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

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Date:   Tue, 18 Oct 2022 08:25:08 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     peterhuewe@....de, jarkko@...nel.org, 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, l.sanfilippo@...bus.com,
        p.rosenberger@...bus.com
Subject: Re: [PATCH v8 08/11] tpm, tpm: Implement usage counter for locality

On Tue, Oct 18, 2022 at 01:57:29AM +0200, Lino Sanfilippo wrote:
> Implement a usage counter for the (default) locality used by the TPM TIS
> driver:
> Request the locality from the TPM if it has not been claimed yet, otherwise
> only increment the counter. Also release the locality if the counter is 0
> otherwise only decrement the counter. Ensure thread-safety by protecting
> the counter with a mutex.
> 
> This allows to request and release the locality from a thread and the
> interrupt handler at the same time without the danger to interfere with
> each other.
[...]
> +static int tpm_tis_release_locality(struct tpm_chip *chip, int l)
>  {
>  	struct tpm_tis_data *priv = dev_get_drvdata(&chip->dev);
>  
> -	tpm_tis_write8(priv, TPM_ACCESS(l), TPM_ACCESS_ACTIVE_LOCALITY);
> +	mutex_lock(&priv->locality_count_mutex);
> +	priv->locality_count--;
> +	if (priv->locality_count == 0)
> +		tpm_tis_release_locality_locked(priv, l);
> +	mutex_unlock(&priv->locality_count_mutex);
>  
>  	return 0;
>  }

Hm, any reason not to use struct kref for the locality counter?
Provides correct memory ordering (no mutex needed) and allows for
calling a release function too upon reaching 0.

Thanks,

Lukas