Message-ID: <1451747.3VlxbhJES4@vostro.rjw.lan>
Date:	Wed, 28 Nov 2012 23:21:59 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Toshi Kani <toshi.kani@...com>
Cc:	linux-acpi@...r.kernel.org,
	Vasilis Liaskovitis <vasilis.liaskovitis@...fitbricks.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Wen Congyang <wencongyang@...il.com>,
	isimatu.yasuaki@...fujitsu.com, 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 Wednesday, November 28, 2012 03:04:52 PM Toshi Kani wrote:
> On Wed, 2012-11-28 at 23:01 +0100, Rafael J. Wysocki wrote:
> > On Wednesday, November 28, 2012 02:40:09 PM Toshi Kani wrote:
> > > On Wed, 2012-11-28 at 22:40 +0100, Rafael J. Wysocki wrote:
> > > > On Wednesday, November 28, 2012 02:02:48 PM Toshi Kani wrote:
> > > > > > > > > > > > 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.
> > > > > > > > > 
> > > > > > > > > Since driver_unbind() calls device_lock(dev->parent) before calling
> > > > > > > > > device_release_driver(), I am wondering if we can call
> > > > > > > > > device_lock(dev->dev->parent) at the beginning of acpi_bus_remove()
> > > > > > > > > (i.e. before calling pre_remove) and fails if dev->driver is NULL.  The
> > > > > > > > > parent lock is otherwise released after device_release_driver() is done.
> > > > > > > > 
> > > > > > > > I would be careful.  You may introduce some subtle locking-related issues
> > > > > > > > this way.
> > > > > > > 
> > > > > > > Right.  This requires careful inspection and testing.  As far as the
> > > > > > > locking is concerned, I am not keen on using fine grained locking for
> > > > > > > hot-plug.  It is much simpler and solid if we serialize such operations.
> > > > > > > 
> > > > > > > > Besides, there may be an alternative approach to all this.  For example,
> > > > > > > > what if we don't remove struct device objects on eject?  The ACPI handles
> > > > > > > > associated with them don't go away in that case after all, do they?
> > > > > > > 
> > > > > > > Umm...  Sorry, I am not getting your point.  The issue is that we need
> > > > > > > to be able to fail a request when memory range cannot be off-lined.
> > > > > > > Otherwise, we end up ejecting online memory range.
> > > > > > 
> > > > > > Yes, this is the major one.  The minor issue, however, is a race condition
> > > > > > between unbinding a driver from a device and removing the device if I
> > > > > > understand it correctly.  Which will go away automatically if the device is
> > > > > > not removed in the first place.  Or so I would think. :-)
> > > > > 
> > > > > I see.  I do not think whether or not the device is removed on eject
> > > > > makes any difference here.  The issue is that after driver_unbind() is
> > > > > done, acpi_bus_hot_remove_device() no longer calls the ACPI memory
> > > > > driver (hence, it cannot fail in prepare_remove), and goes ahead to call
> > > > > _EJ0.  If driver_unbind() did off-line the memory, this is OK.  However,
> > > > > it cannot off-line kernel memory ranges.  So, we basically need to
> > > > > either 1) serialize acpi_bus_hot_remove_device() and driver_unbind(), or
> > > > > 2) make acpi_bus_hot_remove_device() to fail if driver_unbind() is run
> > > > > during the operation.
> > > > 
> > > > OK, I see the problem now.
> > > > 
> > > > What exactly is triggering the driver_unbind() in this scenario?
> > > 
> > > User can request driver_unbind() from sysfs as follows.  I do not see
> > > much reason why user has to do for memory, though.
> > > 
> > > echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind
> > 
> > This is wrong.  Even if we want to permit user space to forcibly unbind
> > drivers from anything like this, we should at least check for some
> > situations in which it is plain dangerous.  Like in this case.  So I think
> > the above should fail unless we know that the driver won't be necessary
> > to handle hot-removal of memory.
> 
> Well, we tried twice already... :)
> https://lkml.org/lkml/2012/11/16/649

I didn't mean driver_unbind() should fail.  The code path that executes
driver_unbind() eventually should fail _before_ executing it.

> > Alternatively, this may actually try to carry out the hot-removal and only
> > call driver_unbind() if that succeeds.  Whichever is preferable, I'd say.
> 
> Greg clarified in the above link that this interface is "unbind", not
> remove.

OK, so that's clear.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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