Date: Wed, 28 Nov 2012 15:10:46 -0800
From: Greg KH <gregkh@linuxfoundation.org>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Toshi Kani <toshi.kani@hp.com>, linux-acpi@vger.kernel.org,
        Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>,
        Wen Congyang <wency@cn.fujitsu.com>,
        Wen Congyang <wencongyang@gmail.com>, isimatu.yasuaki@jp.fujitsu.com,
        lenb@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on
 rebind scenario
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On Thu, Nov 29, 2012 at 12:05:20AM +0100, Rafael J. Wysocki wrote:
> On Wednesday, November 28, 2012 02:46:33 PM Greg KH wrote:
> > > So, it looks like the driver core wants us to handle driver unbinding no
> > > matter what.
> > 
> > Yes.  Well, the driver core does the unbinding no matter what, if it was
> > told, by a user, to do so.  Why is that a problem?  The user then is
> > responsible for any bad things (i.e. not able to control the device any
> > more), if they do so.
> 
> I don't really agree with that, because the user may simply not know what
> the consequences of that will be.  In my not so humble opinion any interface
> allowing user space to crash the kernel is a bad one.  And this is an example
> of that.

This has been in place since 2005, over 7 years now, and I have never
heard any problems with it being used to crash the kernel, despite the
easy ability for people to unbind all of their devices from drivers and
instantly cause a system hang.  So really doubt this is a problem in
real life :)

> > > This pretty much means that it is a bad idea to have a driver that is
> > > exposed as a "device driver" in sysfs for memory hotplugging.
> > 
> > Again, why?  All this means is that the driver is now not connected to
> > the device (memory in this case.)  The memory is still there, still
> > operates as before, only difference is, the driver can't touch it
> > anymore.
> > 
> > This is the same for any ACPI driver, and has been for years.
> 
> Except that if this driver has been unbound and the removal is triggered by
> an SCI, the core will just go on and remove the memory, although it may
> be killing the kernel this way.

Why would memory go away if a driver is unbound from a device?  The
device didn't go away.  It's the same if the driver was a module and it
was unloaded, you should not turn memory off in that situation, right?
Are you also going to prevent module unloading of this driver?

> Arguably, this may be considered as the core's fault, but the only way to
> fix that would be to move the code from that driver into the core and not to
> register it as a "driver" any more.  Which was my point. :-)

No, I think people are totally overreacting to the unbind/bind files,
which are there to aid in development, and in adding new device ids to
drivers, as well as sometimes doing a hacky revoke() call.

> > Please don't confuse unbind with any "normal" system operation, it is
> > not to be used for memory hotplug, or anything else like this.
> > 
> > Also, if you really do not want to do this, turn off the ability to
> > unbind/bind for these devices, that is under your control in your bus
> > logic.
> 
> OK, but how?  I'm looking at driver_unbind() and not seeing any way to do
> that actually.

See the suppress_bind_attrs field in struct device_driver.  It's even
documented in device.h, but sadly, no one reads documentation :)

I recommend you set this field if you don't want the bind/unbind files
to show up for your memory driver, although I would argue that the
driver needs to be fixed up to not do foolish things like removing
memory from a system unless it really does go away...

hope this helps,

greg k-h
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