linux-kernel - Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on rebind scenario

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Date:	Tue, 27 Nov 2012 19:32:45 +0100
From:	Vasilis Liaskovitis <vasilis.liaskovitis@...fitbricks.com>
To:	Toshi Kani <toshi.kani@...com>
Cc:	Wen Congyang <wency@...fujitsu.com>,
	Wen Congyang <wencongyang@...il.com>,
	linux-acpi@...r.kernel.org, isimatu.yasuaki@...fujitsu.com,
	rjw@...k.pl, lenb@...nel.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on
 rebind scenario

On Mon, Nov 26, 2012 at 05:19:01PM -0700, Toshi Kani wrote:
> > >> Consider the following sequence of operations for a hotplugged memory
> > >> device:
> > >>
> > >> 1. echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind
> > >> 2. echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
> > >>
> > >> If we don't offline/remove the memory, we have no chance to do it in
> > >> step 2. After
> > >> step2, the memory is used by the kernel, but we have powered off it. It
> > >> is very
> > >> dangerous.
> > > 
> > > How does power-off happen after unbind? acpi_eject_store checks for existing
> > > driver before taking any action:
> > > 
> > > #ifndef FORCE_EJECT
> > > 	if (acpi_device->driver == NULL) {
> > > 		ret = -ENODEV;
> > > 		goto err;
> > > 	}
> > > #endif
> > > 
> > > FORCE_EJECT is not defined afaict, so the function returns without scheduling
> > > acpi_bus_hot_remove_device. Is there another code path that calls power-off?
> > 
> > Consider the following case:
> > 
> > We hotremove the memory device by SCI and unbind it from the driver at the same time:
> > 
> > CPUa                                                  CPUb
> > acpi_memory_device_notify()
> >                                        unbind it from the driver
> >     acpi_bus_hot_remove_device()
> 
> Can we make acpi_bus_remove() to fail if a given acpi_device is not
> bound with a driver?  If so, can we make the unbind operation to perform
> unbind only?

acpi_bus_remove_device could check if the driver is present, and return -ENODEV
if it's not present (dev->driver == NULL).

But there can still be a race between an eject and an unbind operation happening
simultaneously. This seems like a general problem to me i.e. not specific to an
acpi memory device. How do we ensure an eject does not race with a driver unbind
for other acpi devices?

Is there a per-device lock in acpi-core or device-core that can prevent this from
happening? Driver core does a device_lock(dev) on all operations, but this is
probably not grabbed on SCI-initiated acpi ejects.

thanks,

- Vasilis
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