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Date: Thu, 29 Jan 2004 01:35:24 -0500 (EST)
From: John Wehle <john@feith.com>
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To: bugs@freebsd.org
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Subject: nasty device_delete_child interaction
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device_delete_child works by starting with the grandchildren
working back towards the immediate child.  Several drivers
(i.e. if_xl.c, if_wx.c, iicbb.c) have code similar to:

  xxx_attach()
    {

      ...
      sc->child_dev = device_add_child ...
    }

  xxx_detach()
    {

      bus_generic_detach ();
      if (sc->child_dev)
        device_delete_child ...
    }

The problem is using device_delete_child on one of these
drivers causes the grandchild to be freed twice.  When
device_delete_child is called for xxx, it recurses since
xxx has a child.  The grandchild is detached and deleted.
xxx_detach is then called which calls device_delete_child
for the grandchild a second time causing a panic.

It seems to me that any driver which calls device_delete_child
as part of detaching must also implement something like:

  xxx_child_detached()
    {

    if (child == sc->child_dev)
      sc->child_dev = NULL;
    }

  xxx_detach()
    {

    /*
     * Remember the child so we can delete it (bus_generic_detach
     * indirectly zeroes sc->child_dev).
     */
    child = sc->child_dev;

    bus_generic_detach();
    if (child)
      device_delete_child ...
    }

or am I missing something?

-- John
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