOP_GETATTR is to be
  		 * called with the vnode locked or not.  stat() calls
  		 * it with (vp) locked, and fstat calls it with
  		 * (vp) unlocked.
  		 * In the mean time, compensate here by checking
  		 * the union_node's lock flag.
  		 */
  		if (un->un_flags & UN_LOCKED)
  			FIXUP(un);
  
  		error = VOP_GETATTR(vp, vap, ap->a_cred, ap->a_p);
  		if (error)
  			return (error);
  		union_newsize(ap->a_vp, vap->va_size, VNOVAL);
  	}
  
  	if (vp == NULLVP) {
  		vp = un->un_lowervp;
  	} else if (vp->v_type == VDIR) {
  		vp = un->un_lowervp;
  		vap = &va;
  	} else {
  		vp = NULLVP;
  	}
  
  	if (vp != NULLVP) {
  		error = VOP_GETATTR(vp, vap, ap->a_cred, ap->a_p);
  		if (error)
  			return (error);
  		union_newsize(ap->a_vp, VNOVAL, vap->va_size);
  	}
  
  	if ((vap != ap->a_vap) && (vap->va_type == VDIR))
  		ap->a_vap->va_nlink += vap->va_nlink;
  
  	ap->a_vap->va_fsid = ap->a_vp->v_mount->mnt_stat.f_fsid.val[0];
  	return (0);
  }
  
  int
  union_setattr(v)
  	void *v;
  {
  	struct vop_setattr_args /* {
  		struct vnode *a_vp;
  		struct vattr *a_vap;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  	int error;
  
  	/*
  	 * Handle case of truncating lower object to zero size,
  	 * by creating a zero length upper object.  This is to
  	 * handle the case of open with O_TRUNC and O_CREAT.
  	 */
  	if ((un->un_uppervp == NULLVP) &&
  	    /* assert(un->un_lowervp != NULLVP) */
  	    (un->un_lowervp->v_type == VREG)) {
  		error = union_copyup(un, (ap->a_vap->va_size != 0),
  						ap->a_cred, ap->a_p);
  		if (error)
  			return (error);
  	}
  
  	/*
  	 * Try to set attributes in upper layer,
  	 * otherwise return read-only filesystem error.
  	 */
  	if (un->un_uppervp != NULLVP) {
  		FIXUP(un);
  		error = VOP_SETATTR(un->un_uppervp, ap->a_vap,
  					ap->a_cred, ap->a_p);
  		if ((error == 0) && (ap->a_vap->va_size != VNOVAL))
  			union_newsize(ap->a_vp, ap->a_vap->va_size, VNOVAL);
  	} else {
  		error = EROFS;
  	}
  
  	return (error);
  }
  
  int
  union_read(v)
  	void *v;
  {
  	struct vop_read_args /* {
  		struct vnode *a_vp;
  		struct uio *a_uio;
  		int  a_ioflag;
  		struct ucred *a_cred;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp = OTHERVP(ap->a_vp);
  	int dolock = (vp == LOWERVP(ap->a_vp));
  
  	if (dolock)
  		VOP_LOCK(vp);
  	else
  		FIXUP(VTOUNION(ap->a_vp));
  	error = VOP_READ(vp, ap->a_uio, ap->a_ioflag, ap->a_cred);
  	if (dolock)
  		VOP_UNLOCK(vp);
  
  	/*
  	 * XXX
  	 * perhaps the size of the underlying object has changed under
  	 * our feet.  take advantage of the offset information present
  	 * in the uio structure.
  	 */
  	if (error == 0) {
  		struct union_node *un = VTOUNION(ap->a_vp);
  		off_t cur = ap->a_uio->uio_offset;
  
  		if (vp == un->un_uppervp) {
  			if (cur > un->un_uppersz)
  				union_newsize(ap->a_vp, cur, VNOVAL);
  		} else {
  			if (cur > un->un_lowersz)
  				union_newsize(ap->a_vp, VNOVAL, cur);
  		}
  	}
  
  	return (error);
  }
  
  int
  union_write(v)
  	void *v;
  {
  	struct vop_read_args /* {
  		struct vnode *a_vp;
  		struct uio *a_uio;
  		int  a_ioflag;
  		struct ucred *a_cred;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp;
  	struct union_node *un = VTOUNION(ap->a_vp);
  
  	vp = UPPERVP(ap->a_vp);
  	if (vp == NULLVP)
  		panic("union: missing upper layer in write");
  
