Message-ID: <3950A1C1.72CDA79B@uow.edu.au>
Date:   Wed, 21 Jun 2000 21:06:41 +1000
From:   Andrew Morton <andrewm@uow.edu.au>
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To:     Keith Owens <kaos@ocs.com.au>
CC:     "netdev@oss.sgi.com" <netdev@oss.sgi.com>
Subject: Re: modular net drivers, take 2
References: Your message of "Wed,
            21 Jun 2000 04:45:19 GMT." <3950485F.AD6F58C6@uow.edu.au> <13465.961564584@kao2.melbourne.sgi.com>
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Keith Owens wrote:
> 
> On Wed, 21 Jun 2000 04:45:19 +0000,
> Andrew Morton <andrewm@uow.edu.au> wrote:
> >Plan M:
> >
> >sys_delete_module() doesn't do the vfree().  It schedules it for 5
> >seconds in the future.  Or provides a mechanism for userland to do this.
> 
> Does not fix the problem where module open() code is running at the
> same time as module_exit() code which tears down the kernel structures
> that open needs.  Freeing the code pages while they are in use is only
> one of the possible failure modes for this race.


Aaaah.  I can see it all clearly now.

The fundamental misdesign here is that register_netdevice() and
unregister_netdev() are called from within the module constructor and
destructor functions.

You see, right now a module "owns" a number of netdevices.  This is
wrong.

It should be that a module "owns" a driver (pci_driver) and that driver
"owns" a number of netdevices:

module_init()
    registers the availability of the driver.  refcount remains zero

driver *open_module("eepro100")
    Locates the driver. Increments module refcount.
    No netdevices open yet.
    Shold not be called from within module_init!  That's the current
problem.

device *open_driver(driver *)
    Creates an interface instance. Increments an 'opencount' in the
driver.
    Critically: register_netdevice is called olny now.

close_device(device *)
    Unregister_netdev()

close_driver(driver *)
    Fails if any devices are open.
    Decrements module refcount to zero.

Now, no drivers are registered, so nobody can access "eth0", or
device->open or anything else.

module_exit()
        Unregisters the driver availability.
	Safe.

We just need to prevent module_exit() from racing against
open_driver().  Just a spinlock.


Too late for all of this.