Index: sys/proc.h
===================================================================
RCS file: /usr/repo/src/sys/sys/proc.h,v
retrieving revision 1.457
diff -u -p -r1.457 proc.h
--- sys/proc.h	13 Mar 2006 10:37:24 -0000	1.457
+++ sys/proc.h	28 Apr 2006 14:50:02 -0000
@@ -388,6 +388,7 @@ struct thread {
 #define	TDP_GEOM	0x00010000 /* Settle GEOM before finishing syscall */
 #define	TDP_SOFTDEP	0x00020000 /* Stuck processing softdep worklist */
 #define	TDP_NORUNNINGBUF 0x00040000 /* Ignore runningbufspace check */
+#define	TDP_WRITE	0x00080000 /* Writing to a file system */
 
 /*
  * Reasons that the current thread can not be run yet.
Index: kern/vfs_vnops.c
===================================================================
RCS file: /usr/repo/src/sys/kern/vfs_vnops.c,v
retrieving revision 1.241
diff -u -p -r1.241 vfs_vnops.c
--- kern/vfs_vnops.c	31 Mar 2006 03:54:19 -0000	1.241
+++ kern/vfs_vnops.c	28 Apr 2006 20:36:33 -0000
@@ -873,6 +873,19 @@ vn_start_write(vp, mpp, flags)
 	struct mount *mp;
 	int error;
 
+#ifdef INVARIANTS
+	/*
+	 * When vn_start_write() is called recursively with V_WAIT, it can
+	 * deadlock when vfs_write_suspend() is called in the meantime.
+	 */
+	if ((flags & V_WAIT) && (curthread->td_pflags & TDP_WRITE)) {
+		printf("Recursive vn_start_write() call:\n");
+#ifdef KDB
+		kdb_backtrace();
+#endif
+	}
+#endif
+
 	error = 0;
 	/*
 	 * If a vnode is provided, get and return the mount point that
@@ -906,6 +919,9 @@ vn_start_write(vp, mpp, flags)
 	}
 	if (flags & V_XSLEEP)
 		goto unlock;
+#ifdef INVARIANTS
+	curthread->td_pflags |= TDP_WRITE;
+#endif
 	mp->mnt_writeopcount++;
 unlock:
 	MNT_REL(mp);
@@ -1031,6 +1047,16 @@ vn_finished_write(mp)
 {
 	if (mp == NULL)
 		return;
+#ifdef INVARIANTS
+	if (!(curthread->td_pflags & TDP_WRITE)) {
+		printf("vn_finished_write() called without corresponding "
+		    "vn_start_write() call:\n");
+#ifdef KDB
+		kdb_backtrace();
+#endif
+	}
+	curthread->td_pflags &= ~TDP_WRITE;
+#endif
 	MNT_ILOCK(mp);
 	mp->mnt_writeopcount--;
 	if (mp->mnt_writeopcount < 0)
Index: kern/vfs_subr.c
===================================================================
RCS file: /usr/repo/src/sys/kern/vfs_subr.c,v
retrieving revision 1.669
diff -u -p -r1.669 vfs_subr.c
--- kern/vfs_subr.c	4 Apr 2006 06:46:10 -0000	1.669
+++ kern/vfs_subr.c	28 Apr 2006 20:38:18 -0000
@@ -2823,7 +2823,15 @@ vfs_msync(struct mount *mp, int flags)
 	struct vnode *vp, *mvp;
 	struct vm_object *obj;
 
-	(void) vn_start_write(NULL, &mp, V_WAIT);
+#ifdef INVARIANTS
+	if (!(curthread->td_pflags & TDP_WRITE)) {
+		printf("vfs_msync() called without previous vn_start_write() "
+		    "call:\n");
+#ifdef KDB
+		kdb_backtrace();
+#endif
+	}
+#endif
 	MNT_ILOCK(mp);
 	MNT_VNODE_FOREACH(vp, mp, mvp) {
 		VI_LOCK(vp);
@@ -2854,7 +2862,6 @@ vfs_msync(struct mount *mp, int flags)
 			VI_UNLOCK(vp);
 	}
 	MNT_IUNLOCK(mp);
-	vn_finished_write(mp);
 }
 
 /*