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    SubjectRe: [PATCH] virtio: Don't access device data after unregistration.
    On Mon, Sep 03, 2012 at 04:50:10PM +0200, Sjur Brændeland wrote:
    > Hi Michael,
    >
    > >> Fix panic in virtio.c when CONFIG_DEBUG_SLAB is set.
    > >
    > > What's the root cause of the panic?
    >
    > I believe the cause of the panic is calling
    > ida_simple_remove(&virtio_index_ida, dev->index);
    > when the dev structure is "poisoned" after kfree.
    > It might be the "BUG_ON((int)id < 0)" that bites...
    >
    > >> Use device_del() and put_device() instead of
    > >> device_unregister(), and access device data before
    > >> calling put_device().
    >
    > > Why does this help? Does device_unregister free the
    > > device so dev->index access crashes?
    >
    > Yes, if device ref-count is one when calling unregister
    > the device is freed.

    Interesting. Where exactly? Note that:

    struct rproc_vdev {
    struct list_head node;
    struct rproc *rproc;
    struct virtio_device vdev;
    struct rproc_vring vring[RVDEV_NUM_VRINGS];
    unsigned long dfeatures;
    unsigned long gfeatures;
    };

    kfree(&proc_vdev->vdev) is unlikely to be the right thing to do.

    > > If yes virtio_pci_remove will crash too
    > > as it accesses the device after the
    > > call to unregister_virtio_device so the
    > > fix won't be effective.
    >
    > I discovered this using the remoteproc framework.
    > It might be that device is unregistered with ref-count greater
    > than one normally, in that case this bug will not show up.
    >
    > Regards,
    > Sjur

    It might be remoteproc has an unrelated bug?

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