Re: [PATCH v3 07/11] tpm: Replace device number bitmap with IDR

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Mon, Feb 22, 2016 at 08:15:38PM -0500, Stefan Berger wrote:
> > - Tear down /dev/tpmX, sysfs and all other user accessible entry
> >   points
> > - Do device_del (get rid of all the sysfs stuff)
> > - NULL the IDR (disables kAPI access and allow reuse of the ID)
> > - NULL the ops (flush in-progress access now that new access is impossible)
> 
> Why NULL the ops so late and allow access even after device_del? I'd do that at
> the beginning.

It absolutely has to be done as the very last thing.

None of the sysfs code checks ops, and thus can't fail on null ops
(see the comment I added in the ops patch in tpm-sysfs). All sysfs
must be rendered inaccessible and all in-progress accesses must be
completed before we attempt to drop ops.

Since we are moving to using chip->groups to register sysfs a full
device_del seems necessary to fully fence sysfs access.

> Also the IDR has a two-stages:
>  one making the chip inaccessible on the idr : idr_replace(&dev_nums_idr, NULL,
> chip->dev_num)
>  the other one by freeing the IDR for re-use : idr_remove(&dev_nums_ir, chip->
> dev_num)

Oh, I missed that, yes, that is the way to go for the IDR part,
do the idr_remove after device_del.

If you set to null I expect that means the while loop example I gave
will need adjusting.

> - Free the chip's device number from the IDR (makes IDR re-usable; must happen
> after /dev/tpmX is released to avoid clashes

With this new arrangment the idr_remove can happen directly after
device_del and we can avoid the intermediate state of a NULL'd entry,
which seems slightly simplifying. Before there was the problem because
vtpm was using a struct device, but that is gone now.

Jason

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