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From: Archie Cobbs <archie@whistle.com>
Message-Id: <199611052100.NAA05158@bubba.whistle.com>
Subject: mount panics & hangs
To: freebsd-hackers@freebsd.org
Date: Tue, 5 Nov 1996 13:00:29 -0800 (PST)
Cc: julian@whistle.com (Julian Elischer)
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Here is a way to crash a fairly -current system... I'd be interested
to hear if this works for anyone else, and what the cause may be. The
system eventually either goes into some sort of deadlock, or else panics:


  ervisor read, page not present
  instruction pointer     = 0x8:0xf012ee00
  stack pointer           = 0x10:0xefbfff28
  frame pointer           = 0x10:0xefbfff30
  code segment            = base 0x0, limit 0xfffff, type 0x1b
			  = DPL 0, pres 1, def32 1, gran 1
  processor eflags        = interrupt enabled, resume, IOPL = 0
  current process         = 585 (mount)
  interrupt mask          = 
  kernel: type 12 trap, code=0
  Stopped at      _vfs_busy+0x8:  testb   $0x40,0x1a(%ebx)


I've seen deadlocks wrt. mounting and unmounting before, but the
panic is something new.

The scenario is: I have two idential partitions each containing a
FreeBSD-2.2 image.. one is the root partition, the other gets mounted
under "/mnt". The /etc/fstab contains:


  /dev/wd0a                     /       ufs     ro              1 1
  /dev/wd0e                     /mnt    ufs     rw,noauto       1 2


The /mnt partition has a short file in its root directory called
"foo". Now create these two shell scripts:


  #!/bin/sh
  while /usr/bin/true; do
	  mount /mnt
	  cat /mnt/foo > /dev/null
	  umount /mnt
  done


and


  #!/bin/sh
  while /usr/bin/true; do
	  mount
  done


Then start them both running in the background.. after a few seconds
I get either a deadlock or a panic.

-Archie

___________________________________________________________________________
Archie Cobbs   *   Whistle Communications, Inc.  *   http://www.whistle.com