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SubjectRe: Locking Between User Context and Soft IRQs in 2.4.0
Andi Kleen wrote:
>
> On Sat, Nov 04, 2000 at 12:07:34PM -0500, Jeff Garzik wrote:
> > Andi Kleen wrote:
> > > All the MOD_INC/DEC_USE_COUNT are done inside the modules themselves. There
> > > is nothing that would a driver prevent from being unloaded on a different
> > > CPU while it is already executing in ->open but has not yet executed the add
> > > yet or after it has executed the _DEC but it is still running in module code
> > > Normally the windows are pretty small, but very long running interrupt
> > > on one CPU hitting exactly in the wrong moment can change that.
> >
> > Module unload calls unregister_netdev, which grabs rtnl_lock.
> > dev->open runs under rtnl_lock.
> >
> > Given this, how can the driver be unloaded if dev->open is running?
>
> It does not help, because when the semaphore synchronizes it is already
> too late -- free_module already did the zero module count check and
> nothing is going to stop it from unloading.

aaarrrggh!!!

CPU0 CPU1

rtnl_lock()
dev_ifsioc()
dev_change_flags()
dev_open();
dev->open();
vortex_open()
sys_delete_module()
if (!__MOD_IN_USE)
free_module()
mod->cleanup()
vortex_cleanup()
pci_unregister_driver()
[ time passes ] drv->remove();
vortex_remove_one()
unregister_netdev()
unregister_netdevice()
rtnl_lock() /* blocks */
...
MOD_INC_USE_COUNT;
...
rtnl_unlock()
...
module_unmap(); /* Not good */


We can't even fix this with a lock_kernel wrapped around
the dev->owner stuff in dev_open(), because the netdevice's
open() can sleep.

<subliminalmessage>prumpf's patch</sumliminalmessage>

Perhaps the best thing to do here is to create a system-wide
semaphore for module unloading. So we do a down()/up()
in sys_delete_module() and do this in dev_open:

/*
* Call device private open method
*/

down(&mod_unload_sem); /* sync with sys_delete_module() */
if (dev->owner == 0) {
if (dev->open)
ret = dev->open(dev);
} else {
if (try_inc_mod_count(dev->owner)) {
if (dev->open) {
if ((ret = dev->open(dev)) != 0)
__MOD_DEC_USE_COUNT(dev->owner);
}
} else
ret = -ENODEV;
}
up(&mod_unload_sem);
-
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