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Date: Fri, 8 Nov 1996 01:39:23 +0900 (JST)
From: Michael Hancock <michaelh@cet.co.jp>
To: David Greenman <dg@root.com>
cc: Thomas David Rivers <ponds!rivers@dg-rtp.dg.com>,
        freebsd-hackers@freefall.freebsd.org
Subject: Re: More info on the daily panics... 
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On Thu, 7 Nov 1996, David Greenman wrote:

> In the one case of this problem that I looked into, it was caused by the
> v_usecount being negative. There was clearly one too many vrele's occuring
> somewhere. It was not caused by any sort of "freelist wrap" condition. The
> patch you've provided will kludge around the problem, but it is not in any
> sense a "fix". Each time a vnode is deallocated too many times (and thus
> v_usecount goes negative), you'll end up losing all of the vnodes on the
> freelist that follow it (potentially several thousand) because the condition
> will never go away. This is bad.

Then Thomas would be doing us a favor if he put in the following diff to
help track it down:

Index: vfs_subr.c
===================================================================
RCS file: /jaz/cvs/src/sys/kern/vfs_subr.c,v
retrieving revision 1.50
retrieving revision 1.49
diff -c -r1.50 -r1.49
*** vfs_subr.c	1996/01/02 18:13:20	1.50
--- vfs_subr.c	1995/12/17 21:23:19	1.49
***************
*** 36,42 ****
   * SUCH DAMAGE.
   *
   *	@(#)vfs_subr.c	8.13 (Berkeley) 4/18/94
!  * $Id: vfs_subr.c,v 1.50 1996/01/02 18:13:20 davidg Exp $
   */
  
  /*
--- 36,42 ----
   * SUCH DAMAGE.
   *
   *	@(#)vfs_subr.c	8.13 (Berkeley) 4/18/94
!  * $Id: vfs_subr.c,v 1.49 1995/12/17 21:23:19 phk Exp $
   */
  
  /*
***************
*** 835,846 ****
  	vp->v_usecount--;
  	if (vp->v_usecount > 0)
  		return;
- 	if (vp->v_usecount < 0 /* || vp->v_writecount < 0 */ ) {
  #ifdef DIAGNOSTIC
  		vprint("vrele: negative ref count", vp);
- #endif
  		panic("vrele: negative reference cnt");
  	}
  	if (vp->v_flag & VAGE) {
  		TAILQ_INSERT_HEAD(&vnode_free_list, vp, v_freelist);
  		vp->v_flag &= ~VAGE;
--- 835,846 ----
  	vp->v_usecount--;
  	if (vp->v_usecount > 0)
  		return;
  #ifdef DIAGNOSTIC
+ 	if (vp->v_usecount < 0 /* || vp->v_writecount < 0 */ ) {
  		vprint("vrele: negative ref count", vp);
  		panic("vrele: negative reference cnt");
  	}
+ #endif
  	if (vp->v_flag & VAGE) {
  		TAILQ_INSERT_HEAD(&vnode_free_list, vp, v_freelist);
  		vp->v_flag &= ~VAGE;