Index: head/sys/kern/vfs_vnops.c
===================================================================
--- head/sys/kern/vfs_vnops.c
+++ head/sys/kern/vfs_vnops.c
@@ -1289,7 +1289,6 @@
 vn_truncate(struct file *fp, off_t length, struct ucred *active_cred,
     struct thread *td)
 {
-	struct vattr vattr;
 	struct mount *mp;
 	struct vnode *vp;
 	void *rl_cookie;
@@ -1316,20 +1315,35 @@
 	if (error)
 		goto out;
 #endif
+	error = vn_truncate_locked(vp, length, (fp->f_flag & O_FSYNC) != 0,
+	    fp->f_cred);
+out:
+	VOP_UNLOCK(vp, 0);
+	vn_finished_write(mp);
+out1:
+	vn_rangelock_unlock(vp, rl_cookie);
+	return (error);
+}
+
+/*
+ * Truncate a file that is already locked.
+ */
+int
+vn_truncate_locked(struct vnode *vp, off_t length, bool sync,
+    struct ucred *cred)
+{
+	struct vattr vattr;
+	int error;
+
 	error = VOP_ADD_WRITECOUNT(vp, 1);
 	if (error == 0) {
 		VATTR_NULL(&vattr);
 		vattr.va_size = length;
-		if ((fp->f_flag & O_FSYNC) != 0)
+		if (sync)
 			vattr.va_vaflags |= VA_SYNC;
-		error = VOP_SETATTR(vp, &vattr, fp->f_cred);
+		error = VOP_SETATTR(vp, &vattr, cred);
 		VOP_ADD_WRITECOUNT_CHECKED(vp, -1);
 	}
-out:
-	VOP_UNLOCK(vp, 0);
-	vn_finished_write(mp);
-out1:
-	vn_rangelock_unlock(vp, rl_cookie);
 	return (error);
 }
 
Index: head/sys/sys/vnode.h
===================================================================
--- head/sys/sys/vnode.h
+++ head/sys/sys/vnode.h
@@ -695,6 +695,8 @@
 int	vn_start_write(struct vnode *vp, struct mount **mpp, int flags);
 int	vn_start_secondary_write(struct vnode *vp, struct mount **mpp,
 	    int flags);
+int	vn_truncate_locked(struct vnode *vp, off_t length, bool sync,
+	    struct ucred *cred);
 int	vn_writechk(struct vnode *vp);
 int	vn_extattr_get(struct vnode *vp, int ioflg, int attrnamespace,
 	    const char *attrname, int *buflen, char *buf, struct thread *td);