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Subject: Re: [PATCH v7 07/10] tmp, tmp_tis: Implement usage counter for
 locality
From:   Lino Sanfilippo <LinoSanfilippo@gmx.de>
To:     Jarkko Sakkinen <jarkko@kernel.org>
Cc:     peterhuewe@gmx.de, jgg@ziepe.ca, stefanb@linux.vnet.ibm.com,
        linux@mniewoehner.de, linux-integrity@vger.kernel.org,
        linux-kernel@vger.kernel.org, l.sanfilippo@kunbus.com,
        lukas@wunner.de, p.rosenberger@kunbus.com
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Message-ID: <09eefdab-f677-864a-99f7-869d7a8744c2@gmx.de>
Date:   Sat, 8 Oct 2022 19:05:44 +0200
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Hi Jarkko,

On 28.07.22 at 17:45, Lino Sanfilippo wrote:

>>>
>>> This means switching to a complete new locking scheme which affects ma=
ny
>>> parts of the TIS core code. It is also not directly related to what th=
is patch series
>>> is about, namely activating the interrupts for TPM TIS.
>>>
>>> I suggest to first finish polishing this series especially since there=
 have
>>> only been minor issues in the last versions. Once the interrupts work =
we
>>> still can think of implementing another lock handling in a follow up s=
eries.
>>
>> So what if you would use kref instead here?
>>
>
> Sure, that should not be too difficult to do. I will implement this for =
the next version.
>
> Regards,
> Lino
>

First of all, sorry for this very late reply. Unfortunately in the last we=
eks I was
not able to work further on this series due to my private situation.
Nevertheless I tried to implement your suggestion (using krefs for the loc=
ality counting)
meanwhile. However krefs turned out to be a rather bad fit for this task.

The reason is that for the locality handling we have to perform a certain =
action (i.e.
writing to the access register) on two occasions:

1. When the locality is requested while no locality is active
2. When the locality has been released the number of times it has been req=
uested before

Since a kref is designed to track the lifetime of an object which is freed=
 as soon as the
kref counter hits 0, it starts with a counter of 1 when it is created, not=
 with a counter 0
(as we would need it, since at the beginning nothing has claimed the local=
ity yet).
Furthermore while kref provides a built-in mechanism to execute a function=
 when the counter
hits 0 it does not provide anything similar for the case that the counter =
is increased the
first time (i.e when we want to claim the locality by writing to the acces=
s register).

So although certainly doable I do not see much gain from using krefs in th=
is case. Again,
sorry for this late reply.

Regards,
Lino