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SubjectRe: A udev rule to serve the change event of ACPI container?
Hi Michal,

Sorry for my delay...

On Tue, Jul 25, 2017 at 02:48:37PM +0200, Michal Hocko wrote:
> On Mon 24-07-17 17:29:21, Joey Lee wrote:
> > On Mon, Jul 24, 2017 at 10:57:02AM +0200, Michal Hocko wrote:
> > > On Wed 19-07-17 17:09:10, Joey Lee wrote:
> > > > On Mon, Jul 17, 2017 at 11:05:25AM +0200, Michal Hocko wrote:
> > > [...]
> > > > > The problem I have with this expectation is that userspace will never
> > > > > have a good atomic view of the whole container. So it can only try to
> > > >
> > > > I agreed!
> > > >
> > > > Even a userspace application can handle part of offline jobs. It's
> > > > still possible that other kernel/userland compenents are using the
> > > > resource in container.
> > > >
> > > > > eject and then hope that nobody has onlined part of the container.
> > > > > If you emit offline event to the userspace the cleanup can be done and
> > > > > after the last component goes offline then the eject can be done
> > > > > atomically.
> > > >
> > > > The thing that we didn't align is how does kernel maintains the flag
> > > > of ejection state on container.
> > >
> > > Why it cannot be an attribute of the container? The flag would be set
> > > when the eject operation is requested and cleared when either the
> > > operation is successful (all parts offline and eject operation acked
> > > by the BIOS) or it is terminated.
> > >
> >
> > For the success case, yes, we can clear the flag when the _EJ0 of container
> > is success. But for the fail case, we don't know when the operation is
> > terminated.
>
> Hmm, this is rather strange. What is the BIOS state in the meantime?
> Let's say it doesn't retry. Does it wait for the OS for ever?
>

Unfortunately ACPI spec doesn't mention the detail of BIOS behavior for
container hot-removing.

IMHO, if the BIOS doesn't retry, at least it should maintains a timer
to handle the OS layer time out then BIOS resets hardware(turns off
progress light or something else...).

The old BIOS just treats the ejection event as a button event. BIOS
emits 0x103 ejection event to OS after user presses a button or UI.
Then BIOS hopes that OS(either kernel or userland) finishs all jobs,
calls _EJ0 to turn off power, and calls _OST to return state to BIOS.

If the ejection event from BIOS doesn't trigger anything in upper OS
layer, old BIOS can not against this situation unless it has a timer.

> > > [...]
> > > > Base on the above figure, if userspace didn't do anything or it
> > > > just performs part of offline jobs. Then the container's [eject]
> > > > state will be always _SET_ there, and kernel will always check
> > > > the the latest child offline state when any child be offlined
> > > > by userspace.
> > >
> > > What is a problem about that? The eject is simply in progress until all
> > > is set. Or maybe I just misunderstood.
> > >
> >
> > I agree, but it's only for success case. For fail case, kernel can not
> > wait forever. Can we?
>
> Well, this won't consume any additional resources so I wouldn't be all
> that worried. Maybe we can reset the flag as soon as somebody tries to
> online some part of the container?
>

So, the behavior is:

Kernel received ejection event, set _Eject_ flag on container object
-> Kernel sends offline events to all children devices
-> User space performs cleaning jobs and offlines each child device
-> Kernel detects all children offlined
-> Kernel removes objects and calls power off(_EJ0)

If anyone onlined one of the children devices in the term of waiting
userland offlines all children, then the _Eject_ flag will be clean
and ejection process will be interrupted. In this situation, administrator
needs to trigger ejection event again. Do you think that the race hurts
anything?

Thanks a lot!
Joey Lee

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