  	FIXUP(un);
  	error = VOP_WRITE(vp, ap->a_uio, ap->a_ioflag, ap->a_cred);
  
  	/*
  	 * the size of the underlying object may be changed by the
  	 * write.
  	 */
  	if (error == 0) {
  		off_t cur = ap->a_uio->uio_offset;
  
  		if (cur > un->un_uppersz)
  			union_newsize(ap->a_vp, cur, VNOVAL);
  	}
  
  	return (error);
  }
  
  int
  union_lease(v)
  	void *v;
  {
  	struct vop_lease_args /* {
  		struct vnode *a_vp;
  		struct proc *a_p;
  		struct ucred *a_cred;
  		int a_flag;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_lease), ap));
  }
  
  int
  union_ioctl(v)
  	void *v;
  {
  	struct vop_ioctl_args /* {
  		struct vnode *a_vp;
  		u_long a_command;
  		caddr_t  a_data;
  		int  a_fflag;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_ioctl), ap));
  }
  
  int
  union_poll(v)
  	void *v;
  {
  	struct vop_poll_args /* {
  		struct vnode *a_vp;
  		int a_events;
  		struct proc *a_p;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_poll), ap));
  }
  
  int
  union_mmap(v)
  	void *v;
  {
  	struct vop_mmap_args /* {
  		struct vnode *a_vp;
  		int  a_fflags;
  		struct ucred *a_cred;
  		struct proc *a_p;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_mmap), ap));
  }
  
  int
  union_fsync(v)
  	void *v;
  {
  	struct vop_fsync_args /* {
  		struct vnode *a_vp;
  		struct ucred *a_cred;
  		int  a_waitfor;
  		struct proc *a_p;
  	} */ *ap = v;
  	int error = 0;
  	struct vnode *targetvp = OTHERVP(ap->a_vp);
  
  	if (targetvp != NULLVP) {
  		int dolock = (targetvp == LOWERVP(ap->a_vp));
  
  		if (dolock)
  			VOP_LOCK(targetvp);
  		else
  			FIXUP(VTOUNION(ap->a_vp));
  		error = VOP_FSYNC(targetvp, ap->a_cred,
  					ap->a_waitfor, ap->a_p);
  		if (dolock)
  			VOP_UNLOCK(targetvp);
  	}
  
  	return (error);
  }
  
  int
  union_seek(v)
  	void *v;
  {
  	struct vop_seek_args /* {
  		struct vnode *a_vp;
  		off_t  a_oldoff;
  		off_t  a_newoff;
  		struct ucred *a_cred;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_seek), ap));
  }
  
  int
  union_remove(v)
  	void *v;
  {
  	struct vop_remove_args /* {
  		struct vnode *a_dvp;
  		struct vnode *a_vp;
  		struct componentname *a_cnp;
  	} */ *ap = v;
  	int error;
  	struct union_node *dun = VTOUNION(ap->a_dvp);
  	struct union_node *un = VTOUNION(ap->a_vp);
  
  	if (dun->un_uppervp == NULLVP)
  		panic("union remove: null upper vnode");
  
  	if (un->un_uppervp != NULLVP) {
  		struct vnode *dvp = dun->un_uppervp;
  		struct vnode *vp = un->un_uppervp;
  		struct componentname *cnp = ap->a_cnp;
  
  		FIXUP(dun);
  		VREF(dvp);
  		dun->un_flags |= UN_KLOCK;
  		vput(ap->a_dvp);
  		FIXUP(un);
  		VREF(vp);
  		un->un_flags |= UN_KLOCK;
  		vput(ap->a_vp);
  
  		if (union_dowhiteout(un, cnp->cn_cred, cnp->cn_proc))
  			cnp->cn_flags |= DOWHITEOUT;
  		error = VOP_REMOVE(dvp, vp, cnp);
  		if (!error)
  			union_removed_upper(un);
  	} else {
  		FIXUP(dun);
  		error = union_mkwhiteout(
  			MOUNTTOUNIONMOUNT(UNIONTOV(dun)->v_mount),
  			dun->un_uppervp, ap->a_cnp, un->un_path);
  		vput(ap->a_dvp);
  		vput(ap->a_vp);
  	}
  
