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

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Message-ID: <ZVMZZNyLE0+YtKYz@yilunxu-OptiPlex-7050>
Date:   Tue, 14 Nov 2023 14:53:24 +0800
From:   Xu Yilun <yilun.xu@...ux.intel.com>
To:     Marco Pagani <marpagan@...hat.com>
Cc:     Moritz Fischer <mdf@...nel.org>, Wu Hao <hao.wu@...el.com>,
        Xu Yilun <yilun.xu@...el.com>, Tom Rix <trix@...hat.com>,
        Alan Tull <atull@...nsource.altera.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fpga: remove module reference counting from core
 components

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). 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().

I don't get what's the problem. fpga_mgr_get() would first of all
look for mgr_dev via class_find_device(), if low-level module is
unloaded, then you cannot find the mgr_dev and gracefully error out.

If class_find_device() succeed, mgr_dev got a reference and won't
disappear. Finally we may still found module removed when
try_module_get(), but should be another graceful error out.

Am I missing anything?

Thanks,
Yilun

> 
> (1) 557aafac1153 ("kernel/module: add documentation for try_module_get()")
>