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    SubjectRe: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip
    On Fri, Feb 05, 2021 at 04:50:13PM +0100, Lino Sanfilippo wrote:
    >
    > On 05.02.21 16:15, Jason Gunthorpe wrote:
    > >
    > > No, the cdev layer holds the refcount on the device while open is
    > > being called.
    > >
    > 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).

    No, they are all chained together because they are all in the same
    struct:

    struct tpm_chip {
    struct device dev;
    struct device devs;
    struct cdev cdev;
    struct cdev cdevs;

    dev holds the refcount on memory, when it goes 0 the whole thing is
    kfreed.

    The rule is dev's refcount can't go to zero while any other refcount
    is != 0.

    For instance devs holds a get on dev that is put back only when devs
    goes to 0:

    static void tpm_devs_release(struct device *dev)
    {
    struct tpm_chip *chip = container_of(dev, struct tpm_chip, devs);

    /* release the master device reference */
    put_device(&chip->dev);
    }

    Both cdev elements do something similar inside the cdev layer.

    The net result is during any open() the tpm_chip is guarenteed to have
    a positive refcount.

    Jason

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