Re: [PATCH v5 6/7] module: Improve support for asynchronous module exit code

Ming Lei
2022-09-29T01:10:53Z

On Wed, Sep 28, 2022 at 12:27:07PM -0700, Bart Van Assche wrote:
> On 9/27/22 18:09, Ming Lei wrote:
> > On Wed, Sep 14, 2022 at 03:56:20PM -0700, Bart Van Assche wrote:
> > > Some kernel modules call device_del() from their module exit code and
> > > schedule asynchronous work from inside the .release callback without waiting
> > > until that callback has finished. As an example, many SCSI LLD drivers call
> >
> > It isn't only related with device, any kobject has such issue, or any
> > reference counter usage has similar potential risk, see previous discussion:
> >
> > https://lore.kernel.org/lkml/YsZm7lSXYAHT14ui@T590/
> >
> > IMO, it is one fundamental problem wrt. module vs. reference counting or
> > kobject uses at least, since the callback depends on module code
> > segment.
> >
> > > scsi_remove_host() from their module exit code. scsi_remove_host() may
> > > invoke scsi_device_dev_release_usercontext() asynchronously.
> > > scsi_device_dev_release_usercontext() uses the host template pointer and
> > > that pointer usually exists in static storage in the SCSI LLD. Support
> > > using the module reference count to keep the module around until
> > > asynchronous module exiting has completed by waiting in the delete_module()
> > > system call until the module reference count drops to zero.
> >
> > The issue can't be addressed by the normal mod->refcnt, since user need
> > to unload module when the device isn't used.
>
> Hi Ming,
>
> How about removing support for calling scsi_device_put() from atomic context
> as is done in the untested patch below?

That can't work.

The problem is that no existed mechanism can guarantee that kobject reference
drops to zero inside module_exit().

Thanks,
Ming