Introduce the VV_FORCEINSMQ vnode flag. It instructs the insmnque() function

to ignore the unmounting and forces insertion of the vnode into the mount
vnode list.

Change insmntque() to fail when forced unmount is in progress and
VV_FORCEINSMQ is not specified.

Add an assertion to the insmntque(), requiring the vnode to be
exclusively locked for mp-safe filesystems.

Use the VV_FORCEINSMQ for the creation of the syncvnode.

Tested by:	pho
Reviewed by:	tegge
MFC after:	1 month

2 files changed, 21 insertions, 5 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 7c7b2a5..3bc2341 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -1024,17 +1024,28 @@ int
 insmntque1(struct vnode *vp, struct mount *mp,
 	void (*dtr)(struct vnode *, void *), void *dtr_arg)
 {
+	int locked;
 
 	KASSERT(vp->v_mount == NULL,
 		("insmntque: vnode already on per mount vnode list"));
 	VNASSERT(mp != NULL, vp, ("Don't call insmntque(foo, NULL)"));
+#ifdef DEBUG_VFS_LOCKS
+	if (!VFS_NEEDSGIANT(mp))
+		ASSERT_VOP_ELOCKED(vp,
+		    "insmntque: mp-safe fs and non-locked vp");
+#endif
 	MNT_ILOCK(mp);
 	if ((mp->mnt_kern_flag & MNTK_NOINSMNTQ) != 0 &&
-	    mp->mnt_nvnodelistsize == 0) {
-		MNT_IUNLOCK(mp);
-		if (dtr != NULL)
-			dtr(vp, dtr_arg);
-		return (EBUSY);
+	    ((mp->mnt_kern_flag & MNTK_UNMOUNTF) != 0 ||
+	     mp->mnt_nvnodelistsize == 0)) {
+		locked = VOP_ISLOCKED(vp);
+		if (!locked || (locked == LK_EXCLUSIVE &&
+		     (vp->v_vflag & VV_FORCEINSMQ) == 0)) {
+			MNT_IUNLOCK(mp);
+			if (dtr != NULL)
+				dtr(vp, dtr_arg);
+			return (EBUSY);
+		}
 	}
 	vp->v_mount = mp;
 	MNT_REF(mp);
@@ -3283,9 +3294,13 @@ vfs_allocate_syncvnode(struct mount *mp)
 		return (error);
 	}
 	vp->v_type = VNON;
+	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
+	vp->v_vflag |= VV_FORCEINSMQ;
 	error = insmntque(vp, mp);
 	if (error != 0)
 		panic("vfs_allocate_syncvnode: insmntque failed");
+	vp->v_vflag &= ~VV_FORCEINSMQ;
+	VOP_UNLOCK(vp, 0);
 	/*
 	 * Place the vnode onto the syncer worklist. We attempt to
 	 * scatter them about on the list so that they will go off
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index b55114c..03a0a2e99 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -251,6 +251,7 @@ struct xvnode {
 #define	VV_NOKNOTE	0x0200	/* don't activate knotes on this vnode */
 #define	VV_DELETED	0x0400	/* should be removed */
 #define	VV_MD		0x0800	/* vnode backs the md device */
+#define	VV_FORCEINSMQ	0x1000	/* force the insmntque to succeed */
 
 /*
  * Vnode attributes.  A field value of VNOVAL represents a field whose value