rmmod osst hangs whenever osst gets loaded by hotplug at boot

From: Willem Riede (osst_at_riede.org)
Date: 10/31/04

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    Date:	Sun, 31 Oct 2004 14:24:10 +0000
To: linux-hotplug-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org

    Folks,

    I need some help here. I've been wrecking my brain to understand why
    "rmmod osst" works just fine whenever I perfom the "modprobe osst"
    manually from a terminal but always hangs when I coax hotplug (by a
    modified /etc/hotplug/scsi.agent) to do the loading at boot time...

    [root@fallguy osst]# ps alx
    F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
    4 0 3509 3472 17 0 3124 380 - D+ pts/1 0:00 rmmod osst

    It is the call to "scsi_unregister_driver(&osst_template.gendrv)" that
    doesn't return. AFAICT, this can only be due to "down(&drv->unload_sem)"
    in driver_unregister() at line 111 of linux-2.6.9/drivers/base/driver.c.

    For that semaphore to be free a call to "up(&drv->unload_sem)" in
    driver_release() at line 68 of linux-2.6.9/drivers/base/bus.c is needed.

    That would happen if osst_template.gendrv.bus->subsys.kset.kobj.kref.refcount
    reached zero in put_bus() called from bus_remove_driver() [put_bus translates
    into a kobject_put which does kref_put(&kobj->kref, kobject_release), which
    is documented to "Decrement the refcount, and if 0, call kobject_cleanup()",
    with kobject_cleanup doing "get_ktype(kobj)->release(kobj)"].

    Having traced all that, and created some debug output fromm osst, I'm now even
    more baffled -- the refcount is 14 regardless of when/how osst was loaded at
    the time of the call to scsi_unregister_driver (the only difference is that it
    already reached 14 when osst initialized, if osst gets loaded at boot time
    register_driver makes it 10, with 4 references counted later).

    The other counts must be from the other drivers on the "scsi" bus:
    [root@fallguy ~]# ls /sys/bus/scsi/drivers
    osst sd sr st

    There should be no way that refcount can make it back to zero, which would
    imply that rmmod osst should always hang. But it doesn't. Which must mean that
    I don't realy understand what's going on :-(

    So I'm lost, which is why I'm asking for help :-) . Can somebody explain it?

    (by the way, I can get rmmod st to hang in the same way)

    For the record - I'm doing this testing on a scsi based dual PIII machine
    running Fedora Core release 2.92 (FC3 Test 3) kernel 2.6.9-1.643smp.

    Thanks, Willem Riede.

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