  	return (error);
  }
  
  /* a_dvp: directory in which to link
     a_vp: new target of the link
     a_cnp: name for the link
     */
  int
  union_link(v)
  	void *v;
  {
  	struct vop_link_args /* {
  		struct vnode *a_dvp;
  		struct vnode *a_vp;
  		struct componentname *a_cnp;
  	} */ *ap = v;
  	int error = 0;
  	struct union_node *dun;
  	struct vnode *dvp;
  	struct vnode *vp;
  
  	dun = VTOUNION(ap->a_dvp);
  
  #ifdef DIAGNOSTIC
  	if (!(ap->a_cnp->cn_flags & LOCKPARENT)) {
  		printf("union_link called without LOCKPARENT set!\n");
  		error = EIO; /* need some error code for "caller is a bozo" */
  	} else
  #endif
  	if (ap->a_dvp->v_op != ap->a_vp->v_op) {
  		vp = ap->a_vp;
  	} else {
  		struct union_node *un = VTOUNION(ap->a_vp);
  
  		if (un->un_uppervp == NULLVP) {
  			/*
  			 * needs to be copied up before we can link it.
  			 */
  			VOP_LOCK(ap->a_vp);
  			if (dun->un_uppervp == un->un_dirvp) {
  				VOP_UNLOCK(ap->a_dvp);
  			}
  			error = union_copyup(un, 1, ap->a_cnp->cn_cred,
  						ap->a_cnp->cn_proc);
  			if (dun->un_uppervp == un->un_dirvp) {
  				/* During copyup, we dropped the lock on the
  				 * dir and invalidated any saved namei lookup
  				 * state for the directory we'll be entering
  				 * the link in.  We need to re-run the lookup
  				 * in that directory to reset any state needed
  				 * for VOP_LINK.
  				 * Call relookup on the union-layer to
  				 * reset the state.
  				 */
  				vp  = NULLVP;
  				if (dun->un_uppervp == NULLVP)
  					panic("union: null upperdvp?");
  				/*
  				 * relookup starts with an unlocked node,
  				 * and since LOCKPARENT is set returns
  				 * the starting directory locked.
  				 */
  				if ((error = relookup(ap->a_dvp,
  						      &vp, ap->a_cnp))) {
  					vrele(ap->a_dvp);
  					VOP_UNLOCK(ap->a_vp);
  					return EROFS;
  				}
  				if (vp != NULLVP) {
  					/* The name we want to create has
  					   mysteriously appeared (a race?) */
  					error = EEXIST;
  					VOP_UNLOCK(ap->a_vp);
  					goto croak;
  				}
  			}
  			VOP_UNLOCK(ap->a_vp);
  		}
  		vp = un->un_uppervp;
  	}
  
  	dvp = dun->un_uppervp;
  	if (dvp == NULLVP)
  		error = EROFS;
  
  	if (error) {
  croak:
  		vput(ap->a_dvp);
  		return (error);
  	}
  
  	FIXUP(dun);
  	VREF(dvp);
  	dun->un_flags |= UN_KLOCK;
  	vput(ap->a_dvp);
  
  	return (VOP_LINK(dvp, vp, ap->a_cnp));
  }
  
  int
  union_rename(v)
  	void *v;
  {
  	struct vop_rename_args  /* {
  		struct vnode *a_fdvp;
  		struct vnode *a_fvp;
  		struct componentname *a_fcnp;
  		struct vnode *a_tdvp;
  		struct vnode *a_tvp;
  		struct componentname *a_tcnp;
  	} */ *ap = v;
  	int error;
  
  	struct vnode *fdvp = ap->a_fdvp;
  	struct vnode *fvp = ap->a_fvp;
  	struct vnode *tdvp = ap->a_tdvp;
  	struct vnode *tvp = ap->a_tvp;
  
  	if (fdvp->v_op == union_vnodeop_p) {	/* always true */
  		struct union_node *un = VTOUNION(fdvp);
  		if (un->un_uppervp == NULLVP) {
  			/*
  			 * this should never happen in normal
  			 * operation but might if there was
  			 * a problem creating the top-level shadow
  			 * directory.
  			 */
  			error = EXDEV;
  			goto bad;
  		}
  
