Re: [RFC PATCH v3 0/3] acpi: Introduce prepare_remove deviceoperation

[Toshi Kani]

On Fri, 2012-12-07 at 00:00 +0800, Jiang Liu wrote:
> On 11/29/2012 02:41 AM, Toshi Kani wrote:
> > On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:
> >> On 2012/11/24 1:50, Vasilis Liaskovitis wrote:
> >>> As discussed in https://patchwork.kernel.org/patch/1581581/
> >>> the driver core remove function needs to always succeed. This means we need
> >>> to know that the device can be successfully removed before acpi_bus_trim / 
> >>> acpi_bus_hot_remove_device are called. This can cause panics when OSPM-initiated
> >>> or SCI-initiated eject of memory devices fail e.g with:
> >>> echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
> >>>
> >>> since the ACPI core goes ahead and ejects the device regardless of whether the
> >>> the memory is still in use or not.
> >>>
> >>> For this reason a new acpi_device operation called prepare_remove is introduced.
> >>> This operation should be registered for acpi devices whose removal (from kernel
> >>> perspective) can fail.  Memory devices fall in this category.
> >>>
> >>> acpi_bus_remove() is changed to handle removal in 2 steps:
> >>> - preparation for removal i.e. perform part of removal that can fail. Should
> >>>   succeed for device and all its children.
> >>> - if above step was successfull, proceed to actual device removal
> >>
> >> Hi Vasilis,
> >> We met the same problem when we doing computer node hotplug, It is a good idea
> >> to introduce prepare_remove before actual device removal.
> >>
> >> I think we could do more in prepare_remove, such as rollback. In most cases, we can
> >> offline most of memory sections except kernel used pages now, should we rollback
> >> and online the memory sections when prepare_remove failed ?
> > 
> > I think hot-plug operation should have all-or-nothing semantics.  That
> > is, an operation should either complete successfully, or rollback to the
> > original state.
> > 
> >> As you may know, the ACPI based hotplug framework we are working on already addressed
> >> this problem, and the way we slove this problem is a bit like yours.
> >>
> >> We introduce hp_ops in struct acpi_device_ops:
> >> struct acpi_device_ops {
> >> 	acpi_op_add add;
> >> 	acpi_op_remove remove;
> >> 	acpi_op_start start;
> >> 	acpi_op_bind bind;
> >> 	acpi_op_unbind unbind;
> >> 	acpi_op_notify notify;
> >> #ifdef	CONFIG_ACPI_HOTPLUG
> >> 	struct acpihp_dev_ops *hp_ops;
> >> #endif	/* CONFIG_ACPI_HOTPLUG */
> >> };
> >>
> >> in hp_ops, we divide the prepare_remove into six small steps, that is:
> >> 1) pre_release(): optional step to mark device going to be removed/busy
> >> 2) release(): reclaim device from running system
> >> 3) post_release(): rollback if cancelled by user or error happened
> >> 4) pre_unconfigure(): optional step to solve possible dependency issue
> >> 5) unconfigure(): remove devices from running system
> >> 6) post_unconfigure(): free resources used by devices
> >>
> >> In this way, we can easily rollback if error happens.
> >> How do you think of this solution, any suggestion ? I think we can achieve
> >> a better way for sharing ideas. :)
> > 
> > Yes, sharing idea is good. :)  I do not know if we need all 6 steps (I
> > have not looked at all your changes yet..), but in my mind, a hot-plug
> > operation should be composed with the following 3 phases.
> > 
> > 1. Validate phase - Verify if the request is a supported operation.  All
> > known restrictions are verified at this phase.  For instance, if a
> > hot-remove request involves kernel memory, it is failed in this phase.
> > Since this phase makes no change, no rollback is necessary to fail.  
> > 
> > 2. Execute phase - Perform hot-add / hot-remove operation that can be
> > rolled-back in case of error or cancel.
> > 
> > 3. Commit phase - Perform the final hot-add / hot-remove operation that
> > cannot be rolled-back.  No error / cancel is allowed in this phase.  For
> > instance, eject operation is performed at this phase.  
> Hi Toshi,
> 	There are one more step needed. Linux provides sysfs interfaces to
> online/offline CPU/memory sections, so we need to protect from concurrent
> operations from those interfaces when doing physical hotplug. Think about
> following sequence:
> Thread 1
> 1. validate conditions for hot-removal
> 2. offline memory section A
> 3.						online memory section A			
> 4. offline memory section B
> 5 hot-remove memory device hosting A and B.

Hi Gerry,

I agree.  And I am working on a proposal that tries to address this
issue by integrating both sysfs and hotplug operations into a framework.


Thanks,
-Toshi

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