linux-kernel - Re: [PATCH v8 08/11] tpm, tpm: Implement usage counter for locality

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Message-ID: <Y2ksZXP1dunRyul8@kernel.org>
Date:   Mon, 7 Nov 2022 18:03:49 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Lino Sanfilippo <l.sanfilippo@...bus.com>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jgg@...pe.ca, stefanb@...ux.vnet.ibm.com, linux@...ewoehner.de,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        jandryuk@...il.com, pmenzel@...gen.mpg.de, lukas@...ner.de,
        p.rosenberger@...bus.com
Subject: Re: [PATCH v8 08/11] tpm, tpm: Implement usage counter for locality

On Fri, Nov 04, 2022 at 05:18:21PM +0100, Lino Sanfilippo wrote:
> 
> 
> On 01.11.22 02:06, Jarkko Sakkinen wrote:
> > On Tue, Oct 25, 2022 at 02:15:51AM +0200, Lino Sanfilippo wrote:
> 
> >> Actually thats on me, since it took me much too long to send the v8 after the v7 review.
> >>
> >> However the reason that we need a mutex here is that we not only increase or decrease
> >> the locality_counter under the mutex, but also do the locality request and release by
> >> writing to the ACCESS register. Since in the SPI case each communication over the spi bus
> >> is protected by the bus_lock_mutex of the SPI device we must not hold a spinlock when doing
> >> the register accesses.
> >>
> >> Concerning covering the whole tpm_tis_data struct:
> >> Most structure elements are set once at driver startup but never changed at driver
> >> runtime. So no locking needed for these. The only exception is "flags" and "locality_count"
> >> whereby "flags" is accessed by atomic bit manipulating functions and thus
> >> does not need extra locking. So "locality_count" is AFAICS the only element that needs to be
> >> protected by the mutex.
> > 
> > OK, but you should should still address this in commit message, e.g.
> > by mentioning that in the case of SPI bus mutex is required because
> > the bus itself needs to be locked in the mutex.
> > 
> > I.e. this a claim, definitely not an argument: "Ensure thread-safety by
> > protecting the counter with a mutex."
> > 
> 
> Ok, I will rephrase the commit message accordingly. 
> Thanks for the review!

Yeah, np. I.e. I understand your reasoning but it is easy to intuitively
think it as not the right solution. Thus, it deserves a remark, right?
:-)

BR, Jarkko