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Subject: kern/20632: stacking mount_null causes an error: mount_null: Resource deadlock avoided
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>Number:         20632
>Category:       kern
>Synopsis:       stacking mount_null causes an error: mount_null: Resource deadlock avoided
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Aug 15 20:50:00 PDT 2000
>Closed-Date:
>Last-Modified:
>Originator:     Tanaka Akira
>Release:        FreeBSD 4.1-RELEASE i386
>Organization:
m17n
>Environment:

FreeBSD coulee.m17n.org 4.1-RELEASE FreeBSD 4.1-RELEASE #0: Thu Jul 27 04:44:16 GMT 2000     root@usw4.freebsd.org:/usr/src/sys/compile/GENERIC  i386

>Description:

If null file system is stacked by mount_null, following error is caused.

  mount_null: Resource deadlock avoided

Then, umount cannot unmount the file system with same error and some
commands hangs forever.

>How-To-Repeat:

This machine has freashly installed FreeBSD 4.1-RELEASE with default file
system hierarchy.

coulee# cd /
coulee# df
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/ad0s1a     49583    27200    18417    60%    /
/dev/ad0s1f   5521533   637931  4441880    13%    /usr
/dev/ad0s1e     19815      268    17962     1%    /var
procfs              4        4        0   100%    /proc
coulee# ls
.cshrc          cdrom           kernel          root            usr
.profile        compat          kernel.GENERIC  sbin            var
COPYRIGHT       dev             mnt             stand
bin             etc             modules         sys
boot            home            proc            tmp

Try to mount /home to /mnt.  This is no problem.

coulee# mount_null /home /mnt

Try to mount /home to /mnt once more.  This causes an error.

coulee# mount_null /home /mnt
mount_null: Resource deadlock avoided

Try to unmount it.  This causes same error.

coulee# umount /mnt
umount: unmount of /mnt failed: Resource deadlock avoided

`ls /home' hangs up.

coulee# ls /home
^C^C^C^C

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted:


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