linux-kernel - Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm

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Message-ID: <f6e5dd7d-30df-26d9-c712-677c127a8026@kunbus.com>
Date:   Fri, 5 Feb 2021 16:50:13 +0100
From:   Lino Sanfilippo <l.sanfilippo@...bus.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, stable@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

On 05.02.21 16:15, Jason Gunthorpe wrote:
> 
> No, the cdev layer holds the refcount on the device while open is
> being called.
> 
> Jason
> 

Yes, but the reference that is responsible for the chip deallocation is chip->dev
which is linked to chip->cdev and represents /dev/tpm, not /dev/tpmrm.
You are right, we dont have the issue with /dev/tpm for the reason you mentioned.
But /dev/tpmrm is represented by chip->cdevs and keeping this ref held by the cdev 
layer wont protect us from the chip being freed (which is the reason why we need
the chip->dev reference in the first place).

And yes, the naming dev/devs/cdev/cdevs is quite confusing  :(

Regards,
Lino