linux-kernel - Re: [PATCH] driver core: Ensure proper suspend/resume ordering

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Date:	Tue, 22 Sep 2015 02:26:32 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Monday, September 21, 2015 10:34:54 AM Alan Stern wrote:
> On Mon, 21 Sep 2015, Thierry Reding wrote:
> 
> > > > Force-removing drivers that depend on a device that's being unbound
> > > > would be a possibility to solve the problem where consumers depend on a
> > > > device that could physically go away. It might also be the right thing
> > > > to do in any case. Presumably somebody unloading a module want to do
> > > > just that, and refusing to do so isn't playing very nice. Of course
> > > > allowing random modules to be removed even if a lot of consumers might
> > > > depend on it may not be friendly either. Consider if you unload a GPIO
> > > > driver that provides a pin that's used to enable power to an eMMC that
> > > > might have the root filesystem.
> > > >
> > > > Then again, if you unload a module you better know what you're doing
> > > > anyway, so maybe that's not something we need to be concerned about.
> > > 
> > > I think that it's better to fail module unloads in such cases by
> > > default (to prevent simple silly mistakes from having possibly severe
> > > consequences), but if a "force" option is used, we should regard that
> > > as "the user really means it" and do as requested.  That would be very
> > > much analogous to the hot-unplug situation from the software
> > > perspective.
> > 
> > Sounds very reasonable to me.
> 
> I'm not so sure about this.  For one thing, how are you going to 
> distinguish which module unloads are safe?

Ones that have no dependencies?

Very simple: If the driver being unloaded is explicitly depended on by
something (ie. there is a "device link" to it), we fail the unload with
-EBUSY unless the "force" option is used.

> For another, even if you do make this distinction, don't you think 
> people will get into the habit of always using the "force" option?  My 
> impression is that most module unloads end up causing some device to be 
> unbound from a driver, or even completely deregistered.
> 
> Right now the kernel uses the "You better know what you're doing when
> you unload a module" point of view, and I don't see any good reasons
> for changing.

Well, is that point of view appropriate from the users' perspective?

Thanks,
Rafael

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