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

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Date:   Fri, 30 Sep 2022 12:39:27 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Ming Lei <ming.lei@...hat.com>, Hannes Reinecke <hare@...e.de>,
        John Garry <john.garry@...wei.com>,
        Mike Christie <michael.christie@...cle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v5 6/7] module: Improve support for asynchronous module
 exit code

On Wed, Sep 28, 2022 at 11:17:02AM -0700, Bart Van Assche wrote:
> On 9/27/22 17:02, Luis Chamberlain wrote:
> > On Tue, Sep 20, 2022 at 10:13:40AM -0700, Bart Van Assche wrote:
> > > On 9/14/22 15:56, 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
> > > > 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.
> > > 
> > > Hi Luis,
> > > 
> > > I'd like to know your opinion about this patch since you are the maintainer
> > > of the kernel module system.
> > 
> > See this patch which extends the documentation of try_module_get():
> > 
> > https://lkml.kernel.org/r/20211029184500.2821444-7-mcgrof@kernel.org
> > 
> > You can ignore discussion around the thread as sadly it is just
> > irrelevant stuff not about that patch. But the logic it spells out
> > is still true.
> > 
> > So, in short, using try_module_get() on exit is actually the wrong
> > thing to do and it is no surprise it would fail. I haven't gotten
> > yet around to reviewing Mauro's driver API which let's you unbind
> > drivers, but it sounds related so I CC'd you on that.
> > 
> > So I'd like to ask instead if an alternative to using try_module_get()
> > on exit would be better here and for the future.
> 
> Hi Luis,
> 
> The extended documentation of try_module_get() is very helpful. But please
> note that this patch is not related to try_module_get() at all. See also
> patch 7/7 in this series (https://lore.kernel.org/linux-scsi/20220914225621.415631-8-bvanassche@acm.org/).

I cannot see how this patch set is no way related to try_module_get()
given the 7/7 patch you posted replaces try_module_get() with __module_get().
My point, and hint, is that the original construct that added try_module_get()
on removal was flawed and I'm not sure trying to expand on that idea would
or even *should* be fruitful given the issues / tribal knowledge I tried
extending documentation for.

It would beg the question if instead re-evaluating the goal could be
done in such a way that the new documentation I suggested on try_module_get()
would be seriously taken into account.

  Luis