  		fdvp = un->un_uppervp;
  		VREF(fdvp);
  		vrele(ap->a_fdvp);
  	}
  
  	if (fvp->v_op == union_vnodeop_p) {	/* always true */
  		struct union_node *un = VTOUNION(fvp);
  		if (un->un_uppervp == NULLVP) {
  			/* XXX: should do a copyup */
  			error = EXDEV;
  			goto bad;
  		}
  
  		if (un->un_lowervp != NULLVP)
  			ap->a_fcnp->cn_flags |= DOWHITEOUT;
  
  		fvp = un->un_uppervp;
  		VREF(fvp);
  		vrele(ap->a_fvp);
  	}
  
  	if (tdvp->v_op == union_vnodeop_p) {
  		struct union_node *un = VTOUNION(tdvp);
  		if (un->un_uppervp == NULLVP) {
  			/*
  			 * this should never happen in normal
  			 * operation but might if there was
  			 * a problem creating the top-level shadow
  			 * directory.
  			 */
  			error = EXDEV;
  			goto bad;
  		}
  
  		tdvp = un->un_uppervp;
  		VREF(tdvp);
  		un->un_flags |= UN_KLOCK;
  		vput(ap->a_tdvp);
  	}
  
  	if (tvp != NULLVP && tvp->v_op == union_vnodeop_p) {
  		struct union_node *un = VTOUNION(tvp);
  
  		tvp = un->un_uppervp;
  		if (tvp != NULLVP) {
  			VREF(tvp);
  			un->un_flags |= UN_KLOCK;
  		}
  		vput(ap->a_tvp);
  	}
  
  	return (VOP_RENAME(fdvp, fvp, ap->a_fcnp, tdvp, tvp, ap->a_tcnp));
  
  bad:
  	vrele(fdvp);
  	vrele(fvp);
  	vput(tdvp);
  	if (tvp != NULLVP)
  		vput(tvp);
  
  	return (error);
  }
  
  int
  union_mkdir(v)
  	void *v;
  {
  	struct vop_mkdir_args /* {
  		struct vnode *a_dvp;
  		struct vnode **a_vpp;
  		struct componentname *a_cnp;
  		struct vattr *a_vap;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_dvp);
  	struct vnode *dvp = un->un_uppervp;
  
  	if (dvp != NULLVP) {
  		int error;
  		struct vnode *vp;
  
  		FIXUP(un);
  		VREF(dvp);
  		un->un_flags |= UN_KLOCK;
  		VOP_UNLOCK(ap->a_dvp);
  		error = VOP_MKDIR(dvp, &vp, ap->a_cnp, ap->a_vap);
  		if (error) {
  			vrele(ap->a_dvp);
  			return (error);
  		}
  
  		error = union_allocvp(
  				ap->a_vpp,
  				ap->a_dvp->v_mount,
  				ap->a_dvp,
  				NULLVP,
  				ap->a_cnp,
  				vp,
  				NULLVP,
  				1);
  		vrele(ap->a_dvp);
  		if (error)
  			vput(vp);
  		return (error);
  	}
  
  	vput(ap->a_dvp);
  	return (EROFS);
  }
  
  int
  union_rmdir(v)
  	void *v;
  {
  	struct vop_rmdir_args /* {
  		struct vnode *a_dvp;
  		struct vnode *a_vp;
  		struct componentname *a_cnp;
  	} */ *ap = v;
  	int error;
  	struct union_node *dun = VTOUNION(ap->a_dvp);
  	struct union_node *un = VTOUNION(ap->a_vp);
  
  	if (dun->un_uppervp == NULLVP)
  		panic("union rmdir: null upper vnode");
  
  	if (un->un_uppervp != NULLVP) {
  		struct vnode *dvp = dun->un_uppervp;
  		struct vnode *vp = un->un_uppervp;
  		struct componentname *cnp = ap->a_cnp;
  
