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SubjectRe: [PATCH v7 07/10] tmp, tmp_tis: Implement usage counter for locality
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On 04.07.22 19:45, Lino Sanfilippo wrote:
>
>
> On 01.07.22 01:29, Jarkko Sakkinen wrote:
>
>>
>> I'm kind of thinking that should tpm_tis_data have a lock for its
>> contents?
>
> Most of the tpm_tis_data structure elements are set once during init and
> then never changed but only read. So no need for locking for these. The
> exceptions I see are
>
> - flags
> - locality_count
> - locality
>
>
> whereby "flags" is accessed by atomic bit manipulating functions and thus
> does not need extra locking. "locality_count" is protected by the locality_count_mutex.
> "locality" is only set in check_locality() which is called from tpm_tis_request_locality_locked()
> which holds the locality_count_mutex. So check_locality() is also protected by the locality_count_mutex
> (which for this reason should probably rather be called locality_mutex since it protects both the "locality_count"
> and the "locality" variable).
>
> There is one other place check_locality() is called from, namely the interrupt handler. This is also the only
> place in which "locality" could be assigned another value than 0 (aka the default). In this case there
> is no lock, so this could indeed by racy.
>
> The solution I see for this is:
> 1. remove the entire loop that checks for the current locality, i.e. this code:
>
> if (interrupt & TPM_INTF_LOCALITY_CHANGE_INT)
> for (i = 0; i < 5; i++)
> if (check_locality(chip, i))
> break;
>
> So we avoid "locality" from being changed to something that is not the default.
>
>

I wonder if we need tpm_tis_data->locality at all: the claimed locality is already tracked in
chip->locality and in TPM TIS we never use anything else than locality 0 so it never changes.

Is there any good reason not to remove it?



Regards,
Lino

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