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

[Toshi Kani]

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.  


Thanks,
-Toshi




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