  		FIXUP(dun);
  		VREF(dvp);
  		dun->un_flags |= UN_KLOCK;
  		vput(ap->a_dvp);
  		FIXUP(un);
  		VREF(vp);
  		un->un_flags |= UN_KLOCK;
  		vput(ap->a_vp);
  
  		if (union_dowhiteout(un, cnp->cn_cred, cnp->cn_proc))
  			cnp->cn_flags |= DOWHITEOUT;
  		error = VOP_RMDIR(dvp, vp, cnp);
  		if (!error)
  			union_removed_upper(un);
  	} else {
  		FIXUP(dun);
  		error = union_mkwhiteout(
  			MOUNTTOUNIONMOUNT(UNIONTOV(dun)->v_mount),
  			dun->un_uppervp, ap->a_cnp, un->un_path);
  		vput(ap->a_dvp);
  		vput(ap->a_vp);
  	}
  
  	return (error);
  }
  
  int
  union_symlink(v)
  	void *v;
  {
  	struct vop_symlink_args /* {
  		struct vnode *a_dvp;
  		struct vnode **a_vpp;
  		struct componentname *a_cnp;
  		struct vattr *a_vap;
  		char *a_target;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_dvp);
  	struct vnode *dvp = un->un_uppervp;
  
  	if (dvp != NULLVP) {
  		int error;
  		struct vnode *vp;
  
  		FIXUP(un);
  		VREF(dvp);
  		un->un_flags |= UN_KLOCK;
  		vput(ap->a_dvp);
  		error = VOP_SYMLINK(dvp, &vp, ap->a_cnp,
  					ap->a_vap, ap->a_target);
  		*ap->a_vpp = NULLVP;
  		return (error);
  	}
  
  	vput(ap->a_dvp);
  	return (EROFS);
  }
  
  /*
   * union_readdir works in concert with getdirentries and
   * readdir(3) to provide a list of entries in the unioned
   * directories.  getdirentries is responsible for walking
   * down the union stack.  readdir(3) is responsible for
   * eliminating duplicate names from the returned data stream.
   */
  int
  union_readdir(v)
  	void *v;
  {
  	struct vop_readdir_args /* {
  		struct vnodeop_desc *a_desc;
  		struct vnode *a_vp;
  		struct uio *a_uio;
  		struct ucred *a_cred;
  		int *a_eofflag;
  		off_t *a_cookies;
  		int a_ncookies;
  	} */ *ap = v;
  	register struct union_node *un = VTOUNION(ap->a_vp);
  	register struct vnode *vp;
  
  	if ((vp = un->un_uppervp) == NULLVP)
  		return (0);
  
  	FIXUP(un);
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_readdir), ap));
  }
  
  int
  union_readlink(v)
  	void *v;
  {
  	struct vop_readlink_args /* {
  		struct vnode *a_vp;
  		struct uio *a_uio;
  		struct ucred *a_cred;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp = OTHERVP(ap->a_vp);
  	int dolock = (vp == LOWERVP(ap->a_vp));
  
  	if (dolock)
  		VOP_LOCK(vp);
  	else
  		FIXUP(VTOUNION(ap->a_vp));
  	ap->a_vp = vp;
  	error = VCALL(vp, VOFFSET(vop_readlink), ap);
  	if (dolock)
  		VOP_UNLOCK(vp);
  
  	return (error);
  }
  
  int
  union_abortop(v)
  	void *v;
  {
  	struct vop_abortop_args /* {
  		struct vnode *a_dvp;
  		struct componentname *a_cnp;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp = OTHERVP(ap->a_dvp);
  	struct union_node *un = VTOUNION(ap->a_dvp);
  	int islocked = un->un_flags & UN_LOCKED;
  	int dolock = (vp == LOWERVP(ap->a_dvp));
  
  	if (islocked) {
  		if (dolock)
  			VOP_LOCK(vp);
  		else
  			FIXUP(VTOUNION(ap->a_dvp));
  	}
  	ap->a_dvp = vp;
  	error = VCALL(vp, VOFFSET(vop_abortop), ap);
  	if (islocked && dolock)
  		VOP_UNLOCK(vp);
  
  	return (error);
  }
  
  int
  union_inactive(v)
  	void *v;
  {
  	struct vop_inactive_args /* {
  		struct vnode *a_vp;
  	} */ *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  
  	/*
  	 * Do nothing (and _don't_ bypass).
  	 * Wait to vrele lowervp until reclaim,
  	 * so that until then our union_node is in the
  	 * cache and reusable.
  	 *
  	 * NEEDSWORK: Someday, consider inactive'ing
  	 * the lowervp and then trying to reactivate it
  	 * with capabilities (v_id)
  	 * like they do in the name lookup cache code.
  	 * That's too much work for now.
  	 */
  
