Re: [RFC PATCH] fpga: remove module reference counting from core components

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Nov 10, 2023 at 11:58:37PM +0100, Marco Pagani wrote:
> 
> 
> On 2023-11-08 16:52, Xu Yilun wrote:
> > On Fri, Nov 03, 2023 at 09:31:02PM +0100, Marco Pagani wrote:
> >>
> >>
> >> On 2023-10-30 09:32, Xu Yilun wrote:
> >>> On Fri, Oct 27, 2023 at 05:29:27PM +0200, Marco Pagani wrote:
> >>>> Remove unnecessary module reference counting from the core components
> >>>> of the subsystem. Low-level driver modules cannot be removed before
> >>>> core modules since they use their exported symbols.
> >>>
> >>> Could you help show the code for this conclusion?
> >>>
> >>> This is different from what I remember, a module cannot be removed when
> >>> its exported symbols are being used by other modules. IOW, the core
> >>> modules cannot be removed when there exist related low-level driver
> >>> modules. But the low-level driver modules could be removed freely
> >>> without other protecting mechanism.
> >>>
> >>
> >> My understanding was that we wanted to remove module reference counting
> >> from the fpga core and ease it from the responsibility of preventing
> >> low-level driver modules from being unloaded. 
> > 
> > FPGA core needs to prevent low-level driver module unloading sometimes,
> > e.g. when region reprograming is in progress. That's why we get fpga
> > region driver modules & bridge modules in fpga_region_program_fpga().
> > 
> > But we try best to get them only necessary. Blindly geting them all the
> > time results in no way to unload all modules (core & low level modules).
> > 
> >>
> >> If we want to keep reference counting in the fpga core, we could add a
> >> struct module *owner field in the struct fpga_manager_ops (and others
> >> core *_ops) so that the low-level driver can set it to THIS_MODULE.
> >> In this way, we can later use it in fpga_mgr_register() to bump up the
> > 
> > Yes, we should pass the module owner in fpga_mgr_register(), but could
> > not bump up its refcount at once.
> > 
> >> refcount of the low-level driver module by calling
> >> try_module_get(mgr->mops->owner) directly when it registers the manager.
> >> Finally, fpga_mgr_unregister() would call module_put(mgr->mops->owner)
> >> to allow unloading the low-level driver module.
> > 
> > As mentioned above, that makes problem. Most of the low level driver
> > modules call fpga_mgr_unregister() on module_exit(), but bumping up
> > their module refcount prevents module_exit() been executed. That came
> > out to be a dead lock.
> >
> 
> Initially, I considered calling try_module_get(mgr->mops->owner)
> in fpga_mgr_get(). But then, the new kernel-doc description of
> try_module_get() (1) made me question the safety of that approach.
> My concern is that the low-level driver could be removed right when
> someone is calling fpga_mgr_get() and hasn't yet reached
> try_module_get(mgr->mops->owner).

Can that really happen?  This shouldn't be a real issue, but normally
this should only be needed on an open() like call, don't tie a module
reference to a device reference, those are two different things.

> In that case, the struct mops
> (along with the entire low-level driver module) and the manager dev
> would "disappear" under the feet of fpga_mgr_get().

You should have a lock for handling this anyway, this feels odd that
it's a problem, but I haven't looked at the code in a long time.

try it and see!

greg k-h