  #ifdef UNION_DIAGNOSTIC
  	if (un->un_flags & UN_LOCKED)
  		panic("union: inactivating locked node");
  	if (un->un_flags & UN_ULOCK)
  		panic("union: inactivating w/locked upper node");
  #endif
  
  	union_diruncache(un);
  
  	if ((un->un_flags & UN_CACHED) == 0)
  		vgone(ap->a_vp);
  
  	return (0);
  }
  
  int
  union_reclaim(v)
  	void *v;
  {
  	struct vop_reclaim_args /* {
  		struct vnode *a_vp;
  	} */ *ap = v;
  
  	union_freevp(ap->a_vp);
  
  	return (0);
  }
  
  int
  union_lock(v)
  	void *v;
  {
  	struct vop_lock_args *ap = v;
  	struct vnode *vp = ap->a_vp;
  	struct union_node *un;
  
  start:
  	while (vp->v_flag & VXLOCK) {
  		vp->v_flag |= VXWANT;
  		sleep((caddr_t)vp, PINOD);
  	}
  
  	un = VTOUNION(vp);
  
  	if (un->un_uppervp != NULLVP) {
  		if (((un->un_flags & UN_ULOCK) == 0) &&
  		    (vp->v_usecount != 0)) {
  			/*
  			 * We MUST always use the order of: take upper
  			 * vp lock, manipulate union node flags, drop
  			 * upper vp lock.  This code must not be an
  			 * exception.
  			 */
  			VOP_LOCK(un->un_uppervp);
  			un->un_flags |= UN_ULOCK;
  		}
  #ifdef DIAGNOSTIC
  		if (un->un_flags & UN_KLOCK) {
  			vprint("dangling upper lock", vp);
  			panic("union: dangling upper lock");
  		}
  #endif
  	}
  
  	if (un->un_flags & UN_LOCKED) {
  #ifdef DIAGNOSTIC
  		if (curproc && un->un_pid == curproc->p_pid &&
  			    un->un_pid > -1 && curproc->p_pid > -1)
  			panic("union: locking against myself");
  #endif
  		un->un_flags |= UN_WANTED;
  		sleep((caddr_t)un, PINOD);
  		goto start;
  	}
  
  #ifdef DIAGNOSTIC
  	if (curproc)
  		un->un_pid = curproc->p_pid;
  	else
  		un->un_pid = -1;
  #endif
  
  	un->un_flags |= UN_LOCKED;
  	return (0);
  }
  
  /*
   * When operations want to vput() a union node yet retain a lock on
   * the upper VP (say, to do some further operations like link(),
   * mkdir(), ...), they set UN_KLOCK on the union node, then call
   * vput() which calls VOP_UNLOCK() and comes here.  union_unlock()
   * unlocks the union node (leaving the upper VP alone), clears the
   * KLOCK flag, and then returns to vput().  The caller then does whatever
   * is left to do with the upper VP, and insures that it gets unlocked.
   *
   * If UN_KLOCK isn't set, then the upper VP is unlocked here.
   */
  
  int
  union_unlock(v)
  	void *v;
  {
  	struct vop_lock_args *ap = v;
  	struct union_node *un = VTOUNION(ap->a_vp);
  
  #ifdef DIAGNOSTIC
  	if ((un->un_flags & UN_LOCKED) == 0)
  		panic("union: unlock unlocked node");
  	if (curproc && un->un_pid != curproc->p_pid &&
  			curproc->p_pid > -1 && un->un_pid > -1)
  		panic("union: unlocking other process's union node");
  #endif
  
  	un->un_flags &= ~UN_LOCKED;
  
  	if ((un->un_flags & (UN_ULOCK|UN_KLOCK)) == UN_ULOCK)
  		VOP_UNLOCK(un->un_uppervp);
  
  	un->un_flags &= ~(UN_ULOCK|UN_KLOCK);
  
  	if (un->un_flags & UN_WANTED) {
  		un->un_flags &= ~UN_WANTED;
  		wakeup((caddr_t)un);
  	}
  
  #ifdef DIAGNOSTIC
  	un->un_pid = 0;
  #endif
  
  	return (0);
  }
  
  int
  union_bmap(v)
  	void *v;
  {
  	struct vop_bmap_args /* {
  		struct vnode *a_vp;
  		daddr_t  a_bn;
  		struct vnode **a_vpp;
  		daddr_t *a_bnp;
  		int *a_runp;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp = OTHERVP(ap->a_vp);
  	int dolock = (vp == LOWERVP(ap->a_vp));
  
  	if (dolock)
  		VOP_LOCK(vp);
  	else
  		FIXUP(VTOUNION(ap->a_vp));
  	ap->a_vp = vp;
  	error = VCALL(vp, VOFFSET(vop_bmap), ap);
  	if (dolock)
  		VOP_UNLOCK(vp);
  
  	return (error);
  }
  
  int
  union_print(v)
  	void *v;
  {
  	struct vop_print_args /* {
  		struct vnode *a_vp;
  	} */ *ap = v;
  	struct vnode *vp = ap->a_vp;
  
  	printf("\ttag VT_UNION, vp=%p, uppervp=%p, lowervp=%p\n",
  	    vp, UPPERVP(vp), LOWERVP(vp));
  	if (UPPERVP(vp))
  		vprint("uppervp", UPPERVP(vp));
  	if (LOWERVP(vp))
  		vprint("lowervp", LOWERVP(vp));
  	if (VTOUNION(vp)->un_dircache) {
  		struct vnode **vpp;
  		for (vpp = VTOUNION(vp)->un_dircache; *vpp != NULLVP; vpp++)
  			vprint("dircache:", *vpp);
  	}
  	return (0);
  }
  
  int
  union_islocked(v)
  	void *v;
  {
  	struct vop_islocked_args /* {
  		struct vnode *a_vp;
  	} */ *ap = v;
  
  	return ((VTOUNION(ap->a_vp)->un_flags & UN_LOCKED) ? 1 : 0);
  }
  
  int
  union_pathconf(v)
  	void *v;
  {
  	struct vop_pathconf_args /* {
  		struct vnode *a_vp;
  		int a_name;
  		register_t *a_retval;
  	} */ *ap = v;
  	int error;
  	struct vnode *vp = OTHERVP(ap->a_vp);
  	int dolock = (vp == LOWERVP(ap->a_vp));
  
  	if (dolock)
  		VOP_LOCK(vp);
  	else
  		FIXUP(VTOUNION(ap->a_vp));
  	ap->a_vp = vp;
  	error = VCALL(vp, VOFFSET(vop_pathconf), ap);
  	if (dolock)
  		VOP_UNLOCK(vp);
  
  	return (error);
  }
  
  int
  union_advlock(v)
  	void *v;
  {
  	struct vop_advlock_args /* {
  		struct vnode *a_vp;
  		caddr_t  a_id;
  		int  a_op;
  		struct flock *a_fl;
  		int  a_flags;
  	} */ *ap = v;
  	register struct vnode *vp = OTHERVP(ap->a_vp);
  
  	ap->a_vp = vp;
  	return (VCALL(vp, VOFFSET(vop_advlock), ap));
  }
  
  
  /*
   * XXX - vop_strategy must be hand coded because it has no
   * vnode in its arguments.
   * This goes away with a merged VM/buffer cache.
   */
  int
  union_strategy(v)
  	void *v;
  {
  	struct vop_strategy_args /* {
  		struct buf *a_bp;
  	} */ *ap = v;
  	struct buf *bp = ap->a_bp;
  	int error;
  	struct vnode *savedvp;
  
  	savedvp = bp->b_vp;
  	bp->b_vp = OTHERVP(bp->b_vp);
  
  #ifdef DIAGNOSTIC
  	if (bp->b_vp == NULLVP)
  		panic("union_strategy: nil vp");
  	if (((bp->b_flags & B_READ) == 0) &&
  	    (bp->b_vp == LOWERVP(savedvp)))
  		panic("union_strategy: writing to lowervp");
  #endif
  
  	error = VOP_STRATEGY(bp);
  	bp->b_vp = savedvp;
  
  	return (error);
  }
  
  